Jul 23, 2013 - Summary. The LHC Optics Measurement and Corrections review took place during the 17th and 18th of June 2013 1. This article summarizes ...
EuCARD-REP-2013-003
European Coordination for Accelerator Research and Development
PUBLICATION
Summary of the 2013 LHC Optics Measurement and Correction Review Bruning, O (CERN) et al 25 July 2013
The research leading to these results has received funding from the European Commission under the FP7 Research Infrastructures project EuCARD, grant agreement no. 227579.
This work is part of EuCARD Work Package 4: AccNet: Accelerator Science Networks.
The electronic version of this EuCARD Publication is available via the EuCARD web site or on the CERN Document Server at the following URL : 20%) of beta beating at 3.5 TeV as well as at 4 TeV start after β ∗ =3 m. Clearly, toward 7 TeV this figure could change due to saturation of magnets field and higher energy. MD studies performed in 2012 demonstrated that optics measurements during the ramp can be performed 4
and corrections implemented. This would require good tools and control of the settings. The change of optics during the ramp would require machine protection validations. MD studies performed in 2012 proved that loss maps during the ramp can be performed varying the ADT intensity at given intervals. Eliminating the possibility of punctual and local checks (so far done at the squeeze stop points) reduces our control on transitory phases. Closed orbit at the TCTs could potentially become problematic due to simultaneous reduction of Xing angle and change of bump shape. TCT with BPM would be helpful. Tune and chromaticity corrections could become critical toward 7 TeV. The optics distribution in the ramp has to be done manually with presently available tools. An LSA tool should be developed to allow automatic optimization and calculation of minimum required time. SW used for TCT functions generation would require changes. In order to confirm the feasibility and effectiveness of CRS a series of studies has to be performed (3/4 shifts needed) in addition to a dry run needed once a configuration has been fixed that can be done in the shade of operation. These studies include: test of the mechanics, measurement of the optics at flat-top to spot eventual changes, chroma and tune check, TCT function check, loss maps during the ramp, optics measurements on the fly, optics corrections (if needed). As some optics changes are anyway foreseen during the ramp (IP2/8) it might be worth squeezing even partially IP1/5. The complexity of CRS strongly depends on the final β ∗ value chosen. A baseline could be: Start without CRS, Perform few dedicated tests/MD, Converge on a final β ∗ value and set of optics, Commission CRS after some (6/12) months of operation. Mei Bai showed surprise concerning the worries to implement CRS in LHC: “You should trust your machine and model by now”. Multiple answers were given. Some people worry about collimation, other about stability. Psychology plays an important role for sure. Jorg commented that one could be more aggressive and go for RCS since the beginning.
4.7
FiDeL status & changes in field quality after LS1, Ezio Todesco
FiDeL (Field Model of the LHC) is the recipe to convert current in field/gradients, plus the precycling strategy. It is based on equations with free parameters to fit the measurements. All the knowledge of the magnetic measurements during correction is lumped in these equations plus their coefficients. At 7 TeV many LHC magnet will enter a regime of saturated iron (nonlinear transfer function). All saturations are implemented in the model since 2008. Some components are large, so with a relative effect of 5% one can have some uncertainty. The worst case is the inner triplet quadrupoles with 500 units saturation in MQXA and 180 in MQXB. This can induce large beta beating after squeeze. For the dipole should not be an issue (70 units saturation, error of a few units). Decay of tune and chromaticity are modeled and Corrected during the injection plateau (∆Q ≈0.02, ∆Q′ ≈25. Amplitude of decay depends on top energy. Decay and snapback will increase about 50% from 4 to 7 TeV. A chromaticity decay of 25 units will become of 40 at 7 TeV. Powering history will have to be re-estimated. Hysteresis are implemented in the model, but not used. Switching from one branch to the other is not obvious and creates discontinuities. For MQM, MQY quadrupoles the hysteresis effect can be implemented as a trim. Proper precycling is the key of reproducibility. Previous physics run can be used as a precycle. The precycling time is dominated by IR quads. Further discussiong on the implementation of hysteresis corrections took place. In particular Riccardo de Maria asked about the difficulty involved involved a hysteresis model. Ezio replied that it is not so easy. There are codes (big ones) that can do it. Mike Lamont commented about the chromaticity difference between the model (Q′ ≈ 2) and the machine (Q′ ≈ 12) at injection. Ezio replied that no effort was put on this as it is considered a small discrepancy.
