Plasma current start-up experiments using a dielectric-loaded waveguide array antenna in the TST-2 spherical tokamak

Abstract

Plasma current start-up and ramp-up using the lower hybrid wave (LHW) were investigated on the TST-2 spherical tokamak. The LHW was launched by a dielectric-loaded waveguide array (grill) antenna. The antenna–plasma coupling of this antenna deteriorates as the input power exceeds several kW. This deterioration is believed to be caused by the density depletion due to the ponderomotive force. This conjecture was confirmed by the measurement of density reduction and the result of a non-linear full wave numerical calculation based on the finite element method (FEM). The plasma current was started and ramped up to 10 kA using this antenna. The ability of this grill antenna to excite the LHW with different n∥ = ck∥/ω was used to identify the most favourable n∥ spectrum for plasma current ramp-up. It was found that effective current drive can be achieved by the LHW with n∥ less than 6. However, even in this case, the energetic electrons which account for a large fraction of the driven current, are lost rapidly because the poloidal field generated by this level of plasma current is not sufficient to confine high energy electrons.

Reduction Rate of the MHD Potential Energy by a Toroidal Plasma Current Drive during Tokamak Plasma Formation with DC_CS

“…W M0 depends on not only the plasma size but also the plasma pressure profile ratio n 1 /(n 1 + n 2 ). A coil configuration of TST2 [5,6] is utilized; the configuration comprises a CS coil with 239 coil turns and three pairs of PF coils with 9 (PF1), 6 (PF2), and 40 (PF3) coils turns in one-side which are numbered beginning at the top of the device, as shown in Figs. 1 (A-1) and (C-1).…”

“…A temporally changing current is applied to the central solenoidal coil (CS coil) in the central axis of a usual tokamak, generating a temporally changing magnetic field energy in order to drive a toroidal plasma current for Joule heating in induction heating experiments. On the other hand, no current is applied to the CS coil for non-induction heating experiments because a tokamak plasma formation and its sustainment by non-induction heating have been investigated for steady state operation at a constant equilibrium state and for the elimination of the CS coil [1][2][3][4][5][6]. The CS coil current has little influence on the equilibrium configuration of tokamak plasmas because a solenoidal coil ideally only generates a small leakage magnetic field.…”

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].…”

Generation of Energetic Electrons during Spherical Tokamak Merging in UTST

“…Various current drive schemes such as an RF current drive [2,3] and helicity injection [4,5] are under development in several ST experiments performed worldwide. Another CS-less start-up method is employed in START, MAST (UKAEA) [6], and TS-3/TS-4 [7,8] devices using induction from in-vessel PF coils as well as in the UTST device [9] using induction from ex-vessel PF coils in combination with plasma merging, which can provide high-power initial heating via magnetic reconnection and density increases via compression.…”