Brau, K.

Casey, J.

Coleman, J.

Gerver, M.

Golovato, S.N.

Post, R.S.

DSpace@MIT

PFC/JA-88-34Stability Issues in the Tara Tandem MirrorExperimentbyR. S. Post, K. Brau, J. Casey, J. Coleman,M. Gerver, S. N. Golovato, W. C. Guss,G. Hallock, S. Home, J. H. Irby, J. Kesner,B. Lane, J. Machuzak, R. Myer, E. Sevillano,D. K. Smith, J. D. Sullivan, R. Torti, X. Z. Yao,and J. ZielinskiPlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, Massachusetts 02139September 9,1988This work was supported by the U. S. Department of Energy Contract No.DE-AC02-78ET51013.Will be presented at the Twelfth International Conference on Plasma Physicsand Controlled Nuclear Fusion Research, International Atomic EnergyAgency, Nice, France, October 12-19, 1988, and will be published in theConference Proceedings.~7~4INTERNATIONAL ATOMIC ENERGY AGENCYTWELFTH INTERNATIONAL CONFERENCE ON PLASMA PHYSICSAND CONTROLLED NUCLEAR FUSION RESEARCHNice, France, 12-19 October 1988IAEA-CN-50/ C-3-3STABILITY ISSUES IN THE TARA TANDEM MIRROR EXPERIMENTR.S. Post, K. Brau, J. Casey, J. Coleman, M. Gerver, S.N. Golovato, W.C. Guss,G. Hallock, S. Horne, J.H. Irby, J. Kesner, B. Lane, J. Machuzak, R. Myer,E. Sevillano, D.K. Smith, J.D. Sullivan, R. Torti, X.Z. Yao, J. ZielinskiPlasma Fusion Center, Massachusetts Institute of Technology, Cambridge, Ma., USAThis is a preprint of a paper intended for presentation at a scientific meeting. Because of the provisional nature of itscontent and since changes of substance or detail may have to be made before publication, the preprint is made available on theunderstanding that it will not be cited in the literature or in any way be reproduced in its present form. The views expressed andthe statements made remain the responsibility of the named author(s); the views do not necessarily reflect those of the govern-mont of the designating Member State(s) or of the designating organization(s). In particular, neither the IAEA nor any otherofganization or body sponsoring this meeting can be held responsible for any material reproduced in this preprint.1AbstractExperimental studies were performed to understand the stability of the Taratandem mirror. Central cell fluctuations are dominated by radially rigid, m = 1 flutemodes in which the entire column oscillates about the machine axis. These modescan be stabilized by outboard quadrupole anchors, or by ponderomotive effectsrelated to central cell ion cyclotron heating (ICH) power levels. A magnetic divertorinstalled at the central cell mid-plane has also significantly enlarged the stableoperating parameter regime. Axially localized instabilities are clearly observed inthe axicells during the application of axicell ICH. They also appear to exist duringthe application of axicell electron cyclotron heating (ECH) or neutral beams, andto limit the buildup of hot electrons and beam ions.IntroductionThe Tara tandem mirror configuration' consisted of a 10 m solenoidal centralcell with a "magnetic plateau" at its center where the field rose to a mirror ratioof about two with respect to the "well" regions on either side. Atop the plateauwas a "slot" antenna which launched radio frequency (RF) waves resonant with theion cyclotron frequency at the bottoms of the adjacent wells. Also atop the mag-netic plateau was a gas box2 which was the primary fueling source for the plasma.Between these was mounted a magnetic divertor that diverted a fraction of theplasma around a coil and created an azimuthally symmetric null. This divertor wasinstalled with the purpose of providing stability by enhanced electrical connectionacross field lines.3 Using this configuration a stable central cell plasma (Ti,± = 6002- 1500 eV, T, = 150 - 200 eV, T. = 60 - 100 eV, n, = 4 - 5 x 10 2 cm-3 ) could becreated consistently.The central cell region was terminated by high field (mirror ratio Re !5 12)coils, and flanked by axisymmetric plug cells (axicells) and outboard quadrupoleminimum-B anchor cells. The axicells were