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4.8
Non-linear modelling and machine set-up, Ewen Maclean
Arc skew sextupoles MSS are used for chromatic coupling correction (a3 in arc dipoles). MSS have not been used in operation so far. First commissioning was performed with Beam as an MD in 2012. The corrections should be implemented post-LS1. The sextupolar spool pieces MCS are intended for correction of b3 in arc dipoles. They are used since start of commissioning and have never been checked. Beam based check of MCS should be included in the post-LS1 commissioning. MCO & MCD are nested octupole and decapole spool pieces intended for correction of b4 and b5 errors in arcs. MCO are not pre-cycled and some families were not operational. In 2011 the nonlinear chromaticities were corrected at injection with global trims applied to the MCO and MCD to correct Q” and Q”’. They also reduced amplitude detuning improving the beam decoherence. There are very substantial deficits in the simulated non-linear chromaticites compared with measurements. Between 75% and 100% of measured Q” is missing from the model. MCO hysteresis explains a significant fraction of the deficit. Between 50% and 85% of measured Q”’ is missing from the model (simulations done with MAD-X / PTC including best available knowledge of magnetic and alignment errors). MO octupoles provide Landau damping for stabilization of the beams. In 2012 Q′ shifts correlated with MO powering were observed. Feed-down from MO alignment explains 15-50% of observed ∆Q′ . Another possible source is the observed substantial systematic closed orbit offset in the MOF and MOD, which gives feed-down to ∆Q′ . These two together explain majority of observed ∆Q′ . However, these cannot explain dependence of coupling on MO powering as measured with BBQ. To verify the observed couplings AC-dipole measurements with MO on and off were performed and coupling resonance-driving terms were obtained much smaller than with BBQ. It was concluded that the BBQ coupling in presence of strong octupoles had some systematic error. In June 2012 substantial amplitude detuning, dominated by MO, was measured. Missing b4 (determined by matching first order detunings to measurement) agrees well with Q” deficit in 2011 measured with zero MO. Dynamic aperture at injection was determined from losses on kicking the beam giving 9.3σnominal . Correction of Q′′ and Q′′′ increased it to 11.2σnominal . Coupling amplitude/phase give non-negligible uncertainty in DA simulation. Agreement between model and measurement is better than the factor 2 of safety margin previously specified. In second half of 2012 the MO polarity was reversed. In the new polarity the amplitude detuning at injection is considerably smaller than for the old polarity. This could be interpreted as the MO doing the job of MCO. Non-linear errors in the IRs can have significant impact because low-beta insertions require large beta in the triplets and separation dipoles. Most of the MCXs, the dedicated correctors on both sides of the IPs, should be available after LS1. Ideally, the errors should be corrected by minimization of dominant RDTs. Alternatively, it is possible simply to correct errors locally on either side. It requires accurate magnetic model of NL errors in IR: aim to verify magnetic model with beam-based measurements and need to verify corrections with beam-based measurements. One can identify the NL errors, and check corrections, via feed-down to tune and coupling with varying closed orbit (CO) bumps through IR. In 2011 a test of the reversal of IR2 external crossing angle was done and excellent agreement between MAD-X model and magnetic measurements was found. The NL errors in IR2 are dominated by b3 in D1 separation dipoles. IR2 model also agrees well for higher orders and the errors in IR2 are very well understood. In 2012 dedicated studies of NL errors in IR1 were performed and measurements on Beam 2 during β ∗ =40 cm MD agree well with the model. In Beam 2 a3 and b4 corrections were successful at 60 cm. Also performed dedicated measurements in IR5 at 60 cm and observed a large discrepancy with the model. No corrections were applied since IR5 should be better understood. In conclusion, Ewen would like to include MSS and MCS in post-LS1
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commissioning and he suggests a beam-based non-linear chromaticity correction of bare machine with MCDO, what should simplify operation with MO. There was a comment on the measured resonance phase-space plots looking like simulations. Also a question was asked concerning comparisons between the two LHC beams. Ewen answered that only dynamic aperture of beam 2 could be measured with the kick technique.
5
Beam instrumentation
Chairman: Rhodri Jones Scientific secretary: Ralph Steinhagen This session reviewed the specifications, achieved performance and planned upgrades for the various beam instrumentation systems related to optics measurements during LHC first long shutdown (LS1).
5.1
Requirements from BI and new instruments after LS1, Bernd Dehning
The initial specifications for emittance measurements in LHC and its injector, detailed in [2, 3], requests bunch-by-bunch emittance resolution and accuracies in the order of 5 % and for an integration period of less than 0.1 s. The implied assumptions was that the targeted 5 % uncertainty on emittance is split in equal parts between the uncertainties deriving from the beta function and beam size measurement, corresponding to an absolute uncertainty of 1.8 % on the beam size measurement (N.B. dσ/σ = 1/2dε/ε). Tests at the CERN PSB, PS, SPS accelerators and complemented by lab laser-beam-based tests indicated fluctuations of the beam width estimation in the order of 100 µm, intrinsic to the principle of the existing scanner hardware [4]. In response, two new systems are foreseen as an upgrade during LS1: 1. A new all-in-vacuum rotary wire-scanner (WS) design with a single axis and high-resolution position encoders that is target to improve the fluctuation of the beam size measurements down to the 5 µm level. The secondary showers created by the wire are planned to be measured by biased diamond detectors in order to accommodate the required large dynamic range (≈ 104 ) [5]. 2. A new Beam-Gas-Vertex-Detector (BGV), inspired and similar to the existing Velo detector used in the LHCb experiment. The beam size is reconstructed through tracking the scattered protons of the beam-rest-gas interactions using silicon-vertex detectors close to the vacuum beam pipe. The system requires the injection of a small amount of gas into the vacuum chamber, but intrinsically can measure the beam profile bunch-by-bunch, however within a given integration interval. The resolution depends on the actual beam size and number of tracks per vertex (i.e. amount of rest-gas in the vacuum chamber). For the planned setup and based on the existing LHCb experience, relative accuracies in the order 5 % for bunch-by-bunch and about 2 % for the average beam width measurements are expected for an integration period of less than a minute [6]. Using either system, to obtain the specified emittance accuracy of 5 % a beta function absolute accuracy of 3.5 % needs to be obtained at the location of the WS and BGV. In response to the questions by Richard Talman (Cornell University) on the larger choice of BPMs and meaning of errors (slide 14), R. Tomas replied that the different error scenarios depend on the specific selection of BPM combinations and that the table rows in slide 14 shows the estimated absolute value β(s) of the beta-function, its systematic σβ (sys.) and random σβ (rnd.) component.
