Formation of spherical tokamak equilibria by ECH in the LATE device

Abstract

Main objective of the LATE (Low Aspect ratio Torus Experiment) device is to demonstrate formation of ST plasmas by electron cyclotron heating (ECH) alone without center solenoid. By injecting a 2.45 GHz microwave pulse up to 30 kW for 4 seconds, a plasma current of 1.2 kA is spontaneously initiated under a weak steady vertical field of B v = 12 Gauss, and then ramped up with slow ramp-up of B v for the equilibrium of the plasma loop and finally reaches 6.3 kA at B v = 70 Gauss. This currents amount 10 percents of the coil currents of 60 kAT for the toroidal field. Magnetic measurements show that an ST equilibrium, having the last closed flux surface with an aspect ratio of R 0 /a 20.4 cm/14.5 cm 1.4, an elongation of κ = 1.5 and q edge = 37, has been produced and maintained for 0.5 s at the final stage of discharge. The plasma center locates near the second harmonic EC resonance layer and the line averaged electron density significantly exceeds the plasma cutoff density, suggesting that the second harmonic EC heating by the mode-converted electron Bernstein waves (EBW) support the plasma. Spontaneous formation of ST equilibria under steady B v fields, where plasma current increases rapidly in the time scale of a few milliseconds, is also effective and a plasma current of 6.8 kA is spontaneously generated and maintained at B v = 85 Gauss by a 5 GHz microwave pulse (130 kW, 60 ms).

Recent progress on spherical torus research

“…1 and 2), there are a large number of significant ST results which are unique to STs, while at the same time reaffirming many common physics features with conventional tokamaks. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] As a member of the tokamak family, 19 ST research contributes to advancing conventional tokamaks such as ITER 20,21 by providing data that extends into a unique plasma and device parameter space. Since ITER represents a significant extrapolation from present day tokamaks, ST research provides a different set of tokamak-related data for the improvement of predictive capabilities by providing leverage over a wide range of parameter space.…”

Two Dimensional Li Beam Imaging to Study the Magnetic Field Configuration Effects on Plasma Confinement in Spherical Tokamak CPD

“…In fact, for ITER start-up with a multi-pole null magnetic field configuration, addition of an extra 3 MW RF power is planned along with Ohmic power to facilitate reliable breakdown over a wide range of error field and pressure conditions [3]. The possibilities of plasma current generation in RF plasma have been demonstrated in CDX-U [4] LATE [5] and TST-2@K [6] under a weak magnetic mirror configuration. Such current formation as well as ramp up under a steady B z field is considered to be responsible for the change of magnetic field topology from an open field equilibrium to a closed field equilibrium.…”

Visualization of Magnetic Surfaces during Current Ramp-Up Phase Using Thermal Lithium Sheet Beam in CPD

“…In CDX-U, DIII-D and LATE the electron cyclotron waves have been used to ramp-up the plasma current under open field configuration with the combination of the toroidal and vertical magnetic field lines [1,2]. However, the physics of current ramp-up mechanism, that is, magnetic reconnection, is still not clear.…”

“…The plasma current can be driven by RF waves itself in a weak mirror configuration and a clear change is observed in plasma boundary as well as magnetic field topology associated with the transition of the current from low (∼1 kA) to high (∼3 kA) value. In CDX-U, DIII-D and LATE the electron cyclotron waves have been used to ramp-up the plasma current under open field configuration with the combination of the toroidal and vertical magnetic field lines [1,2]. However, the physics of current ramp-up mechanism, that is, magnetic reconnection, is still not clear.…”