Crossing Intrinsic Depolarizing Resonances in ELSA with Pulsed Betatron. Tune Jump Quadrupoles. C. Steier, W. v. Drachenfels, F. Frommberger, M. Hoffmann, ...
Proceedings of the 1999 Particle Accelerator Conference, New York, 1999
Crossing Intrinsic Depolarizing Resonances in ELSA with Pulsed Betatron Tune Jump Quadrupoles C. Steier, W. v. Drachenfels, F. Frommberger, M. Hoffmann, D. Husmann, J. Keil, Bonn University, Germany; S. Nakamura, T. Nakanishi, Nagoya University, Japan; H. Sato, T. Toyama, KEK, Japan
DORIS cavity
superconducting solenoid
GDH
PETRA cavity
Tune jump quadrupole
Compton Polarimeter
MSE1 B2
MSE2
B1
ELSA
MSE3
bending magnet beamlines for SR experiments
0.5 - 3.5 GeV
booster synchrotron
K29 B3
K2
Tune jump quadrupole
DORIS cavity
BN1
K3
(20 MeV)
pol. e source (120 keV)
Skew quadrupoles
pol. e source (50 keV)
electron gun
LINAC 2 (20 MeV)
γ a= 2+Q z
γ a= 0+Q z
γ a= -2+ Qz Injection
γ a= -4+ Qz
Qz
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Figure 2: Fast tune jumping of all resonances in ELSA.
BN0
half cell of ELSA M
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Q BPM
SR beamline
detector tests
0m
5m
10 m
15 m
Figure 1: The ELSA facility at Bonn University. In ELSA depolarization is caused mainly by intrinsic and imperfection resonances [3][4]. The spin originally oriented nearly perpendicular to the accelerator plane precesses around the direction of the magnetic field in the 0-7803-5573-3/99/$10.00@1999 IEEE.
Pf Pi
= 2e, 2 , 1 j j2
:
(1)
In this formula the influence of synchrotron radiation is neglected, so it cannot be used for resonances at high energies in electron accelerators. Spin tracking studies show P especially that a total spin flip ( Pfi , ) is impossible there. Small polarization losses can be obtained for a small res_
a _ Q z onance strength or a high crossing speed !0 which can be achieved by fast shifting the vertical betatron tune Qz for intrinsic resonances using pulsed quadrupoles. For the resonances in ELSA an absolute tune shift of Qz : is needed with a rise time of about 10 s. This tune jumping is illustrated in Fig. 2. Before crossing the resonance, the vertical betatron tune is shifted very fast by the tune jump quadrupoles.
=
FZK laboratory
K3
bending magnets. During circulation of the particles in the synchrotron only the polarization component parallel to the guiding field is conserved while the other components are lost. So, horizontal magnetic fields may cause polarization loss in the case of a resonance with the spin precession frequency. Intrinsic resonances are driven by the vertical betatron motion of the electrons, characterized by the vertical betatron tune Qz . Depolarization caused by the linear crossing of an isolated resonance is quantified by the Froissart-StoraFormula [5]
= 1
BN3 BN2 injection septa
2.5 GeV
LINAC 1
z 12-Q γ a=
Dipole (horizontal) Dipole (vertical) Quadrupole Skew Quadrupole Sextupole Combined Function Magnet Solenoid Radio Frequency
extraction septa
Moller Polarimeter
z 10-Q γ a=
SAPHIR
K1
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INTRODUCTION
At ELSA [1] external fixed target experiments with longitudinally polarized electrons or circularly polarized photons (produced by Bremsstrahlung) are carried out. For this purpose a polarized electron source is used. With a GaAs superlattice crystal a polarization level of about 66% at the source has been achieved [2]. The polarized electron beam is preaccelerated in a LINAC and a fast cycling booster (50 Hz). After injection into the main ring further acceleration up to 3.5 GeV is possible. The polarization of the extracted beam can be determined with a Møller polarimeter. medium energy physics experiments
4-20 ms
Qz z 8-Q γ a=
1
4-14 µ s
z 6-Q γ a=
At the electron stretcher accelerator ELSA of Bonn University an external fixed target experiment using a polarized electron beam has been started. To provide a polarized beam with sufficient intensity a dedicated source for polarized electrons is used (with a GaAs-AlGaAs superlattice photocathode). To prevent depolarization during acceleration in the circular accelerators several depolarizing resonances have to be corrected for. One type of resonances (the intrinsic ones) is compensated using two pulsed betatron tune jump quadrupoles. This method is applied because adiabatic crossing techniques that became popular in proton accelerators in the last years cannot be used in an electron accelerator due to synchrotron radiation effects at higher energies.