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In his presentation Bernd Dehning mentioned plans to improve the beta-functions in LSS4 around the WS and BGVs. J. Wenninger asked why the beta-functions are not yet sufficient. B. Dehning replied that this was a request driven by BI to increase and/or to shift the location of the maximum beta-functions in favour of the various beam instrumentation systems. J.J. Gras added that this was in response to an offer from ABP (S. Fartoukh) in view of re-tuning of the ATS optics but (within limits) could be also applied to the nominal LHC optics. Some preliminary discussions took place between ABP, BI, OP and RF (for the ADT and RF systems). R. Tomas clarified that changes to the nominal LHC optics are priori decoupled from the new ATS optics. J. Uythoven indicated that other non-BI/RF devices (such as MKA etc.) installed at point 4 might be also affected by these changes. M. Wendt asked about the maximum beam intensity allowed for cross-calibration of the various emittance measurement devices. B. Dehning explained that this depends on the beam energy: limit at high energy being potential quenches of the superconducting magnets, and at low energy damage of the wire. R. Jones added that a practical cross-calibration across all beam types is not possible. B. Dehning, in response to Mei Bai (BNL) question on effective wire life-times, indicated that the typical life-time under nominal conditions is about a year. However, he observed some early wire breakage due to non-nominal operation and heating issues caused by trapped RF modes inside the WS tank.
5.2
AC dipole upgrades, Nicolas Magnin
The AC dipole system shares the same MKQ/MKA magnet with the aperture and tune kicker systems but deploys a different current driver. The specification, functional principle and performance are outlined in [7, 8, 9, 10]. The system is thoroughly interlocked by procedure and interlock hardware to prevent accidental firing under nominal beam operation. Presently the driven oscillations are ramped up, kept constant at flat-top levels for about 200 ms, before being ramped down. Due to power limitations (heating of the driver) the AC dipole firing period is limited to about once per minute. Some issues related to synchronisation of the AC-dipole and BPM turn-by-turn acquisition with respect to the same beam phase were observed resulting in a undesired one turn uncertainty between both systems. Upgrades during LS1 foresee to move the existing controls infrastructure to standard CO front-ends, allow local pulsing for commissioning and tests during technical stops. The possibility to increase flat-top duration from 200 to 600 ms is being investigated. Also, 50 Hz interferences potentially being introduced by the AC-dipole driver are being investigated. R. Tom´as in response to a question by Mei Bai replied that the turn/phase synchronisation between the AC-dipole and BPM system is needed to compensate for systematics and to simplify the averaging of measurements across several acquisitions. R. Jones proposes to use the same BST-based trigger for the AC-dipole as being used by the BPMs. This would compensates for cable delays and guarantee the required synchronisation between both system. ACTION: BE-BI, EN-ABT R. Steinhagen asked whether lowering the flat-top amplitude by e.g. a factor 10 would alleviate the power limitations and possibly allow continuous wave operation? Nicolas Magnin confirmed the concept and added that this would imply mostly changes to the FPGA code as the ’ramp-up, flat-top, ramp-down’ sequence is presently hard-coded. Other similar firmware options are possible but probably not feasibly w.r.t. the other LS1 constraints. R. Tomas welcomes the possible improvement of the flat-top duration from 200 to 600 ms as this would already provide three times more measurements compared to the present situation.
5.3
Future operation of the aperture kicker MKA, Jan Uythoven
The MKA uses the same magnet but with a different generator and operational use-case as the MKQ and the AC-dipole magnet, with a main difference being the severity of the impact on machine 8
protections. These lead to limiting the kick strengths compared to the initial MKA design (5.2 σ at 7 TeV and 20.1 σ at 450 GeV) [9, 10, 11]. The parameters being used were: • until-2012: maximum voltage 864 V, corresponding to a kick of 5.3 σ at 450 GeV and 1.4 σ at 7 TeV • from 2012: maximum voltage 1.8 kV, corresponding to a kick of 14.5 σ at 450 GeV and 3.7 σ at 7 TeV (ǫn = 3.75 µm, [11]) Similar to the AC-dipole, the MKA is thoroughly interlocked by both operational procedures and hardware interlocks to prevent accidental firing under nominal beam operation. In 2012, during regular operation the MKA is disabled with pad-locks in tunnel (consigne) and implies a mandatory access prior and after its use. J. Uythoven elaborates the following three possible scenarios for Post-LS1 operation of the MKA: • use-case 1: similar to what has been used in 2012, with a double-kick (2x half-machine) option available which is presently not being exploited, • use-case 2: to explore higher beam energies, multiple-kick option possibly interesting for tune and chromaticity measurements using 1 σ kicks (would imply pre-2012 setup), • use-case 3: if encountering dynamic aperture issues at 6.5 TeV, one may need to re-discuss stronger kicks going beyond 1.8 kV. This implies a driver hardware upgrade and of the related machine protection items. The present TE-ABT working and MP assumptions for Post-LS1 operation is to maintain the a ’status quo’ situation with operating the the MKA within the limits used in 2012 and outlined in [11]. Future upgrade could foresee the installation of a Beam Energy Tracking System (BETS) on the MKA which would allow a maximum MKA kick dependence on energy, i.e. to inhibit the kick if the requested kick is too large ACTION: ABP to choose/confirm desired configuration for the post-LS1 start-up: • kicker voltage of 1.8 kV with access constraints (N.B. 15 σ kick at 450 GeV), • kicker voltage of 0.6 kV without access constraints (N.B. 5 σ kick at 450 GeV), or • kicker voltages beyond 1.8 kV, which are however presently not foreseen due to MKA and MPS limits. J. Uythoven added that the 1 minute pause between pulses can be investigated upon request if multiple, small kicks at higher energies are of a interest. R. Tom´as inquired whether with the BETS on the MKA the system can be considered as ’safe’, or more specifically with the BETS being in place, whether the access before and after MDs could be omitted? J. Uythoven replied that the BETS primarily would add safety. Dropping of the procedural mandatory access prior and after an MD would need to be further discussed within MPP.
