Suppression of internal disruptions in inductively driven tokamak discharges by lower hybrid wave current drive

Chu, T. K.; Bell, R. E.; Bernabei, S.; Cavallo, A.; Guharay, S. K.; Hooke, W. M.; Hosea, J. C.; Jobes, F. C.; Mazzucato, E.; Mcneill, D. H.; Meservey, E.; Motley, R. W.; Stevens, J.; Goeler, S. von

doi:10.1088/0029-5515/26/5/012

Cited by 35 publications

(20 citation statements)

References 7 publications

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“…The sawtooth period increases to at most twice its initial value for A<f> = +90° phasing, whereas for A# = -90° phasing the sawtooth repetition time rises only by a modest amount. In contrast to the results of low density stabilization experiments on PLT and ASDEX [7,8], in our Alcator C experiments the sawtooth period increases monotonically with RF power and does not decrease when the RF power is just below the stabilization threshold. The sawtooth inversion radius inferred from the soft X-ray profile data does not differ substantially from the inversion radius of an Ohmic plasma for any phasing or RF power level at which sawteeth are still observed.…”

Section: On Sawtooth Stabilizationcontrasting

confidence: 99%

“…However, in the PLT low density experiments when sawteeth were suppressed, a residual m = 1 oscillation was often observed, suggesting that the q = 1 surface may still have been present at rf powers sufficient to stabilize the sawteeth. However, at higher rf powers the m = 1 oscillation was also eliminated [7]. Sawteeth have also been suppressed by lower hybrid current drive in considerably higher density plasmas [6,9].…”

Section: Introductionmentioning

confidence: 99%

“…Suppression of sawteeth by lower hybrid waves has been reported in current drive plasmas in which all or part of the current is driven by lower hybrid waves [5][6][7][8][9]. In all cases, the sawteeth are suppressed when Present address: JET Joint Undertaking, Abingdon, Oxfordshire, United Kingdom sufficient RF power is launched in the current drive mode, namely with the waveguide grill phased to launch a travelling wave spectrum predominantly in the direction of the electron drift.…”

Section: Introductionmentioning

confidence: 99%

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Stabilization of sawteeth by lower hybrid waves in the Alcator C tokamak

1988

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Sawteeth have been suppressed by lower hybrid current drive in high density Alcator C plasmas. At a density of 1.1 x 10 1 4 cm-3 and q(a) = 4.7 the sawteeth were eliminated by application of P,.f = 600 kW at 4.6 GHz, which represents an estimated 30% of the rf power required to fully sustain the discharge in the absence of ohmic power. Strong electron heating is observed with both current drive and anti-current drive phasings of the waveguide arrays but sawtooth stabilization occurs only in the former case. For the current drive phasing, the sawtooth period increases with rf power until the sawteeth are stabilized, while the period remains unchanged for rf injection with anti-current drive phasing. The internal inductance of the plasma increases during current drive relative to the anti-current drive case. The disappearance of sawteeth and the increase in internal inductance suggests that the rf current is driven near the q = 1 surface. At high power (Pf, > 850 kW), the sawteeth return 40 -60 ms into the rf pulse, indicating that an optimal power range, or rf deposition profile, exists for successful sawtooth suppression.

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Section: On Sawtooth Stabilizationcontrasting

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Stabilization of sawteeth by lower hybrid waves in the Alcator C tokamak

1988

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“…Based on these T e profiles, the current-density perturbation of Aj/jo>0. 2 is easily expected since Ki oop (r) -const in the steady-state discharges. On the other hand, in our theoretical model, the required current modification for stabilization is Aj/j > 0.05, consistent with the observed result.…”

Section: ^2mentioning

confidence: 90%

Sawtooth stabilization by localized electron cyclotron heating in a tokamak plasma