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Abstract
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Proceedings of the 1999 Particle Accelerator Conference, New York, 1999 190
The Power Supplies 000 111 000 111 111 000 000 111 000 111 000 111
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Ferrit FXC 8C11
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The pulsed power supplies (one for each magnet) were constructed by PPT1 . A schematic drawing is shown in Fig. 4. The design concept is similar to the pulsed power supplies used for COSY in J¨ulich [8]. Each supply consists of three main components, which could be realized in semiconductor technology. First a 1 kV high voltage power supply charges a capacitor bank. The charging is controlled by a PLC (programmable logic controller). An insulated gate bipolar transistor (IGBT) connects the capacitor bank with the load. The IGBT is controlled by the PLC to generate a linear rising edge, which can be varied from 4 to 14 s. The second part consists of a 50 V power supply and generates the falling edge. It is connected by a MOSFET switch to the load. The falling edge can be varied between 4 and 20 ms. The third part is a H-bridge, necessary for polarity changes. The PLC is connected by a VME computer to the control system of the accelerator.
fixing outside
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1111 0000 0000 1111 0000 1111 1111 0000
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27 R
fixing inside 1111 0000 0000 1111 0000 1111 1111 0000
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a
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1 0 0 1 0 1 0 1 1 0 1 0
ceramic vacuum chamber with 10 µ m titanium coating
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0110 1010 101 0 1111000 0000 111
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1111111111 0000000000 1111 0000 0000 1111 0000 1111
coil ends a b
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0000 1111 1111 0000 1111111111 0000 0000000000 1111
b
1111111111 1111 0000000000 0000 0000 1111 0000 1111
500 540 558 600
c
650
supply 50 V
Figure 3: Design of the tune jump quadrupoles: the end cap design consisting of three layers (right), sectional and side view of the magnets (left).
MOSFETswitch
Thyristor H bridge (polarity switch) Monitor
Control fall time (milliseconds)
Control rise time (microseconds)
Control
2
tune jump quadrupole
TUNE JUMP MAGNETS
PLC
For ELSA a fast tune jump system was designed. It consists of two quadrupoles which can be pulsed up to 500 A in 4-14 s. These quadrupoles are of Panofsky type with square shaped ferrite yokes (see Fig. 3) [6]. Calculations for optimization of the design to achieve a low inductance have been applied [7]. Therefore a special end cap design (a,b and c) was carried out. The vacuum chamber is made of Al2 O3 ceramics with a 10 m thin titanium coating on the inner surface. This coating shields the electromagnetic fields of the beam but does not distort the pulsed magnetic field. The main parameters of the quadrupoles are:
resistance: inductance: max. pulse current: max. field gradient:
supply 1 kV
IGBT
(4:298 0:001) m (=) (9.00.1) H (=) 500 A (1.12410.005) T/m.
Because of the high resistance of the ferrites there are no significant eddy currents inside the yoke. Frequency dependencies of the impedance lead to a slight dispersion of the pulse shape which has to be compensated by the power supply.
network connection to control system
Figure 4: Schematic drawing of the pulsed power supplies.
