INSTITUTE OF PHYSICS PUBLISHING and INTERNATIONAL ATOMIC ENERGY AGENCY Nucl. Fusion 45 (2005) 1085–1093
NUCLEAR FUSION doi:10.1088/0029-5515/45/9/008
LHCD and coupling experiments with an ITER-like PAM launcher on the FTU tokamak V. Pericoli Ridolfini, Ph. Bibet1 , F. Mirizzi, M.L. Apicella, E. Barbato, P. Buratti, G. Calabr`o, A. Cardinali, G. Granucci2 , L. Panaccione, S. Podda, C. Sozzi2 and A.A. Tuccillo Associazione EURATOM-ENEA sulla Fusione, Via Enrico Fermi 45, 00044 Frascati Roma, Italy 1 Association Euratom-CEA sur la Fusion, CEA/DSM/DRFC-CEA Cadarache, 13108 St Paul lez Durance, France 2 Associazione EURATOM-ENEA-CNR sulla Fusione, IFP-CNR, Via R. Cozzi 53, 20125 Milano, Italy E-mail:
[email protected]
Received 9 December 2004, accepted for publication 20 June 2005 Published 23 August 2005 Online at stacks.iop.org/NF/45/1085 Abstract A prototype passive active multijunction (PAM) launcher for lower hybrid (LH) waves conceptually similar to that foreseen for ITER has been successfully tested on FTU at frequency f = 8 GHz. The power routinely and safely managed for the maximum time allowed by the LH power plant (0.9 s) without any fault in the transmission lines is 250 kW, corresponding to 75 MW m−2 across the antenna active area and very close to the design value of 270 kW or 80 MW m−2 . The achieved value is at least 1.4 times larger than the ITER request, which would be only 52 MW m−2 , if the 33 MW m−2 required to the ITER grill in order to couple 20 MW to the plasma, are scaled up linearly with f , from fITER = 5 GHz. This linear scaling of the power handling capability of the LH antennae is indeed conservative with respect to the available data. The test results validate also the other two main expectations relevant to ITER, foreseen by the codes, namely to operate with the grill entirely in the vessel shadow and to still preserve good current drive (CD) efficiency. Even with the grill mouth retracted 2 mm inside the port shadow and with density in front of the launcher very close or even lower than the cut-off value, the PAM reflection coefficient is always 2.5%, if the antenna has been properly conditioned. The CD efficiency is comparable to that of a conventional grill, once the lower directivity is taken into account. Flexibility in determining the N|| spectrum is also maintained, according to hard x-rays and electron cyclotron emission spectra. Conditioning the PAM in order to operate at the ITER equivalent power level required only one day of radio-frequency operation, without a previous baking of the waveguides. PACS numbers: 52.35.Hr, 52.50.Sw
(Some figures in this article are in colour only in the electronic version)
1. Introduction The possibility of launching lower hybrid (LH) radiofrequency (RF) waves into the ITER plasma to control the current radial profile depends on the capability of the launcher to withstand the harsh environment at the plasma border. Since the antennae at present in operation in tokamaks do not satisfy this request, a new concept, called passive active multijunction (PAM) [1] started to be developed jointly by ENEA and CEA (Cadarache, France) EURATOM associations over the last few years. 0029-5515/05/091085+09$30.00
© 2005 IAEA, Vienna
The antenna has to satisfy the following fundamental requests. (1) To operate in the full shadow of the vessel port to avoid damage from the large particle flow inside the scrape-off layer (SOL) plasma; (2) to tolerate the heating due to the neutron flux and plasma radiation losses; (3) to preserve a level of power handling and current drive (CD) efficiency (ηCD ) acceptable for ITER. The envisaged plasma scenarios fix the minimum power to be coupled through one equatorial port at PLH 20 MW, with ηCD 0.3 × 1020 m−2 A W−1 , close to the best value obtained so far in the world.
