Solenoid-free current drive via ECRH in EXL-50 spherical torus plasmas

Abstract

As a new spherical tokamak (ST) designed to simplify engineering requirements of a possible future fusion power source, the EXL-50 experiment features a low aspect ratio (A) vacuum vessel (VV), encircling a central post assembly containing the toroidal field coil conductors without a central solenoid. Multiple electron cyclotron resonance heating (ECRH) resonances are located within the VV to improve current drive effectiveness. Copious energetic electrons are produced and measured with hard X-ray detectors, carry the bulk of the plasma current ranging from 50kA to 150kA, which is maintained for more than 1s duration. It is observed that over one Ampere current can be maintained per Watt of ECRH power issued from the 28-GHz gyrotrons. The plasma current reaches Ip>80kA for high density (>51018m-2) discharge with 150kW ECHR heating. An analysis was carried out combining reconstructed multi-fluid equilibrium, guiding-center orbits of energetic electrons, and resonant heating mechanisms. It is verified that in EXL-50 a broadly distributed current of energetic electrons creates smaller closed magnetic-flux surfaces of low aspect ratio that in turn confine the thermal plasma electrons and ions and participate in maintaining the equilibrium force-balance. effectiveness. Copious energetic electrons are observed via hard X-ray detectors, carry the bulk of the plasma current ranging from 50kA to 150kA, which is maintained for more than 1s duration. It is observed that over one Ampere current can be maintained per Watt of ECRH power issued from the 28-GHz gyrotrons. The plasma current with high line-density (approaching 〖10〗^19 m^(-2)) has been achieved for plasma currents as high as 76kA. An analysis was carried out combining reconstructed multi-fluid equilibrium, guiding-center orbits, and resonant heating mechanisms. It is verified that in EXL-50 a broadly distributed current of energetic electrons creates smaller closed magnetic-flux surfaces of low aspect ratio that in turn confine the thermal plasma electrons and ions and participate in maintaining the equilibrium force-balance.

Generation of energetic electrons by an electron cyclotron wave through stochastic heating in a spherical tokamak

“…further supports this finding, demonstrating that the current carried by energetic electrons plays a dominant role in shaping the closed magnetic flux surface (Shi et al. 2022; Wang et al. 2022).…”

“…The EXL-50 uses two sets of 28 GHz ECW systems (O-mode) to heat the plasma and drive plasma current (Shi et al. 2022; Wang et al. 2022).…”

“…As the incident power increases, the plasma current also rises, and the average slope exceeds (Shi et al. 2022).…”

“…This remarkable progress, highlighted by an amps-to-watts ratio of (Shi et al. 2022), underscores the importance of energetic electron involvement in driving the plasma current.…”

Performance assessment of helicon wave heating andcurrent drive in EXL-50 spherical torus plasmas

“…It has a major radius of 0.58 m. One of the critical experimental goals of EXL-50 tokamak is to assess the effectiveness of electron cyclotron current drive (ECCD) and electron cyclotron resonance heating (ECRH) in the absence of a center solenoid (CS) magnet. It has been demonstrated that the ECRH can achieve steady-state high-efficiency current drive and obtain the highest RF current drive record without a CS magnet [21][22][23]. Recently remarkable experiments have also shown plasma currents as high as 80-100 KA at line densities over 0.5 × 10 19 m −3 .…”