5.4
BPM software and longest acquisitions, Lars Jensen
The functionality of the digital acquisition board (DAB) system as being used by the BPMs is outlined in [12]. The BPM DAB card provides an asynchronous (auto-triggered or first-in-first-out ’FIFO’ mode) and a synchronous acquisition that needs to be phased-in to bunch slot ’1’ (N.B. bunch ’slot’ covering 25 ns, or 10 2.5 ns spaced RF buckets). The synchronous mode provides on a specific bunch 9
(/group) gated orbit, post-mortem data and bunch-by-bunch capture data measurements. The maximum amount of capture data is presently limited in software to 3000 turns for 1 bunch, while the electronics is capable of storing the product of ’number of selected bunches’ and ’number of turns’ being less than 128 kSamples per BPM. Presently the BPM DAB firmware provides only one buffer for injection quality checks (IQC) and other MD purposes. Both cases rarely use the maximum available 128 kSamples per BPM. The planned LS1 upgrades related to the SW infrastructure include an upgrade of front-end CPU upgrade which is expected to improve the memory and network IO-performance of the system, and a re-design of the various software layers involved in response to operational experience and previous use-cases. This should allow the request of storing 10000 turns for a single bunch for all BPMs to be fulfilled. R. Jones suggested that since various BPM DAB firmware upgrades are being discussed, that every BPM could have its own IQC buffer. Lars Jensen confirmed this but mentioned that it would imply changes to the firmware. ACTION: BE-BI-QP (E. Calvo-Giraldo, JJ. Savioz), BE-BI-SW (L. Jensen)
5.5
New LHC Diode ORbit and OScillation System (DOROS), Marek Gasior
Initially, the primary DOROS application was to precisely centre the TCT collimator, using BPM buttons that are integrated into the collimator jaws. Later it was also considered to use the same system to complement the LHC orbit measurement with regular BPMs. DOROS is a new beam position electronics, a spin-off and based on the good experience with previous diode-based detectors used in the BBQ, and deploys compensated diode-detectors measuring the peak-amplitude of each BPM button signal independently, and performs the required post-processing in the digital domain [13, 14]. The system takes advantage of the ’only orbit measurement’ requirement (in comparison to the standard LHC BPM system that needs to measure bunch-by-bunch), and demonstrated in initial lab tests and beam-based measurement at the LHC sub-micrometer resolutions and accuracy better than 10 um over about 5 hours. Based on earlier proof-of-concept in the SPS and LHC [15, 16], a dedicated oscillation detection channel has been added that allows sensitive BPM-to-BPM phase advance measurements and subsequently enabling continuous measurements of the beta-function. The combined system will be primarily optimised for orbit measurements and the sensitivity is expected to be less compared to the existing stand-alone BBQ system. Thus, it is expected that the measurement of the phase-advance using this system will still require driven excitations in the order of 10 um (N.B. AC-Dipole/BPMbased measurements excite at the level of 1 mm). For the start-up, it is planned to install DOROS as the TCTP collimator BPMs acquisition system (some 16 collimators), for the planned two new BPMs in IR4 dedicated for the BGI calibration, and the Q1 BPM before and after each experiment for the immediate post-LS1 operation. In the longterm the same system could be deployed for all relevant BPMs ranging between Q7.L up to Q7.R of each LHC point. In response to B. Dehning asking why 24bit-ADCs are being required for DOROS, M. Gasior replied that the button signals are being acquired independently and thus the large number of bits is needed to accommodate the intensity related signal. The small position signal is only extracted after the digitisation. G. Arduini and J. Wenninger inquired why only the DOROS acquisition chain is being foreseen at the BGI BPMs. M. Gasior replied that this is motivated by the fact, that these will be new BPM pick-up installations located directly next to the BGI, and that these are primarily needed for BGI cross-calibration purposes only. The regular BPM electronics hasn’t been requested by the BGI responsible. R. Jones added, that also further splitting of the BPM button signal is less ideal for the button pick-ups, since compared to the strip-lines pick-ups in the other IRs these provide less 10
signal. Mei Bai asked whether k-modulation is regularly being used to estimate the BPM to quadrupole centre offsets? J. Wenninger replied that this had only been done periodically but that no significant drifts had been observed.
5.6
BPM system upgrades and temperature regulation, Eva Calvo Giraldo
A system overview and Wide-Band-Time-Normaliser (WBTN) acquisition principle are outlined in [17, 18]. The main measurement uncertainty contributions have been identified as: geometric pick-up non-linearities, WBTN-intrinsic non-linearities and temperature related drifts of the analog electronics [19, 20, 21]: Initially the geometric non-linearities have been compensated only for the on-axis terms (i.e. no cross-plane dependences) leading to errors up to 400 (800) µm for nominal on-axis (vs. diagonal) beam position offsets of 10 mm. It is planned to deploy the cross-term corrections for post-LS1 operation which is expected to reduce the errors below the 100 (200) um level (on axis vs. diagonal for large offsets). The intrinsic WBTN electronics non-linearity is presently compensated by a simplified third order polynomial that is based on the average of all individually measured WBTN cards. This results in residual relative errors of up to 0.5 % w.r.t. the BPM half-aperture. All BPMs racks are being replaced by temperature controlled racks during LS1 to mitigate the known dependence of the electrical BPM offset on temperature [19, 20]. E. Calvo pointed out a remaining (under certain circumstances important) systematic affecting the strip-line BPMs in locations with a common beam pipes: the reduced strip-line directivity for these pick-ups may cause cross-talk between the B1/B2 measurements for some beam patterns despite their locations having been already optimised to mitigate this during construction. It was found that the residual measured directivity is reduced to some -20 to -29 dB depending on the chosen bandwidth. For orbit measurements this effect can be alleviated by gating on bunches that are sufficiently separated w.r.t. parasitic beam crossings. E. Calvo recommends that for precise optics measurements non-colliding bunch schemes with sufficiently separated parasitic bunch crossings should be used. R. Tom´as inquired why the specific by-WBTN electronics calibration has not been applied operationally. E. Calvo explained that this was not foreseen as part of the baseline operations, as these effects are only important for large orbit offsets and changes, No further upgrades are planned. N.B. Remaining WBTN non-linearity is affecting rather dynamic aperture studies than phase advance measurements needed to determine the machine optics. E. Calvo estimated in response to M. Wendt question that the achieved strip-line directivity depends on the chosen analog bandwidth of the system and ranges between -20 (high bandwidth) and -29 dB (low bandwidth).
