Giant Sawtooth Research Articles

Magnetic safety factor profile before and after sawtooth crashes investigated with toroidicity and ellipticity induced Alfvén eigenmodes

A study of toroidicity and ellipticity inducedAlfvén eigenmodes (TAEs and EAEs) that are excited before and after sawtoothcrashes during ion cyclotron range of frequency (ICRF) heating in JT-60U is presented.From the TAEs that are observed before sawteeth and that reside inside the q = 1surface an upper limit has been set for q in the plasma centre at the timeof the crash. After the sawtooth crash, EAEs that reside at the q = 1 surfaceare often observed. In a number of discharges the start of the EAE activityis delayed to up to 150 ms after the crash. In some cases TAE activity wasobserved between the sawtooth crash and the onset of the EAE activity. TheseTAEs could be modelled successfully only when it was assumed that the centralsafety factor (q0) rises above unity after the giant sawtooth crash.The appearance of the TAE activity immediately after the giant sawtooth crashis a strong indication that the fast particle drive remains in the plasmacentre. This is consistent with theoretical estimates for the confinement ofdeeply trapped ICRF ions. The delayed appearance of the EAEs is also consistent with thedisappearance of the q = 1 surface from the plasma at the giant sawtoothcrash. The only way to obtain agreement between the experimentally measured EAEfrequencies and the NOVA-K simulations is to assume that the q = 1 surfacereappears in the plasma at the start of the EAE activity. The delayedappearance of the EAE activity seems to be correlated with the electrontemperature just before the crash.

Effective temperatures, sawtooth mixing, and stochastic diffusion ripple loss of fast H+ minority ions driven by ion cyclotron heating in the Tokamak Fusion Test Reactor

This paper presents studies of the H+ minority ions driven by Ion Cyclotron Radio Frequency (ICRF) heating in the Tokamak Fusion Test Reactor (TFTR) [R. J. Hawryluk et al., Phys. Plasmas 5, 1577 (1998)] deuterium plasmas using primarily passive H° flux detection in the energy range of 0.2–1.0 MeV. The measured passive H+ energy spectra are compared with active (Li pellet charge exchange) results. It is shown that in the passive mode the main donors for the neutralization of H+ ions in this energy range are C5+ ions. The measured effective H+ tail temperatures range from 0.15 MeV at an ICRF power of 2 MW to 0.35 MeV at 6 MW. Radial redistribution of ICRF-driven H+ ions was detected when giant sawtooth crashes occurred during the ICRF heating. The redistribution affected ions with energy below 0.7–0.8 MeV. The sawtooth crashes displaces H+ ions outward along the plasma major radius into the stochastic ripple diffusion domain where those ions are lost in ∼10 msec. These observations are consistent with the model of the redistribution of energetic particles developed previously to explain the results of deuterium-tritium (DT) alpha-particle redistribution due to sawtooth oscillations observed in TFTR. The experimental data are also consistent with numerical simulations of H+ stochastic ripple diffusion losses.

Active control of 2/1 magnetic islands in a tokamak

Closed and open loop control techniques were applied to growing m/n=2/1 rotating islands in wall-stabilized plasmas in the High Beta Tokamak-Extended Pulse (HBT-EP) [J. Fusion Energy 12, 303 (1993)]. HBT-EP combines an adjustable, segmented conducting wall (which slows the growth or stabilizes ideal external kinks) with a number of small (6° wide toroidally) driven saddle coils located between the gaps of the conducting wall. Two-phase driven magnetic island rotation control from 5 to 15 kHz has been demonstrated powered by two 10 MW linear amplifiers. The phase instability has been observed and is well modeled by the single-helicity predictions of nonlinear Rutherford island dynamics for 2/1 tearing modes including important effects of ion inertia and finite Larmor radius, which appear as a damping term in the model equations. The closed loop response of active feedback control of the 2/1 mode at moderate gain was observed to be in good agreement with the theory. Suppression of the 2/1 island growth has been demonstrated using an asynchronous frequency modulation drive which maintains the inertial flow damping of the island by application of rotating control fields with frequencies alternating above and below the natural mode frequency. This frequency modulation control technique was also able to prevent disruptions normally observed to follow giant sawtooth crashes in the plasma core.