designed to provide electrostatic plug-ging of the central cell endlosses through the use of neutral beams and gyrotrons4or, alternately, through the use of RF resonant with the ion cyclotron frequency atthe mid-plane of the axicell.'Interchange ModesEarly experiments in Tara showed that the central cell could be maintainedstably by the central cell slot ICH.' This stability is attributed to ponderomotiveeffects from the RF heating.' However the introduction of auxiliary sources of power,such as beams or gyrotrons, during endloss reduction experiments caused the centralcell to become unstable in the parameter regimes of interest.' 0 It was found thatthese modes could be stabilized at moderate levels of axicell ECH and ICH powerthrough the use of the outboard anchors as originally proposed for Tara, althoughthe theoretically predicted enhancement of stability due to the presence of hotelectrons in the anchors8 was never observed.' The outboard anchors, however, didnot provide sufficient stability during higher power ECH pulses. Furthermore, thecutoff of the plasma stream to the adjoining anchor during axicell plugging reducedthe anchor ion beta, and a loss of MHD stability often ensued, particularly duringECH plugging.This problem was ameliorated by the introduction of the magnetic divertor atthe central cell mid-plane. The divertor allowed a significant enlargement of theparameter range in which the central cell could operate stably without anchors andallowed higher gyrotron powers to be employed in plugging experiments."Under unstable conditions the dominant characteristics of the coherent modeseen in Tara were: (a) low frequency 3-15 kHz; (b) axially flute-like; (c) radiallycoherent. We observed a sharp threshold behavior with respect to ECH power.3Below a particular power the plasma was quiescent and the fluctuation level wasindependent of ECH power. Above this level fluctuations were large, but again thepower dependence was weak. We interpret this behavior as evidence of a stabilityboundary.In Figure 1 we show the stability boundary relative to the divertor null radiusand ICH power level for several power levels of ECH heating in the axicell. We notethat higher ICH power and smaller divertor null radius were stabilizing, while higherECH power was destabilizing. Figure 1 demonstrates the interplay of divertor andICH stabilization on two widely separated run days. The scaling of these stabilityboundaries with divertor current and ICH power was reproducible. More detaileddescription of the ponderomotive stabilization appears elsewhere. 7Scatter Plot at Stability Threshold .16 17Null Radius (cm)18Figure 1. The multi-parameter nature of the sta-bility boundary is shown at constant fueling rate.Plug ECH drives the plasma unstable. Enhancedstability is achieved by mapping the null deeperinto the plasma, raising the central cell ICH powerlevel, or both.AILCELL MIFR MATIO S.S.3 - oeSatI340.2AXICELL MINWR FRTIO * 4.6.3 9atI5341.2-AXICELL MIRRO RATIO *3.2.3 Sb.tiS342.2-0.0* 20 30*T@.40Figure 2. Series of shots exhibiting the dampingof the north axicell sawteeth as the central cellmirror ratio R,, is decreased.400je 300300.LA.S2000100. - . . , . .I " I . I , -, , IDate - ECH Power Level5/14/87 - 160kW12/12/96 - 150 kW* 5/14/07 -115 kW12/12/66 -120kW5/14/67 - 70kwSTABLEUNSTABLE15'i i i i i i i e iioe4Axially Localized ModesTara was seen to exhibit modes of instability which were axially localized.' 