5.7
Automatic coupling correction, Tobias Persson
The presented coupling measurement method has been initially prototyped at RHIC and been tested over the course of 2012 at the LHC [22, 23, 24]. The method being similar to what is being used operationally by the BBQ PLL system [25], uses two rather than one single pick-up to estimate the coupling. Using two pick-ups also enables the independent measurement of the sum ’f1010 ’ (negligible for LHC due to specific tune working point) and difference ’f1001 ’ coupling resonance as well as reduces the systematic measurement error for very small coupling values. The method has been tested mainly using injection oscillations and standard LHC BPMs, and as a cross-check using the ADT and two neighbouring BBQ stations. The signal with the ADT was
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insufficient in most cases, and the BBQ-based measurements required kicks or injections to synchronise the acquisition between the independent stations. Continuous operation to estimate the coupling with BBQ limited by the lack of turn-by-turn synchronisation between individual BBQ stations, and for B1 the non-ideal phase advance between the given pick-ups. N.B. The initial BBQ system design was rather aimed at frequency than phase measurements. While ADCs are clocked on the revolution frequency, the absolute and frequency dependend relative phase response between BBQ front-ends is not synchronised. The fact that the BBQs use single-plane pick-ups and mostly monitors random burst-like not necessarily coherent oscillations, introduces some important systematics compared to using dual-plane pick-ups and AC-dipole type oscillations [26]. The BBQ synchronisation issue is being addressed by the above mentioned DOROS system which is being designed for synchronised BPM-to-BPM phase-advanced measurements and which should provide better phase-alignment. T. Persson and R. Tom´as asked whether DOROS could be gated e.g. to pilot bunches? This may allow measurements during regular beam operation similar to 2012 where 6 bunches per beam were excluded from the high ADT feedback loop. M. Gasior replied that DOROS is not gated for the time being but could be if needed and necessity is justified. R. Jones inquired about the 2000 turn limitation at injection? Limited by filamentation time constant or settings compatibility with other use cases? L. Jensen clarified that this was more an operational settings issue since the typical injection settings are driven by IQC requirements (20 turns). During injections, the 2000 turn settings were only used during special studies.
6
Optics measurement and corrections
Chairman: Rogelio Tom´as Scientific secretary: Yngve Levinsen The goal of this session was to review the improvements and developments in optics measurement and correction algorithms and techniques.
6.1
Improvements in optics measurements and error reconstruction, Andy Langner
The current algorithm to determine LHC β functions uses the phase advances between three adjacent BPMs. This is shown to not be optimal for the arcs and very inconvenient for the IRs where the phase advances can be very small. Exploring combinations of 3 BPMs within a set of 7 adjacent BPMs gives much better results. An illustration in IR4 shows an order of magnitude improvement in the resulting error bar of the beta function. In the arcs using BPMs which are further away deteriorates the systematic error reflecting the lack of knowledge of the model. To mitigate this the systematic quadrupolar error (b2 ) of the dipoles is introduced in the model when computing β functions. The new algorithm using 7 BPMs together with dipole b2 error in the model gives better resolution measurements of the arc β functions. Richard Talman suggested the development of an equation that would take the phase advance between all the 7 BPMs or to compute weighted average and rms errors from all the possible combinations of 3 BPMs among the initial 7. The segment-by-segment technique is used to propagate beta functions at elements of interest and to compute corrections. The planned improvements to this technique consist in choosing two complementary starting points to minimize the detrimental effects of error propagation with the aid of analytical equations. A Monte Carlo approach proves very efficient in finding lattice errors, like misalignments or gradient errors, and has enough flexibility to include constrains from other measurements, like observables from the ALFA detector or k-modulation. Witold Kozanecki remarked the importance of accurate β ∗ measurements as this directly impacts Van der Meer scans. Basically
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the orbit deformation during the scans affects the measured luminosity by introducing an extra offset which is directly proportional to the β ∗ .
6.2
Optics corrections in RHIC, Mei Bai
RHIC has achieved a β ∗ of 65 cm with close to 100% β-beating in some planes of the Yellow and Blue rings. AC dipoles are routinely used for optics measurements. An Independent Component Analysis (ICA) has demonstrated more powerful than the traditional SVD in RHIC. The systematic error arising from the AC dipole is cancelled by using driving frequencies at both sides of the tune and averaging the resulting β function measurements. RHIC implements a rather fast combined ramp and squeeze without any difficulty. The β-beating starts to be significant for β ∗ below 2 m. Finding optimal global corrections reduced β-beating by a factor between 2 and 3. A recent proposal to use orbit bumps at sextupoles to correct the residual βbeating proved very successful reaching 5% (if an average modulation along the rings is subtracted). A peak orbit excursion of 8 mm was used in the corrections. Gianluigi Arduini and Rama Calaga asked about the implications of this orbit excursion in the physical and the dynamic apertures. Mei Bai replied that since these corrections were not used in operation a complete assessment is not possible, but no detrimental effect was observed during the experimental session.