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CALIBRATION AND MEASUREMENTS
Using a stored beam in ELSA the pulse amplitude of the tune jump magnets was calibrated by jumping onto a half integer betatron resonance (Qz = 4:5) from various distances with various pulse heights. The betatron tune was measured by analyzing the spectrum of the vertical beam oscillations. When jumping onto the resonance significant beam loss was observed, reducing the lifetime of the beam from 4 h to less than 10 min. The measured value of QIz = (1:77 0:08) 10,4 A,1 at 2.3 GeV is in good agreement with the calculations. Calculations and measurements concerning closed orbit distortions and emittance growth due to the change of the optical functions were carried out [9].
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1 Puls
Plasma Technik, Dortmund, Germany
Proceedings of the 1999 Particle Accelerator Conference, New York, 1999 amplitude broadens the window and therefore minimizes sensitivity to time shifts. However, too large amplitudes, may cause beam loss, because of jumping onto a neighbouring betatron resonance.
4.454 4.452 -4
∆Qz = (1.77 +/- 0.08) 10 / A (at 2.3 GeV)
Vertical tune before jump
4.45 4.448 4.446 4.444
1
4.442 0.8
4.44 Pf/Pi
4.438 4.436 4.434 120 130 140 150 160 170 180 190 200 210 220 230 Current [A]
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Figure 5: Calibration of the pulsed quadrupoles using a jump onto a half integer betatron resonance.
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Fig. 6 shows the maximum distortion of the closed orbit caused by a tune jump of Qz : at 1.5 GeV during the energy ramp. The rise time of the pulse was 10 s and the fall time 20 ms. The distortions are all smaller than 1 mm and can therefore be neglected.
= 01
0+
Figure 7: Jumping over the ( Qz )-resonance: Polarization dependency of the jump time. The pulse amplitude was -110 A with each magnet, corresponding to a Qz , : .
= 0 047
4 0.6
Polarized electrons have been accelerated at ELSA without significant losses in polarization degree. In the case of crossing intrinsic resonances a fast betatron tune jump magnet system has provided the conservation of polarization. First measurements have shown that the depolarization due to the strong intrinsic resonance at 2.0 GeV can be reduced to less than 2% with the tune jump magnets. Further investigations will follow soon to adjust the amplitudes and trigger times to minimize the influence of the remaining intrinsic resonances. Together with a harmonic correction of the imperfection resonances in ELSA a polarized beam will be provided up to the maximum energy in near future.
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SUMMARY
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Figure 6: Change of the horizontal and vertical closed orbit during a tune jump for the uncorrected closed orbit. The successful operation of the tune jump magnets was tested first on the intrinsic resonance at 2.0 GeV [9]. Without tune jump the resonance causes a depolarization of 25%. Using the pulsed quadrupoles the depolarization could be minimized to less than 2%. Variation of the trigger time of the tune jump magnets leads to the curve shown in Fig. 7. If the jump is triggered too early stronger depolarization could be observed, which is expected because the resonance crossing speed is lower on the falling edge of the pulse than without any pulse. The width of the optimal time window changes with the pulse amplitude. A higher pulse 3100
REFERENCES
[1] K.H. Althoff et al., Part. Acc. 27 (1990) 101 [2] S. Nakamura et al., AIP Conf. Proc. 421, Urbana (1997) 497 [3] C. Steier, D. Husmann, Proc. 1997 Part. Accel. Conf., Vancouver [4] S. Nakamura et al., Nucl. Instr. & Meth. A 441 (1998) 93 [5] M. Froissart and R. Stora, Nucl. Instr. & Meth. 7 (1960) 297 [6] M. Hoffmann, diploma thesis, BONN-IB-98-10 [7] T. Toyama, C. Steier et al., BONN-ME-97-01 [8] A. Lehrach, PhD thesis, J¨ulich, IKP-98-12-034 (1997) [9] C. Steier, PhD thesis, BONN-IR-99-07 (1999)