3 Atthe highest divertor currents and at high R,, ICRF heating in the north axicell wasobserved to drive only the axicell unstable. A buildup and dump in density anddiamagnetism (0) was observed to occur with 1 to 2 ms periodicity. During eachdump the neutral density increased in the axicell indicating that the plasma wasscraping off in the axicell. These "sawteeth" and the increases in neutral densitywere not observed in the central cell.The sawteeth were accompanied by increased low frequency noise detected bythe gyrotron scattering diagnostic' 4 and by Langmuir probes. We observed from thegyrotron scattering signal that the axicell diamagnetism rise triggered a coherentlow frequency burst (; 30 kHz) which built up until the dump. A correlated burstcould be detected by the central cell heavy ion beam probe only in shots where theinstability was strongest. The 30 kHz signal was generally not seen on the axicelllight detector array, a line integrated diagnostic that should be insensitive to highm modes. Since the axicell light detector did pick up m = 1 and m = 2 flutelikemodes, this suggests that the 30 kHz mode had m > 3. Double-tipped Langmuirprobes, used in some shots, showed 30-100 kHz modes with m =3-10.Figure 2 shows that the sawteeth decreased in amplitude and frequency asthe mirror ratio between the central cell and axicell was decreased. Variation of theouter axicell high field coils did not affect the appearance of the sawteeth, supportingthe view that it was the connection to the central cell that was important, not theaxicell mirror ratio.The relatively small axicell beta (less than 1%) precludes MHD ballooning asan explanation for the instability, and the passing fraction dependence implies thatwe were observing a trapped particle mode. Theoretically,' 2 cancellation betweenthe ion finite Larmor radius (FLR) stabilization term and the collisionless trappedparticle mode charge separation term should lead to an unstable band of modes forazimuthal mode numbers m - 4 to 5, similar to what was observed. The data inFig. 2 shows that the sawteeth decreased in amplitude when the passing fraction5nTpa,/n, was increased (by decreasing the mirror ratio between the central cell andaxicell). This data is consistent with theory, which predicts that the azimuthalmode number m at which trapped particle modes are unstable is an increasingfunction of passing fraction. Modes with higher m should cause less rapid radialtransport, since the radial loss rate is probably roughly -y/m 2 (where -y is the lineargrowth rate). Although the mechanism causing sawteeth is not fully understood,it appears to be related to the release of gas when plasma scrapes off, so we expectthat the sawtooth amplitude will decrease when the radial loss rate decreases.ECH PluggingIn the original Tara operating scenario,1plugging would be achieved by neutralbeams which produce a density that peaks at the 10 kG point, and by ECH at boththe barrier (5 kG) and plug (10 kG) points which would create a thermal barrierto further enhance the potential at the peak density point. In the experiment theneutral beams were never able to build up a density of greater than 4 x 10"cm-3at the 10kG point (as inferred from the diamagnetism), and the population of hotelectrons (E, > 100 keV) that were localized to the barrier region never built up tomore than 2% of the total density, although a large population of warm nonlocal-ized electrons (E, ::: 2-8 keV) was observed. These components were theoreticallyinsufficient to produce a plugging potential. Nevertheless, ECH often produced areduction in end loss, even without neutral beams, and even with only barrier ECH,in apparent contradiction to plugging theory. This is illustrated in Figure 3 whichshows data from a discharge in which only the south barrier gyrotron was used. Thisdata resembles plugging since the south endloss was cut off entirely. However therewas no increase in north end loss, and the central cell density rise (correspondingto a trapping rate of 2A) cannot account for the 25A reduction in south end loss.We conclude that the reduction in south end loss was due not to plugging, but toenhanced ambipolar radial losses in the south axicell. Furthermore in dischargeswhere the end loss was not cut off completely, potential peak measurements madeby gridded end loss analyzers showed an overall rise in potential when ECH wascn0-ECZQ-E611.0 0 JiIH 'T - I II 1 I I I I I500- North total ton current80 South total ton current40'O-JIK Nor th