6.3
Chromatic coupling correction, Yngve Levinsen
The chromatic β-beat (Montague functions) were observed during the commissioning in 2012, and were well corrected with the correction of linear β-beat. For the chromatic transverse coupling, there are 8 families of four skew sextupoles (MSS) installed in the LHC per beam. For the measurements in 2012, one corrector for beam 1 was out of order. The measurement of chromatic coupling agrees well with model predictions. A new response matrix correction algorithm has been implemented, taking a similar approach as the correction of linear coupling. The correction calculated from this algorithm has a similar performance as the older algorithm which corrects more locally, but has the advantage that the magnet strengths are significantly lower. The correction was applied during an MD in 2012, and an improvement of about 20 units was observed for both beams. Mei Bai asked if a reduction in Q” was observed. This has not yet been checked, but is expected. The impact on machine performance was not studied, these measurements were parts of a larger MD. J¨org Wenninger commented that the corrections from FiDeL were active during parts of the 2012 run.
6.4
Plans for K-modulation, Maria Kuhn
The K-modulation measurement is being reviewed to allow for high resolution measurements in the IR4 instrumentation and in the Interaction Points (IPs). The β-function measurement uncertainty is dominated by the tune noise. Mike Lamont asked Ralph Steinhagen about the plans for commissioning the PLL. Ralph replied that this is planed for 2015 but that PLL might not provide a better tune measurement. Oliver Br¨uning added that the PLL could only look at the K-modulation frequency to actually give a better resolution. The K-modulation application will be upgraded with a collection of new features as automatic estimate of required strength modulation, a sinusoidal modulation, data cleaning capabilities and on-line results. Stephane Fartoukh commented that the tune feedback could be used during Kmodulation measurements to allow for very large modulations without being limited by resonances. The change in the tune trim quadrupoles is then used to estimate the tune change.
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Oliver Br¨uning recommended to check with the QPS experts what is the highest modulation frequency and profit from the LS1 in case there could be actions to increase this frequency. From previous experience a frequency of 0.1 Hz was used. Oliver also asked Rogelio Tom´as about previous inconsistencies between AC dipole and K-modulation measurements. Rogelio answered that indeed partial inconsistencies in some β ∗ values were observed and no time was allocated for further measurements. Stephane commented that the modulation cannot exactly be a sine in the unipolar power supplies as the voltage reduction occurs as a free fall.
6.5
OMC Codes, Thomas Bach and Viktor Maier
The OMC codes are undergoing extensive debugging and improvements yielding to zero severe problems and a considerably speed-up by about a factor 10. Special emphasis is being put in maintainability (moving to github and developing a style guide) and unit tests (with nightly tests) to ease future developments and newcomers. In fact the LS1 break is being used to refactor the Python codes and to start a new phase of developments. On a humorous tone it was stated that physicist do “bad code” which works and that computer scientists turn it into “good code”, the underlying message being that the collaboration of physicists and computer scientists is fundamental for this project. J¨org Wenninger asked about the Eclipse version and Viktor answered that the CERN BE-CO Eclipse version was used. Kajetan Fuchsberger commented that BE-CO can help with code quality and maintainability and that probably tighter links to BE-CO would be needed. The CERN git/JIRA will be considered later staying with github for the moment.
6.6
Observations on dispersion (2011-2012), Kevin Li
Dispersion was only corrected in 2012. A closer look at differences between the dispersion across the IRs in 2011 and 2012 was presented. Basically, in 2011 all possible combinations of positive, negative and zero dispersion beating is observed in the IRs due to the residual uncorrected errors and crossing angle bumps. In 2012 dispersion follows the model prediction (with spurious dispersion from bumps) as expected after correction. A plan was presented evaluating the impact of IR dispersion in basic observables and possibilities to apply correction towards a different target dispersion. The main goal is to answer the following questions: Is it advantageous to have zero dispersion in the IRs? Is it possible? Stephane commented that the main motivation for controlling the dispersion would not be for single particle dynamics but in combination with long-range beam-beam effects. He mentioned that possible chromaticity shifts of 2 units could happen with large dispersion. He also mentioned that the ATS scheme has the capability to correct dispersion in a single phase thanks to orbit bumps in the arcs. Rogelio commented that both Nominal and ATS optics have some flexibility to act on dispersion as shown during the talk and that indeed ATS has that extra knob using orbit bumps so limits and potential should be assessed.
6.7
Measurement of amplitude detuning with AC dipoles, Simon White
A newly developed theory and recent LHC measurements of amplitude detuning using AC dipoles were presented. Theory predicts a factor of 2 difference between the direct terms of the free amplitude detuning and the forced amplitude detuning. Cross terms remain equal between the two oscillation types. Taking this into account the measurement performed at β ∗ =0.6 m in 2012 reveals a discrepancy with the model by about a factor 2-3. However this measurement was performed after
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correcting IR non-linearities. Therefore the amplitude detuning is dominated by small distributed errors which are hard to model. Laurent Deniau commented that actually pure PTC should be able to reproduce the factor 2 from the theory. Currently it is not possible to use this feature of PTC from MADX. He said he plans to make this upgrade within one year.
7
Early commissioning stage
Chairman: Mike Lamont Scientific secretary: Verena Kain This session reviews commissioning plans, tools and procedures aiming to answer the following questions: Are we ready to face a commissioning coming out of a long shut down with major machine modifications? Will there be sector tests?