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These points imply solving the problem of good power coupling to the main plasma in conditions of almost vanishing plasma in front of the grill, while maintaining CD performances similar to a usual multijunction (MJ). The innovative proposal was to locate cooling ducts in between the LH fed waveguides in the final part of the launcher, except at the front end, where passive waveguides interposed between the active ones restore the common MJ periodicity [1]. In this way the N|| spectrum (N|| is the parallel index of refraction) and hence the CD efficiency, can be compared to a standard MJ, provided a strong cross coupling exists between the active and passive waveguides. This, in turn, occurs if the plasma density in front of the grill is close to the cut-off value or if a thin vacuum layer is present. In this way the very low density plasma existing in the shadow of the port favours a good power coupling, rather than inhibiting it, as in the case of either an MJ or a conventional grill. The route to a final robust proposal for an ITER PAM is made up of several steps. The first one is the validation of the PAM concept by means of appropriate experimental tests that the EURATOM-ENEA association proposed some years ago to carry out on FTU. The leading points were that an experimental check could be completed in a rather short time at moderate cost in FTU. Indeed, the existing LHCD system required only minor changes; the machine is compact (major radius R = 0.935 m, minor radius a = 0.3 m) and the LH pulse length is short (max 1 s) so that even at very high LH power density (close to 100 MW m−2 ) no cooling is needed and standard materials can be employed. In addition, the presence of other independent conventional grills that can be phased to give N|| spectra very similar to those calculated for the PAM, could allow a direct comparison with the well-assessed performances of common launchers. The general layout of the FTU PAM and the main technical solutions adopted are described in a recent paper [2]. Here we recall that the working LH frequency in FTU is fLH = 8 GHz and, for the reader’s ease, also propose a picture of the mouth facing the plasma in figure 1.
Figure 1. Front view of the FTU PAM, made of a 4 × 3 matrix of independent modules. One module is evidenced with a black frame at the top right of the grill. The passive waveguides show as bright rectangles, the active ones as black rectangles. Fixed phase shift between active waveguides in a module = 270˚. Normally no-phase shift in the vertical direction is applied.
2. Conditioning and cleaning procedures Before using the PAM antenna in the experimental campaign, a conditioning procedure had to be followed to remove the gas adsorbed onto the internal surfaces of the waveguides. Only direct wave-guide feeding by LH power was utilized for the purpose, no vacuum baking in situ at the typical temperatures of 150–200˚C was applied, because of some simplifying technical arrangements. The baking, however, even though not strictly necessary, would have facilitated the experiment by removing the local character of the adopted procedure, as described below. The waveguides are fed during the plasma pulses by ON/OFF modulated LH power over a total permitted LH time length of 0.9 s, by increasing alternatively the power level and the ON time. In this way only about 30 discharges (one operational day) were enough to attain the power of 170 kW (52 MW m−2 ) that is considered equivalent to that foreseen for ITER when the different frequencies are taken into account (see next section). Instead, to reach the projected target value of 270 kW, with a 0.2/0.08 s ON/OFF cycle for a total ON time of ≈0.7 s, we were required to double this 1086
Figure 2. Time traces of (from top to bottom) the coupled power, the average reflection coefficient of the PAM grill, the electron density in front of the grill, averaged over the measurements of 4 Langmuir probes aside the grill, the ECE from a bit inner than half the minor radius, r/a ≈ 0.4.
