Suppression of Sawtooth Oscillations by Lower-Hybrid Current Drive in the ASDEX Tokamak

Abstract

The sawtooth oscillations in tokamak discharges with Ohmic and neutral-beam heating could be suppressed when a large part of the plasma current was driven by lower hybrid waves (IHF/IP ^ 0.5). The stabilization is due to a flattening of the current profile j(r) and an increase of q(Q) above 1. Higher central electron temperatures are obtained with neutral-beam heating if the sawteeth are stabilized. The increase in total energy content in this case was 30% higher than in the presence of sawteeth.PACS numbers: 52.50.Gj, 52.35.Hr, 52.55.FaThe sawtoothlike oscillations usually observed in tokamaks have been considered for a long time as beneficial for stable and clean discharges. The timeaveraged power loss due to the sawtooth instability is small during Ohmic heating (OH) and is still not serious with additional neutral-beam heating. The situation has changed drastically with high-power ioncyclotron heating where giant sawteeth, with modulation of the central electron temperature by up to 50%, have been observed. 1 The resulting clamping of the central energy content could strongly impair core ignition in large fusion experiments. Methods for stabilization of the sawteeth are therefore of greatest interest.Application of lower hybrid (LH) waves may lead to the suppression of sawteeth in OH discharges. 2 * 3 On the ASDEX tokamak we have studied the influence of LH waves on the sawtooth period. Appropriate wave spectra and power requirements for suppression of sawteeth are discussed. Local current-profile measurements allowed clarification of the mechanism of sawtooth stabilization. Application to neutral-beam heating finally demonstrates the possible gain in total energy content in sawtooth-free discharges.The experiments reported here were performed in the divertor tokamak ASDEX (i? = 165 cm, a = 40 M). LH waves were injected with powers of up to F LH = 1 MW and pulse lengths of up to 1.5 s. Details of the LH system and of the plasma behavior with LH heating and current drive are presented elsewhere. 4 " 6

Radiofrequency current generation by lower hybrid slow waves in the presence of fusion generated alpha particles in the reactor regime

“…by Eqs (2a) and (2b), with T eo = (2-30) keV, T ea = 0.5 keV (except for T e0 = 2 keV where T ea = 0.05 keV), Xn e0 = T e (r) 1 n eo = 1.7 X10 2 °m ea -3 n ea = eo , 1 n eo = 1.5 X iT e , 7 n = 1.21 and 7 t = 1.0. Other parameters are: a = 1.24 m, R o = 3 m, I p = 10 MA, q o = 1.0, B0 = 6 T, 50%/50% D-T gas mixture, cj o /27r = 4.6 GHz, T x = T e , and the electron tail confinement time is TL = (1 s) 7 3 , where y is the relativistic factor. The injected RF power spectrum is calculated according to…”

“…One beneficial aspect of electron Landau damping at r/a > 0.5 is the mitigation of the effect of alpha particle damping due to the drop-off of n a (r) with radius. The LH slow wave would then be useful in applications such as current profile control, including sawtooth stabilization by broadening of the current density profile and hence raising of q(r) to more than 1 everywhere, as demonstrated on ASDEX [3]. Furthermore, the slow wave still shows promise in lower temperature reactor applications such as plasma startup and/or in combination with electron cyclotron resonance (ECR) current drive or fast wave current drive.…”

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

“…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.…”

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

“…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.…”

“…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. In the PETULA and ASDEX low density experiments [6,8], the internal inductance is observed to decrease during RF current drive experiments. This suggests that the radial current profile has flattened and sawtooth stabilization is achieved when the safety factor q(r) rises above unity everywhere in the plasma.…”