7.1
Outline of 2014/15 plans, Verna Kain
Re-commissioning the LHC for beam in 2015 foresees the following phases. • Dry runs and individual system tests will start in March 2014 (LBDS reliability runs are foreseen for March 2014.) • Hardware commissioning will provide the opportunity to test all power converter functionality along with operational settings. • A sector test is planned beginning of November 2014, • The whole month of January 2015 is allocated for machine checkout. • Initial beam commissioning through to first low intensity stable beams will take place in February and March 2015. The commissioning strategy will follow the approach of the commissioning in 2012 adding the time required for full machine protection and individual system re-commissioning following the major interventions in LS1. Dry runs Each dry run block will be dedicated to one or several topics. All interfaces that are required to test a certain system need to be operational for the dry run. Dummy inputs and test modes have to be foreseen. A more-or-less complete set of machine settings will be required. Systems to be included in the dry runs include: LBDS; Timing; Beam instrumentation, kickers and feedbacks; Injection; Beam interlock system, software interlock system and safe machine parameters; Collimators, TCDQ and roman pots; RF and transverse dampers; interface to experiments; power converters; post mortem and XPOC. Sector test The sector test represents a full-blown integration test and is an important first milestone and first debugging after LS1. The test foresees taking beam 2 into point 8 and through sectors S78 + S67. If possible the beam dump will be used in inject and dump mode. Preparation is critical and will require: • LHC pilot in injectors with SPS extraction commissioned • Optics for transfer line + arc uploaded
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• Sequence – “prepare LHC for injection” operational • LHC injection system fully operational • BI (FIFO BPMs, screens, BLMs, BCT in dump line) operational • TDI, TCDIs, collimators in point 6,7 operational • BETS Sim set to 450 GeV • beam dump connected to BIS loop (only required inputs enabled) and tested • Power converters • TEDs and access system tested, machine closed. Proposed tests include: • synchronization of injection kickers and beam dump • threading • dispersion measurement on injected beam dispersion matching • kick response checks of BPM and corrector polarity and checks of the linear optics • aperture measurements in injection region and arc • checks of reproducibility after pre-cycle to 6.5 TeV • MSI hysteresis check • transfer line stability • MKE4 waveform scan • automatic TCDI setup. Machine checkout Machine checkout will include the following. • HWC should finish with an extend heat run. • All circuits should ready and in a state to test the final functions • Energy Tracking Tests • Final MPS and equipment to BIS user input tests • Close beam permit loop with all user inputs connected. With the machine closed, all circuits should be OK, BLMs operational, and the vacuum valves open • Final tests of beam dump with permit loop • Test injection kickers: real conditions • Run the LHC through full cycle including all equipment
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Commissioning with beam Commissioning with beam will follow the usual program of: • Establishing the baseline cycle with low intensity beam • Measurement of key machine characteristics • Commissioning of beam instrumentation • Commissioning of individual systems with beam • Preparation for higher intensity (machine protection tests, collimator set-up etc.) Commissioning combined ramp and squeeze and/or β ∗ leveling or colliding squeeze was not discussed. Details will have to be worked out if these scenarios become LHC run 2 baseline. If planned to be used, the combined ramp & squeeze commissioning would have to be scheduled early on during the LHC start-up, before detailed setup of collimators. 25 ns operation should be the goal of the commissioning after LS1. 50 ns operation will however certainly be a stage during the re-commissioning. It has been re-iterated that 25 ns operation is the desired operational mode for the LHC experiments; 50 ns operation should be seen as an early commissioning option only.
7.2
Transfer lines, injection and extraction re-commissioning , Chiara Bracco
SPS extraction: • Solutions to improve the SPS orbit stability via correcting the extraction region are under investigation. There appears to be a need for orbit corrections at high energy for improving stability. Interlocking strategies for the novel use of orbit correctors at high energy in the SPS must be defined. • There are possibilities to improve the absolute orbit with Q20 by finding a quadrupole alignment that reduces the orbit RMS for both Q20 and Q26 at the same time. A combined correction for Q20 and Q26 implies treating the SPS as a 2 beams machine and will be, at least initially, deployed as an expert tool. • Correction at the SPS extraction point is to be pursued. The idea is to fit a betatron oscillation around each extraction point and interpolate to the extraction point. This method is less sensitive to individual BPM errors. • Improvement works foreseen in LS1 to reduce the MSE ripple. • Non local extraction to be studied and tested and risk analysis is to be given serious consideration. The approach for setting up the transfer lines, the transfer line collimators, LHC injection and LHC extractions will be the same after LS1: • A tool is ready for TCDI automatic setup. • A new tool for quasi-online analysis will be studied and developed. • Optics checks with virtual β ∗ to catch TCDI wrong settings will be deployed.
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LHC injection and extraction system commissioning: There is the usual list of machine checkout and beam commissioning tests that span both injection and extraction. Injection tests include: injection protection collimator setup and validation with a new quasi-online tool; studies of injection losses; and aperture, emittance, mismatch and optics measurements. Extraction tests include: aperture measurements; beam loss studies; TCDQ-TCSG setup and validation (with perhaps a new optics); abort gap cleaning checks with an automatic link to BSRA possible after LS1. The new LHCb crossing-separation scheme will not change polarity in the vertical plane during an LHCb polarity flip. Only the vertical separation will be increased instead of switching to vertical crossing. Regarding moving SPS fixed target operation to Q20 it was pointed out that rather than deteriorate the FT operation it would be better to invest in a good correction strategy with quadrupole alignment once a year which satisfies the needs of Q20 and Q26.