et al. 1991

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YSawtooth oscillations (STO) in the ohmically heated WT-3 tokamak are strongly modified or suppressed by localized electron cyclotron resonance heating (ECH) near the q = 1 surface, where q refers to the safety factor. The effect of ECH is much stronger when it is applied on the high field side (the inner side of the tokamak) as compared to the low field side (outer side). Complete suppression of the STO is achieved for the duration of the ECH when it is applied on the high field side, in a low density plasma, provided the ECH power exceeds a threshold value. The STO stabilization is attributed to a modification of the current density profile by hot electrons generated by ECH, which reduces the shear in the q = 1 region. "Present address: Physics Laboratory, College of Liberal Arts and Science, Kyoto University.• t Japan Atomic Energy R_search Institute, Naka Fusion Research Establishment, , Ibaraki-ken, Japan. tOsaka Institute of Technology, Osaka, Japan 1 '! D Sawtooth oscillations (STO) in tokamaks have long been the subject of intense study. Recently it has been recognized that they can be stabilized by _ , a variety of means, including: lower hybrid current drive, 1-4 ICRF heating 5 artd electron cyclotron resonance heating, s-9 Several models 1°-12 have been proposed as possible stabilization mechanisms.However, a comparison of these models with experiments has been inconclusive, partly due to the difficulty of monitoring and or controlling the plasma parameters near the q = 1 surface, where q refers to the plasma safety factor. In the WT-3 tokamak, STO are observed to be strongly modified and/or stabilized by ECH. The sharply localized nature of the ECH provides an effective tool for modifying plasma parameters locally near the q = 1 surface. These observations as well as plausible theoretical explanations for the stabilization are the subject of this letter.The experiments were carried out in the WT-3 tokamak, 13 with major and minor radii of R = 65 and a = 20cms, respectively. The toroidal field was BT <_ 1.75T, and the plasma current was lp _< 150kA. Microwaves from a gyrotron (w/27r = 56GHz, PEeH <_200kW) were transferred _hrough circular waveguides to a Vlasov antenna with an elliptic reflector placed along the major radius and injected into the plasma from the low field side. This sharply focussed wave, propagating as the X-mode with an angle of 0 = 88°t o the toroidal field, is absorbed at the second harmonic ECH resonance layer, rec H.Sawtooth oscillations were observed when the current was such that the safety factor at the limiter was in the range qL --'-2.2 ,-, 6.0. The sawtooth period rs, as well as the amplitude, increase when the ECH power, PECg, is applied. These modifications of the STO are very sensitive to the field BT which determines the location where the ECH is localized. The ECH power is held constant and BT is adjusted so that the resonance occurs at different surfaces, the geometric configuration is shown in Fig. l(a), while the changes in the soft X-ray (SXR) signal, I,, due to t...

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“…Such a regime is expected to extend up to even higher densities (up to the current drive density limit [25]), and may prove to be an attractive way to enhance energy confinement. Recent sawtooth stabilization experiments [26][27][28] were carried out in this mode of operation. Energy confinement in combined experiments using lower hybrid and other forms of auxiliary heating methods remains a subject of future study.…”

Section: Discussionmentioning

confidence: 99%

Energy confinement studies of lower hybrid current driven plasmas in the Alcator C tokamak

et al. 1987

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ABSTRACT. Energy confinement is studied in lower hybrid current driven (LHCD) plasmas in Alcator C in the density range ii = (1-8) x 1013 cm-3 . In LHCD plasmas, the stored energy in the electron tail Wail can be a significant fraction of the total stored energy W"*, especially at lower densities. At sufficiently low densities, the energy confinement time of the high energy electrons is expected to become shorter than their collisional energy slowing down time, and direct energy losses from the electron tail can become important in the overall power balance. The global energy confinement time, defined as rE** = W**t/Ptot, is found to be comparable to or exceed that in ohmically heated (OH) plasmas at low densities n, < 3 x 1013 cm-3 , where a steady state current can be maintained with * present address : University of Texas, Austin, Texas, U.S.A. 1 relatively low rf power. However, at higher densities where substantially more rf power is needed (relative to the ohmic power required to maintain a similar plasma), a deterioration of rtot relative to ohmic confinement, similar to that predicted by the neutral beam heated L-mode scaling, is observed.Theoretical modeling with the aid of a ray tracing-Fokker Planck-transport code suggests that the deteriorated confinement in this high density, high power regime may be attributed to an enhanced bulk electron thermal diffusivity. In a combined OH-LHCD plasma, a value of r' greater than the ohmic value is obtained as long as the applied rf power does not significantly exceed the ohmic power.2

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Suppression of internal disruptions in inductively driven tokamak discharges by lower hybrid wave current drive

Cited by 35 publications

References 7 publications

Stabilization of sawteeth by lower hybrid waves in the Alcator C tokamak

Stabilization of sawteeth by lower hybrid waves in the Alcator C tokamak

Sawtooth stabilization by localized electron cyclotron heating in a tokamak plasma

Energy confinement studies of lower hybrid current driven plasmas in the Alcator C tokamak

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