time. The conditioning is considered completed when the sudden increase of the reflection coefficient (Rc ) due to internal outgassing becomes only sporadic. At completion, the average Rc is always below 3%, independently of the antenna position. The need to eliminate outgassing is not only for lessening Rc , but also for avoiding that the gas released absorbs most of the incident power. This latter deduction is supported by the graphs presented in figure 2 referring to a conditioning discharge. At the start of the LH pulse the density in front of the waveguide, initially even below the cut-off value, ne,cut-off = 0.79 × 1018 m−3 for fLH = 8 GHz, increases considerably
LHCD and coupling experiments on the FTU tokamak
(third frame), because of direct ionization. The ability of the LH waves to ionize the neutral gas in front of the grill and to affect the coupling has been recently evidenced on the JET LH launcher [3]. However, when Rc jumps to 10% level (second frame) the density drops again and the effects on the plasma bulks are also softened, as the trace of the electron cyclotron emission (ECE) from the core plasma shows in the bottom frame. The small reduction of the power injected, due to the higher reflection only, cannot account for the observed amplitude of the phenomena. This method, however, appears to be fully effective only for the regions where the electron multipactoring effect is higher, so that the cleaning had to be repeated after changing the location of the electron cyclotron resonances along the RF lines. This occurs if it is varied: either the antenna position with respect to the vessel (by more than 5 mm), or the total magnetic field, even the poloidal component only. Thermal gas desorption from the waveguides walls is unimportant, since the temperature increase due to the LH power alone is always 40˚C, even for the longest pulses tLH ≈ 0.9 s. No degrading of the cleanliness of the waveguides is observed over one month. No additional pumping was installed, because the poor conductance of the waveguides would make it scarcely useful on the time scale of the FTU pulse duration, even for large outgassing [4].
Figure 3. Performances of the PAM in terms of the coupled and reflected power. The top two frames refer to the achievement of the design target and to a quasi-continuous feeding (limited only by the power plant constraints). The bottom two frames give the respective reflection coefficients that are always 90 keV, energies pertaining to electrons travelling faster than a LH wave with N|| ≈ 1.9 (relativistic γ = 1.165), very close to where the slower spectrum power drops to negligible value. The effect is slightly smaller when G4 is also energized for the lower weight of the PAM power. The unchanged loop voltage in the two discharges ensures that the distinction between the two spectra is not guided by the residual electric field. Indeed, Vl is much lower than the Dreicer threshold, V1 = 2.6 V in FTU, for a free acceleration of an electron with the same speed as a LH wave with N|| = 1.7. The Vl invariance is also consistent with a balance between the increase of the CD efficiency for the faster N||,pk and the decrease of its directivity, as anticipated above. Figure 12 shows the ECE spectra for two discharges with the same density but different N|| spectra, namely the fastest possible one, = 0˚, N||,pk ≈ 1.7 and the best suited for the FTU PAM design, i.e. = 135˚, N||,pk ≈ 2.24. The large difference between them is a clear sign that the
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Figure 12. ECE emission in the microwave spectral range 150–500 GHz, corresponding to f 2fce for a central toroidal field BT0 = 7.1 T, for two different N|| spectra of the PAM. The supra-thermal spectral window is indicated in the figure.
plasma does feel different spectra and that the faster spectrum is more effective in building a fast tail. More precise deductions from the absolute intensity are not straightforward due to the optical thinness of the plasma at the related frequencies that makes the radiation essentially delocalized in space. It is more instructive to consider the intensity ratio at two frequencies, since this measures the ratio of the total relevant emitting electrons, if the thermal emission is negligible. The behaviour in OH phase indicates that this is indeed the case for f1 = 227 GHz and f2 = 297 GHz, which are the most apart frequencies within the second harmonic downshifted range. In addition, their large spectral separation causes the radiation to come mostly from distinct and separate parts of the electron tail distribution function, assuming that emission at these frequencies is dominated by downshifted second harmonic and provided the e− fast tail is bounded within half minor radius. This is ensured by the FEB radial profiles and the calculation of the fast ray-tracing code (FRTC [13]), whose reciprocal consistency is particularly satisfactory for n¯ e > 0.4 × 1020 m−3 , where the radial diffusion of the fast electrons can be neglected, see [14]. For electrons located half way between the torus major radius and the outer walls, the radiation at 297 GHz and 227 GHz is associated to γ = 1.09 and γ = 1.43, respectively or to LH wave parallel velocities of N|| = 2.5 and 1.4. These intensity ratios, representative then of the slower and faster part of the tail, are plotted versus density in figure 13 for the three tested N|| spectra. Clearly, a faster spectrum generates a faster electron population all over the density range. Hence the different phasing of the PAM modules does produce a change in the N|| spectrum in the expected direction.