well line denstty20~ Total net current00 10 t 20 30 40 50 60Ttme, msecFigure 3. Time histories during an ECH plugging discharge (#13092) in the southplug. Note that although the endloss is entirely cut off in the south, no reflectedstream appears in the north and no density increases are seen in the central cell.Gyrotron timing is indicated by the arrows.applied to one axicell, but no significant potential difference between the two ends,again showing that the reduction in end loss was not due to plugging.The same radial loss rate can account for poor buildup of neutral beam ions, andof hot electrons. The anomalous electron and ion loss rates are comparable to boththe theoretical and observed y/m 2 for trapped particle modes. The observed y/m 2is based on the measured correlation time and m for the 30-100 kHz modes seenby Langmuir probes in the axicells during ECH or neutral beam injection.'"(Thesemodes apparently saturated before the plasma scraped off and did not cause saw-teeth, as similar modes did during axicell ICH.) High frequency (6-30 MHz) ionmicroinstabilities have also been observed by Langmuir probes in the plug duringneutral beam injection, but only at very low amplitudes, far too low to account forthe lack of buildup of neutral beams. The failure of Tara to achieve plugging (and7central cell buildup) may thus be a consequence of the bad curvature of the plugcells.Acknowledgments:We would like to thank the excellent team of technicians, engineers and ad-ministrative support staff. The reference list is incomplete and is meant primarilyto point the interested reader towards more detailed discussions of Tara results.This work was supported by the Department of Energy under contract numberDE-AC02-78ET51013.8References:1J. Kesner, R. S. Post, B. D. McVey, D. K. Smith, Nucl. Fus. 22, 549 (1982).2 Brau, K., Goodrich, P., Post, R.S., Sevillano, E., J. Vac. Sci. Technol. A 4, 1079(1986).3 J. Kesner, B. Lane, and R. S. Post, Nucl. Fus. 27, 217 (1987).4 D. E. Baldwin and B. G. Logan, Phys. Rev. Lett. 43, 1318 (1979).'D. K. Smith and B. D. McVey, Bull. Am. Phys. Soc. 27, 915 (1982), and Bull.Am. Phys. Soc. 28, 1182 (1983).'S. N. Golovato, K. Brau, J. Casey, M. J. Gerver, S. Horne, J. Irby, J. Kesner, B.Lane, J. Machuzak, R. Myer, R. S. Post, E. Sevillano, and L. Wang, "Stability ofPlasmas Sustained by ICRF in the Central Cell of the Tara Tandem Mirror", MITReport PFC/JA-88-20, submitted to Phys. Fluids.7 J. R. Myra and D. A. D'Ippolito, Phys. Rev. Lett. 53, 914 (1984); D. A. D'Ippolitoand J. R. Myra, Phys. Fluids 28, 1895 (1985); J. R. Myra and D. A. D'Ippolito,Nuclear Fusion 26, 361 (1986); D. A. D'Ippolito and J. R. Myra, Phys. Fluids 29,2594 (1986).8J. Kesner, R. S. Post, D. K. Smith, D. E. Baldwin, Y. C. Lee, Nucl Fus. 22, 577(1982)."K. Brau, S. Golovato, B. Lane, S. Horne, J. Irby, J. Kesner, R.S. Post, E. Sevillano,D.K. Smith, Nucl. Fusion 28, 759 (1988).10J. H. Irby, B. G. Lane, J. A. Casey, K. Brau, S. N. Golovato, W. C. Guss, S.F. Horne, J. Kesner, R. S. Post, E. Sevillano, J. D. Sullivan, D. K. Smith, Phys.Fluids 31, 902 (1988).11J. A. Casey, B. G. Lane, J. H. Irby, K. Brau, S. N. Golovato, W. C. Guss, J.Kesner, R. S. Post, and E. Sevillano, Phys. Fluids 31, 2009 (1988).912 H. L. Berk, M. N. Rosenbluth, H. V. Wong, T.. M. Antonsen and D. E. Baldwin,Sov. J. Plasma Phys. 9, 108 (1983)."M. J. Gerver, S. N. Golovato, J. H. Irby, J. Kesner, W. C. Guss, S. F. Horne, B. G.Lane, J. S. Machuzak, R. S. Post, E. Sevillano, J. Zielinski, "Observation of TrappedParticle Modes in a Tandem Mirror", MIT Report PFC/JA-88-11, submitted toPhys. Fluids."J. S. Machuzak, P. Woskoboinikow, W. J. Muligan, D. R. Cohn, M. Gerver, W.C. Guss, M. Mauel, R. S. Post, and R. J. Temkin, Rev. Sci. Instrum. 57, 1983(1986).

Stability issues in the Tara tandem mirror experiment

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Stability issues in the Tara tandem mirror experiment

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