Observation of wall stabilization and active control of low-n magnetohydrodynamic instabilities in a tokamak (original) (raw)

Active-Feedback Control of the Magnetic Boundary for Magnetohydrodynamic Stabilization of a Fusion Plasma

Physical Review Letters, 2006

Stable operation with control on magnetohydrodynamic modes has been obtained in the modified reversed field experiment employing a set of 192 feedback controlled saddle coils. Improvements of plasma temperature, confinement (twofold), and pulse length (threefold) and, as a consequence of the magnetic fluctuation reduction, strong mitigation of plasma-wall interaction and mode locking are reported.

How Rotation affects Instabilities and the Plasma Response to Magnetic Perturbations in a Tokamak Plasma

2014

How Rotation affects Instabilities and the Plasma Response to Magnetic Perturbations in a Tokamak Plasma Bryan DeBono This thesis presents the first systematic study of the multimode external kink mode structure and dynamics in the High-Beta Tokamak Extended-Pulse experiment (HBT-EP) when the plasma rotation is externally controlled using a source of toroidal momentum input. The capabilities of the HBT-EP tokamak to study rotation physics was greatly extended during a 2009-2010 major upgrade, when a new adjustable conducting wall, a high-power modular control coil array system, and an extensive set of 216 poloidal and radial magnetic sensors were installed on the machine. HBT-EP was additionally equipped with a biased edge electrode which made it possible to adjust the plasma ion and plasma magnetohydrodynamics (MHD) mode rotation frequencies by imparting an electromagnetic torque on the plasma. The design of this biased edge electrode, and its capability to torque the plasma is described. The rotation frequency of the helical kink modes was directly inferred from analysis of the magnetics dataset. To directly measure the plasma ion acceleration as the plasma was torqued by the biased electrode, a novel high-throughput and fast-response spectroscopic rotation diagnostic was installed on HBT-EP. This spectroscopic rotation diagnostic was designed to measure the velocity of He ions, therefore when conducting experiments using the spectroscopic rotation diagnostic a gas mixture of 90%D and 10%He was used. With its current power supplies the bias probe is capable of accelerating the primary m/n=3/1 helical kink mode (which has a natural rotation frequency between +7→+9kHz) to somewhere between-50kHz→+25kHz depending on the probe bias. At a probe voltage of +175V the He impurity ions were seen to accelerate by 3km/sec. Biorthogonal decomposition (BD) analysis was applied to the large magnetics dataset and used to determine the multimode m/n spectrum of the helical kink modes present in HBT-EP. The dominant helicities present as revealed by the BD are the m/n=3/1 and m/n=6/2 modes, which represent about 85% and 8% of the total MHD activity respectively. This percentages remain consistent across the entire range of 3/1 mode rotation frequencies obtainable from the bias probe, (-50kHz→25kHz). The Hilbert transform technique was also applied to magnetic sensor data to determine the instantaneous amplitude and frequency of the total MHD activity. The total MHD amplitude was seen to decrease with increasing plasma rotation, a 35% reduction as the 3/1 mode was accelerated from +6→+24kHz. Active MHD spectroscopy experiments using a "phase-flip" resonant magnetic perturbation (RMP) are able to excite a clear three-dimensional plasma response. Plasma rotation is theoretically expected to increase plasma stability to external resonant error fields, and in HBT-EP the plasma amplitude response to a m/n=3/1 RMP increases by a factor of 2.7 when the plasma rotation is decreased from +25kHz to ± 2kHz. As the RMP amplitude increases, slower plasmas are seen to disrupt at a lower perturbation amplitude than unperturbed or rapidly rotating modes. The 6/2 helical kink mode also shows an amplitude and phase response to the 3/1 RMP, and like the 3/1 mode the amplitude response is largest when the plasma is slowly rotating. The ratio between the plasma 6/2 amplification and the 3/1 amplification to a 3/1 RMP is nearly constant, regardless of the plasma rotation or the RMP amplitude.

Stabilization of the external kink and control of the resistive wall mode in tokamaks

Physics of Plasmas, 1999

One promising approach to maintaining stability of high beta tokamak plasmas is the use of a conducting wall near the plasma to stabilize low-n ideal magnetohydrodynamic instabilities. However, with a resistive wall, either plasma rotation or active feedback control is required to stabilize the more slowly growing resistive wall modes ͑RWMs͒. Previous experiments have demonstrated that plasmas with a nearby conducting wall can remain stable to the nϭ1 ideal external kink above the beta limit predicted with the wall at infinity. Recently, extension of the wall stabilized lifetime L to more than 30 times the resistive wall time constant w and detailed, reproducible observation of the nϭ1 RWM have been possible in DIII-D ͓Plasma Physics and Controlled Fusion Research ͑International Atomic Energy Agency, Vienna, 1986͒, p. 159͔ plasmas above the no-wall beta limit. The DIII-D measurements confirm characteristics common to several RWM theories. The mode is destabilized as the plasma rotation at the qϭ3 surface decreases below a critical frequency of 1-7 kHz ͑ϳ1% of the toroidal Alfvén frequency͒. The measured mode growth times of 2-8 ms agree with measurements and numerical calculations of the dominant DIII-D vessel eigenmode time constant w . From its onset, the RWM has little or no toroidal rotation ( mode р w Ϫ1 Ӷ plasma ), and rapidly reduces the plasma rotation to zero. These slowly growing RWMs can in principle be destabilized using external coils controlled by a feedback loop. In this paper, the encouraging results from the first open loop experimental tests of active control of the RWM, conducted in DIII-D, are reported.

Control of the resistive wall mode in advanced tokamak plasmas on DIII-D

Nuclear Fusion, 2000

Resistive wall mode (RWM) instabilities are found to be a limiting factor in advanced tokamak (AT) regimes with low internal inductance. Even small amplitude modes can affect the rotation profile and the performance of these ELMing H-mode discharges. Although complete stabilization of the RWM by plasma rotation has not yet been observed, several discharges with increased beam momentum and power injection sustained good steady-state performance for record time extents. The first investigation of active feedback control of the RWM has shown promising results: the leakage of the radial magnetic flux through the resistive wall can be successfully controlled, and the duration of the high beta phase can be prolonged. The results provide a comparative test of several approaches to active feedback control, and are being used to benchmark the analysis and computational models of active control.