4. Numerical analysis To complete the analysis, the consistency between experiment and calculations has been tested, by checking whether the code simulations can reproduce the experimental features. The
Figure 13. Ratio of the ECE signals at the two downshifted frequencies 227 and 297 GHz versus line density for three different PAM N|| spectra. The faster spectrum always has the larger ratio. a.u. 1.2 1
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Figure 14. Calculated (with FRTC) radial LH driven current density profiles and measured radial perpendicular HXR emission in the photon energy range 20–120 keV. n¯ e ≈ 3.5 × 1019 m−3 . Figure taken from [14].
first part of this work has been carried out recently. Barbato and Saveliev [14] demonstrate how the main macroscopic features of a FTU discharge with the PAM energized can be reproduced by ASTRA code plus FRTC. The consistency between the radial profile of the perpendicular HXR emission and FRTC calculations, with the limitations quoted in the previous paragraph, is proposed again here in figure 14 for a fairly low density discharge, n¯ e ≈ 3.5 × 1019 m−3 . As a further step we calculated the perpendicular bremsstrahlung emission along the FEB viewing chords to check whether the measured HXR spectrum was consistent with the distortion of the electron distribution function caused by the LH waves. The calculation of the HXR emission is carried out from the temperature and density radial profiles measured and from the current profiles calculated by the equilibrium reconstruction code. Assumptions must instead 1091
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Figure 15. Measured and simulated HXR spectra along the central chord of the FEB. The values are normalized to their maximum value to be free from geometrical factors.
be made for the e− fast tail structure. This is supposed to extend in the velocity space from about w1 = 3.5 · vth,e , as proposed in [15], to w2 = c/N||,pk (c = light speed, vth,e = electron thermal velocity) and to contain a fraction of electrons that accounts for the driven current. This fraction is ne,tail /ne,bulk = 0.001 for the case considered here and shown in figure 15. Here, the experimental and computed spectra along the central chord are plotted. The values are normalized to their maximum to get rid of geometrical factors. The combined PAM + G4 phase is chosen as an example to diminish the statistical error on the FEB counts, but also for the PAM alone the agreement between code and experiment is satisfactory enough.
to the divertor configuration (FTU has a toroidal limiter) and the magnetic lines length, and in turn could affect the PAM coupling properties, even though to a lesser extent than it does the value of the plasma density. The CD efficiency is not degraded with respect to a conventional grill launching a similar N|| spectrum, taking into account the lower power directivity. This is derived from the change in the macroscopic plasma parameters and from the analysis of the electro-magnetic emission by the LH generated fast e− tail. This latter is studied in the HXR range (bremsstrahlung radiation) and in the sub-millimetre wave region (relativistically downshifted ECE). The spectra of both these kinds of radiation give evidence of the PAM flexibility in the N|| spectrum. All these features give reasonable confidence in using a PAM antenna in ITER, even though other very crucial aspects will be investigated in the near future on Tore Supra tokamak at Cadarache, CEA, France. In particular the response to large heat loads and to a very prolonged RF excitation, up to 1000 s, which is larger than the time scale of the particle wall saturation and the thermalization of the whole device, will be studied. Total power flows across the last closed magnetic surface of Tore Supra are foreseen to be up to 0.19 MW m−2 for 30 s or 0.12 MW m−2 for 1000 s [16], to be compared with 0.22 MW m−2 predicted for ITER. Other important PAM features, such as the effect of the edge density on the spectrum directivity, are unfortunately beyond the accuracy of our measurements due to the low available power and the limited spectrum directivity.
Acknowledgments The authors wish to thank the technical staff: M. Aquilini, S. Di Giovenale and P. Petrolini for their hard work on the PAM and the overall LH system.