Investigation of high-n TAE modes excited by minority-ion cyclotron heating in JT-60U

Toroidicity-induced Alfven eigen (TAE) modes are observed during minority-ion cyclotron resonance heating (ICRH) in the JT-60U. The toroidal mode numbers of TAE modes are identified as 7, 8, 9, 10 and 11 from the Doppler shift in the TAE modes with scanning toroidal rotation at a plasma current of 3 MA. The toroidal mode number of TAE modes tends to increase during a giant sawtooth by ICRH with a decreasing safety factor for the central region. The TAE mode number increases with plasma current, so that nine TAE modes are observed sequentially during a giant sawtooth at a plasma current of 4 MA, where the maximum toroidal mode number is estimated to be at least 13. There are no Alfven continuum gaps for TAE modes in the safety-factor ranges of i-1/2n<q<i+1/2n, (i=1, 2, 3, ...), except for the gaps in ellipticity-induced Alfven eigen (EAE) modes, where n is the toroidal mode number of TAE modes. Therefore, control of the q profile might provide a means of avoiding TAE modes, as long as the pressure gradient of the high-energy ions is localized.

Excitation of high n toroidicity-induced Alfvén eigenmodes and associated plasma dynamical behaviour in the JT-60U ICRF experiments

High frequency MHD activities observed during second harmonic ICRF heating are identified to be toroidicity-induced Alfvén eigenmodes (TAE) driven by MeV protons produced by ICRF heating. Correlation between MeV protons and TAE modes is clearly observed. TAE mode amplitude increases exponentially with increasing toroidal mode number up to more than ten. The tendency cannot be explained by present local TAE stability theories. Long suppression of TAE modes after a giant sawtooth crash can be explained by fast ion loss due to the sawtooth crash and evolving q-profile.

Recent ion cyclotron range of frequencies experiments in JT-60U

Recent results on the minority ion second harmonic heating on JT-60U are presented. Maximum coupled power reached 6.4 MW. Good antenna-plasma coupling capability and a small fraction (less than 10%) of an incremental radiation loss to r.f. power are confirmed. Power absorption rate increases with increasing r.f. power and is saturated around unity at r.f. powers higher than 3 MW. The sawtooth stabilization by minority ion second harmonic heating was realized over a wide parameter range, i.e. Ip = 0.9 MA−4 MA, q95 = 2.3−8.6, ne = (1.3−5) × 1019m−3 and PIC ⩾ 2.2 MW. A figure of merit VP〈ne〉Ptot for efficiency of the sawtooth stabilization is about 50% higher than those in other devices where fundamental resonance minority ion heating is employed. The longest stable period reached 2.33 s. Attainable sawtooth-free periods scale with the resistive diffusion time. It was found that the energy confinement is further improved by 25% during the reheating phase after the giant sawtooth crash. The electron temperature profile became more peaked at the improved confinement phase. Those phenomena were observed only in low q discharges (q95 ⩽ 2.9).

Major results from JT-60 experiments.

Studies on plasma confinement, steady-state tokamak operations and boundary plasmas are main subjects in tokamaks relating to fusion plasmas. Emphases of recent JT-60 experiments from 1987 to the shutdown in October, 1989 were put on these studies. Investigations of L-mode improved modes such as pellet injection, high Ti mode, giant sawtooth by 3rd harmonics of ICRF etc., and H-mode were carried out as confinement studies. Current drive by LHCD made a large progress by using a new launcher. Bootstrap current reached 80% of plasma current, which has brought a new idea of steady-state tokamaks. Results on particle confinement, particle and heat control studies with divertor are also presented

Present status and prospect of tokamak plasma as revealed by JET.

Present status of JET plasma as a typical example of the large tokamak plasma (up to the end of 1985) is reported. Surveys are done on 1) Impurity and radiation problems, 2) Magnetics, 3) Plasma density, n, the density limit and particle control problems (fueling and exhausting), 4) Plasma temperature, T, in connection to the ICRF wave heating and giant sawtooth activity and 5) Energy confinement time, τE, and the scaling laws. The highest value of nT τE has been obtained during the ohmic discharges and it will be a hard task to obtain an objective nT τE value from the present scaling. Subjectives are discussed in relation to the near future plan of the experiments, i. e., 1) Input power upgrade, 2) Separatrix operation, 3) Combined operation of heating and current drive, 4) Surpression of sawtooth activities by RF, 5) Pellet injection and 6) inner limiter discharges. Miscellaneous thoughts concerning on the plasma disruption (self-selection) and the profile consistency (self-organization) are made from the view point of a theorist.

Giant sawtooth oscillations in the Doublet III tokamak

Large-amplitude sawtooth oscillations have been observed in the soft-X-ray emission, central electron temperature, neutron production rate, radiated power, Dα emission, fast-neutral flux and one-turn voltage during beam-heating experiments in the Doublet III tokamak. The necessary conditions for the appearance of giant sawteeth seem to be operation at low qa and high βT. Many features of the giant sawteeth have been simulated with a transport code to which a sawtooth mixing model and several diagnostic models were added. Substantial redistribution of particles and energy evidently occurs over the plasma interior. Global energy confinement, however, is only moderately degraded by the sawteeth.