Development of a plasma current ramp-up technique for spherical tokamaks by the lower hybrid wave

Abstract

Spherical tokamaks (STs) have the advantage of superior stability at high beta, but to realize a compact fusion reactor, the central solenoid (CS) must be eliminated. The plasma current could be maintained mainly by the self-driven current during the steady-state burning phase, but there is no established method of non-inductive current ramp-up from zero to a high enough level required for fusion burn. The lower-hybrid wave (LHW) is commonly used in tokamaks, but this technique is believed to be unusable in ST plasmas with very high dielectric constant. Wave excitation, propagation and damping were evaluated by numerical modeling for the TST-2 spherical tokamak (R = 0.68 m, a = 0.25 m, B = 0.3 T, I = 100 kA). Wave excitation is calculated by the RF antenna simulation tool based on the finite element method (COMSOL). The plasma is modeled as a medium with cold plasma dielectric response and artificially enhanced loss. Excitation of a traveling fast wave (FW) by the TST-2 combline antenna was confirmed by COMSOL calculation. However, the FW must be mode converted to the LHW to achieve efficient current drive. The TORLH full-wave solver, which can treat diffraction effects properly, was applied to the TST-2 LH current drive experiment. It is shown that core current drive by LHW is possible in the low density, low current plasma formed by ECH. It is important to keep the density low during current ramp-up, and the wavenumber must be reduced as the current increases in order to maintain core current drive. The results of these calculations and the initial experiment on TST-2 will be used to design an optimized LHW antenna with appropriate polarization and wavenumber spectrum controllability for the current ramp-up experiment. Taking a conservative value for the current drive figure of merit, the steadystate driven current by 200 kW of RF power is estimated to be 150 kA, which should be adequate for evaluating the usefulness of this technique up to a current level of 100 kA in TST-2. This technique should be usable during the low-density, low-current, non-burning start-up phase of a reactor to reach a sufficient current level needed for further heating. Initial results from the TST-2 experiment demonstrated that initial current formation and ST equilibrium formation can be achieved by RF power in the LH frequency range. Further current ramp-up by directly excited LHW will be investigated on TST-2. The success of the TST-2 experiment would provide a scientific basis for quantitatively evaluating the required CS capability for a low aspect ratio reactor.

Fully non-inductive plasma start-up with lower-hybrid waves using the outboard-launch and top-launch antennas on the TST-2 spherical tokamak

“…On the other hand, establishment of an efficient and reliable non-inductive plasma start-up method is an active area of research [4][5][6][7][8][9][10][11]. Since neutral beam current drive requires finite plasma current for confinement of fast ions, plasma current ramp-up from no current needs to be performed by other means such as coaxial helicity injection [4] or radio frequency (RF) waves [5][6][7][8][9][10][11].…”

“…Formation of an advanced tokamak by switch-over to neutral beam injection has also been observed [12]. The possibility of plasma startup with LH waves on STs has been investigated on TST-2 [8][9][10][11]. Formation of stable ST equilibria only with LH current drive has been demonstrated [8].…”

“…The possibility of plasma startup with LH waves on STs has been investigated on TST-2 [8][9][10][11]. Formation of stable ST equilibria only with LH current drive has been demonstrated [8]. Although there is no fundamental reason to prohibit application of LH waves to STs, operation at high density is more challenging than conventional tokamaks due to the lower toroidal field.…”

Current drive in recombining plasma

“…[11][12][13][14] from the work done on current drive schemes in other time-varying conditions, e.g., during plasma current ramp-up in tokamaks, where the current and magnetic field are time-varying [16][17][18][19][20][21], is the direct influence of the time-variation on wave-particle processes in the plasma. Time variation in neither the application of rf power nor changes in the current or poloidal magnetic field alters the underlying wave dynamics or the particle collisionality.…”

Observation of High Impurity Ion Temperatures in ECW/EBW Sustained Plasmas on LATE

“…12.1202041 In order to realize a compact spherical tokamak (ST) reactor, the elimination of the massive central solenoid is necessary. RF waves such as electron cyclotron waves and lower hybrid waves are often used to start-up and sustain ST plasmas [1,2]. It is believed that these RF sustained plasmas comprise high energy electrons which carry the plasma current and dominate equilibrium [3,4], low temperature bulk electrons [5,6] and very low temperature ions.…”