5. Conclusions The tests carried out at FTU on a prototype PAM antenna, proposed as LH wave launcher for ITER in order to overcome the harsh environment, have validated this innovative launcher concept on the physics side and also for some technical aspects. From the technical side the power handling capability is well above the minimum of 33 MW m−2 required for ITER: a possible safe target value is larger than 50 MW m−2 , according to the experimental frequency scaling from 8 GHz in FTU to 5 GHz envisaged for ITER. Conditioning the waveguides, even though essential for using the PAM, does not require lengthy or particular techniques and the waveguide cleanliness persists over a long time. Finally no damage has been detected on the PAM mouth after its removal from the FTU port. Very good coupling is obtained throughout the SOL plasma, provided the antenna is properly conditioned to avoid gas release from the internal waveguide walls. On average the reflection coefficient Rc is ≈1.3%, and never exceeds 2.5%, even with almost evanescent plasma in front of the grill, as when this is fully retracted inside the port (up to 3 mm) to simulate the ITER operation and the density at its mouth is equal or even lower than the cut-off value. The SOL gradients, however, will probably differ in ITER due 1092
Annexure to references—papers used for composing figure 4 FTU(433 MHz)—Aquilini M. et al 2004 Fusion Sci. Technol. 45 459–82 PLT(0.8 & 2.45 GHz)—Stevens J.E., Bell R.E., Bernabei S., Cavallo A. and Chu T.K. 1988 Nucl. Fusion 28 217–30 Wega(0.8 GHz)—Gormezano C. 1985 RF coupling structures for lower hybrid resonance heating Proc. 4th Int. Symp. on Heating in Toroidal Plasmas (Roma, Italy, 21–28 March 1984) vol II (International School of Plasma Physics and ENEA) pp 1255–60 T-7(0.9 GHz)—Gormezano C. 1985 RF coupling structures for lower hybrid resonance heating Proc. 4th Int. Symp. on Heating in Toroidal Plasmas (Roma, Italy, 21–28 March 1984) vol II (International School of Plasma Physics and ENEA) pp 1255–60 Petula B(1.25 GHz)—Gormezano C. 1985 RF coupling structures for lower hybrid resonance heating Proc. 4th Int. Symp. on Heating in Toroidal Plasmas (Roma, Italy, 21–28 March 1984) vol II (International School of Plasma Physics and ENEA) pp 1255–60 Asdex(1.3 GHz)—Eckhartt D. et al 1985 Lower hybrid experiment in Asdex Proc. 4th Int. Symp. on Heating in Toroidal Plasmas (Roma, Italy, 21–28 March 1984) vol I (International School of Plasma Physics and ENEA) pp 501–12 JT-60U(2 GHz)—Naito O. and the JT-60 Team 1993 Plasma Phys. Control. Fusion 35 B215–22
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FT(2.45 & 8 GHz)—Alladio F. et al 1987 Transmission and coupling at high power density of 8 GHz lower hybrid on FT Proc 7th Topical Conf. on Applications of Radio-Frequency Power in Plasmas Kissemee (Florida, USA, 1–3 May 1987) (New York: AIP) AIP Conf. Proc. 159 127–30 Alc-A(2.45 GHz)—Schuss J.J. et al 1981 Nucl. Fusion 21 427–52 Asdex(2.45 GHz)—Pericoli Ridolfini V., Giannone L. and Bartiromo R. 1994 Nucl. Fusion 34 469–81 JET(3.7 GHz)—Rimini F.G. et al 1996 High power LHCD experiment in JET Proc. 11th Topical Conf. on Radiofrequency Power in Plasmas (Palm