7.3
Initial Optics Checks, Kajetan Fuchsberger
An overview of the linear optics checks performed during the early commissioning in 2008/9 was presented together with a look forward to 2014/2015. Broadly the subject breaks down into: sector test; tests with circulating beam; and the available tools. The sector test will use, in the main, kick response and dispersion matching. Similar measurements to those of 2008/9 will have to be re-done this time to ensure proper machine configuration. This will allow identification of optics errors, measurement of BPM gains, and COD gains. Circulating beam will allow systematic checks of BPM gains, BPM and COD polarities, and β-beat measurements via phase advance measurements. Tools required will include: • YASP: threading, orbit, dispersion, kick response, BPM checks • Aloha + JMad • Multiturn application: acquisition for phase advance analysis • Beta beat GUI: analysis of phase advance measurements The tools are available and working. The availability of manpower to maintain and use the tools was questioned. Some required specialist knowledge to be used. • With kick response measurements and corrections proposed by ALOHA, polarity inversions of power converters, sources of the dispersion mismatch for beam 2 at injection and BPM gain issues due to uncorrected electronics could be identified during the sector tests. • ALOHA uses the SVD or MICADO algorithm to find corrections for dispersion, kick-response or multi-turn phase-advance measurements. • New and faster techniques for circulating beam will available after LS1 to test BPM and COD gains and polarities. • Kajetan proposes to use software tools for automatic test tracking for re-commissioning the LHC such as being prepared for hardware commissioning in the framework of accTesting. Tracking the progress of the different machine preparation tests already starting with the dry runs would be beneficial. The tests would not have to necessarily be automated, but checkboxes and summaries could be helpful.
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7.4
On-line model, Ghislain Roy
The LHC online model and its implementation in the LHC control system was discussed. The current implementation is based on JMAD and its model description. Errors are not included. The online model acts as JAVA interface to MAD-X for optics generation and simulation in LSA. Knob generation, optics upload, optics simulation through squeeze etc. are covered. The online model software cannot interact with the machine directly, only through the path generating knobs. Possible future implementations such as a close-to-real-time online model of the LHC are still under investigation. • It was noted that the LHC knobs are purely linear and assume this over the range used. Nonlinear knobs had been used in LEP but this functionality was not in use at the LHC. • The LHC aperture information is now kept in the layout database and was therefore always up-to-date. Alignment data is not available as part of the aperture model.
7.5
Polarity Checks of non-linear circuits, Meghan McAteer
The results of polarity checks of non-linear circuits obtained during an MD in August 2012 were presented. The results were consistent with those from the sector tests in 2008. Only the skew sextupole circuits seem to have reversed polarity. The measurements have to be repeated in 2015 after the repair of the interconnects. Automation of the tests with beam could be envisaged with pilot intensity.
7.6
Aperture checks, Stefano Redaelli
The techniques and results of aperture measurements during LHC run 1 were summarized and the requirements of tools and procedures for the startup in 2015 examined. The methods applied so far will still be valid in 2015. • Measurements are done at injection and top energy (since mid 2011). • The LHC aperture is excellent and this allowed to achieve β ∗ =60cm. A beam-based understanding of the aperture was crucial to achieve this result. Several methods were established to measure the aperture in appropriate units. • Methods rely on various types of collimator scans to achieve directly the aperture and protection settings in sigma units used for collimator gaps! • Important work on controlled beam blow-up allows measurement at top energy. Preliminary plans for post-LS1 commissioning were discussed. • The requirements will be similar to what we have done so far (see 2012 case). However, will need to be very careful with measurements at top energy! with even pilot intensity close to the damage threshold for certain equipment techniques will have to be adapted however. • No strong requirements in terms of new tools, although some improvement welcome. • It is important to keep alive the ApertureMeter. A new owner of the aperture meter application will have to be nominated. • New operational scenarios will require new measurements • More details local triplet aperture at injection if smaller beta* considered 19
• Foresee some checks at flat-top in case of combined ramp and squeeze. R. Tom´as asked whether re-centering the orbit at aperture bottleneck locations was considered during run 1. Stefano replied that the aperture measurements had always been carried out after the reference orbit had been established. As the aperture bottlenecks were not limiting the LHC performance, the orbit was not re-centered at that stage of the commissioning. O. Bruning asked why the bottlenecks at injection energy and top energy were not the same during LHC run 1. Stefano answered that the top energy measurements were done after the squeeze, so the optics was different.
7.7
Collimation setup, Belen Salvachua Ferrando
The nominal collimation setup procedure was presented and the collimators with BPMs, which will be available for the TCTs and the TCSG in point 6 after LS1, introduced. The setting up techniques and tools are mature and will be reused after LS1 after some consolidation. It was stressed that collimation setup should only come after the orbit and optics are well corrected to reduce the time spent on re-checking alignment. The requirements for setting up for cycles with combined ramp & squeeze, colliding squeeze or β ∗ leveling have not been worked out in detail, no show stoppers are evident for the time being. The decision on the baseline cycle after LS1 should however come soon to verify the new scenarios in simulation. • The setup beam limit at top energy was still under discussion. • The settings of the collimators at top energy for optics measurements after LS1, where the collimators should normally be slightly more retracted, still need to be defined.
8
Closeout
Chairman: Oliver Br¨uning The last session of the 2013 OMC workshop on Tuesday afternoon featured only one presentation: the close out summary by Gianluigi Arduini.The summary went through all sessions of the workshop and highlighted the main points for a closing discussion. While there was common consensus that the post LS1 operation would focus on the operation with 25 ns and would require leveling via dynamic β ∗ adjustments, the choice of the best optics configuration for nominal post LS1 operation was not yet fully settled. For example the use of Ramp & Squeeze and the two optics options for the post LS1 operation: the nominal optics configuration from the LHC Run I period and the new ATS optics for the HL-LHC period. Other important issues of concern for the post LS1 operation are saturation and hysteresis effects which are going to be an issue for operation at 6.5 TeV and the decay and snapback effects of the field errors, which will be 50% larger as compared to the LHC Run I operation. Another challenge for the post LS1 operation will be the further development of the non-linear machine model and the emittance measurement and preservation throughout the full LHC cycle. A review of the startup procedure of the post LS1 LHC machine indicated that required tools are ready and ’in place’ for a re-commissioning of the ’new’ machine and the MADX Online model is in the process of getting a final revision in preparation of the post LS1 startup.
Acknowledgments We thank all the speakers and workshop attendants for the high quality presentations and subsequent discussions.
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