Springs, CA, 17–19 May 1995) (New York: AIP) AIP Conf. Proc. 355 110 Tore Supra(3.7 GHz)—2000 Report CEA EUR(00) CCE-FU 7/8.9a CIMES 1—Application for preferential support phase I, Cadarache, France, p 117 TdeV(3.7 GHz)—Report CEA EUR(00) CCE-FU 7/8.9a CIMES 1—Application for preferential support phase I, Cadarache, France, p 117 Alc-C(4.6 GHz)—Parker R.R., Greenwald M., Luckhardt S.C., Marmar E.S., Porkolab M. and Wolfe S.M. 1985 Nucl. Fusion 25 1127–36 FTU(8 GHz)—Tuccillo A.A. et al 1994 8 GHz experiment on FTU Strong Microwaves in Plasmas: Proc. Int. Workshop on Strong Microwaves in Plasmas (Nizhny Novgorod, Russia, 15–22, August 1993) vol 1 (Institute of Applied Physics Russian Academy of Sciences, Cheboksary, Russia) p 47 FTU-PAM (8 GHz)—This paper and: Pericoli Ridolfini V. et al 2004 LHCD and coupling experiments with an ITER-like PAM launcher on the FTU tokamak Proc. 20th FEC Conf. (Vilamoura, Portugal, 1–6 November 2004) paper EX/5-5
References [1] Bibet Ph., Litaudon D. and Moreau D. 1995 Nucl. Fusion 35 1213–23 [2] Mirizzi F. et al 2003 Experiment with the PAM Launcher for FTU Proc. 20th IEEE/NPSS Symp. on Fusion Engineering (SOFE 2003) (San Diego, USA, 14–17 October 2003) ed IEEE, pp 512–5 (catalog no. 03CH37469)
[3] Pericoli Ridolfini V. et al 2004 Plasma Phys. Control. Fusion 46 349–68 [4] Apicella M.L. 2003 Considerazioni sul vuoto nelle guide d’onda della struttura di lancio a multigiunzione per FTU ENEA Technical Report RT/2003/80/FUS (in Italian) [5] Tuccillo A.A. et al 1994 8 GHz experiment on FTU strong microwaves in plasmas: Proc. Int. Workshop on Strong Microwaves in Plasmas (Nizhny Novgorod, Russia, 15–22 August 1993) vol 1, Institute of Applied Physics Russian Academy of Sciences, Cheboksary, Russia, p 47 [6] Pericoli Ridolfini V. et al 2004 Experimental test of an ITER-like passive active multijunction lower hybrid RF launcher on the FTU tokamak 31st EPS Conf. on Plasma Physics (London, GB, 28 June–2 July 2004) vol 28G (ECA) P-2.105 http://eps2004.clf.rl.ac.uk/#P2/P2.105 [7] Aquilini M. et al 2004 Fusion Sci. Technol. 45 459–82 [8] Litaudon X. and Moreau D. 1990 Nucl. Fusion 30 471 [9] Irzak M.A. and Shcherbinin O.N. 1995 Nucl. Fusion 35 1341 [10] Leigheb M., Pericoli Ridolfini V. and Zag´orski R. 1997 J. Nucl. Mater. 241–243 914 [11] Pericoli Ridolfini V. et al 1999 Phys. Rev. Lett. 82 93 [12] Podda S. et al . 2002 Proc. 19th Int. Conf. on Fusion Energy 2002 (Lyon, France, 14–19 October 2002) (Vienna: IAEA) CD-ROM file PD/P-07 http://www.iaea.org/programmes/ ripc/physics/fec2002/html/fec2002.htm [13] Esterkin A.R. and Piliya A.D. 1996 Nucl. Fusion 36 1501–12 [14] Barbato E. and Saveliev A. 2004 Benchmarking of LHCD numerical modelling on FTU discharges and application to ITER-FEAT scenarios 31th EPS Conf. on Plasma Physics (London, GB, 28 June–2 July 2004) vol 28G (ECA) P 2.104 http://eps2004.clf.rl.ac.uk/#P2/P2.104 [15] Barbato E. 1998 Plasma Phys. Control. Fusion 40 A63–A76 [16] EURATOM-CEA Association 2000 Report EUR (00) CCE-FU 7/8.9a Application for preferential support-phase 1 Upgrade of the Heating and Refuelling Systems of Tore Supra—Stage 1 (Cadarache, France, June 2000)
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