Achievement of field-reversed configuration plasma sustainment via 10 MW neutral-beam injection on the C-2U device

Abstract

Tri Alpha Energy's experimental program has demonstrated reliable field-reversed configuration (FRC) formation and sustainment, driven by fast ions via high-power neutral-beam (NB) injection. The world's largest compact-toroid device, C-2U, was upgraded from C-2 with the following key system upgrades: increased total NB input power from ~4 MW (20 keV hydrogen) to 10+ MW (15 keV hydrogen) with tilted injection angle; enhanced edge-biasing capability inside of each end divertor for boundary/stability control. C-2U experiments with those upgraded systems have successfully demonstrated dramatic improvements in FRC performance and achieved sustainment of advanced beam-driven FRCs with a macroscopically stable and hot plasma state for up to 5+ ms. Plasma diamagnetism in the best discharges has reached record lifetimes of over 11 ms, timescales twice as long as C-2. The C-2U plasma performance, including the sustainment feature, has a strong correlation with NB pulse duration, with the diamagnetism persisting even several milliseconds after NB termination due to the accumulated fast-ion population by NB injection. Power balance analysis shows substantial improvements in equilibrium and transport parameters, whereby electron energy confinement time strongly correlates with electron temperature; i.e. the confinement time in C-2U scales strongly with a positive power of T e .

Formation of hot, stable, long-lived field-reversed configuration plasmas on the C-2W device

“…The main goals of the C-2W experimental program to be accomplished are as follows: (i) demonstrate plasma ramp-up by NB heating and current drive; (ii) improve edge/divertor plasma performance to achieve high electron temperature both at the plasma edge and inside the core; (iii) develop plasma control on the time scale significantly longer than L/R vessel-wall time and plasma confinement times, and demonstrate controllable plasma ramp-up; and (iv) explore a wide range of plasma parameters such as plasma temperature, magnetic field and plasma size to confirm the previously emerged/obtained energy confinement scaling [6,11]. There are also several key intermediate milestones to accomplish in scientific and engineering aspects on the C-2W experimental program in order to ensure that each subsystem of the machine operates within its designed parameters as well as to accelerate the program towards the main goals: for instance, producing a robust FRC formation and translation through inner divertors; establishing adequately controllable magnetic-field structure in the inner divertor area to change from guiding straight magnetic field for FRC translation to flared field structure, as can be seen in Figs.…”

“…The upgraded NBI and edge biasing systems enabled significant plasma performance advances and had a profound impact on C-2U performance such as: (i) rapid accumulation of fast ions (about half of the initial thermal pressure replaced by fast-ion pressure); (ii) fast-ion footprint largely determines FRC dimensions; (iii) double-humped electron density and temperature profiles (indicative of substantial fast-ion pressure); (iv) FRC lifetime and global plasma stability scale strongly with NB input power; and (v) plasma performance correlates with NB pulse duration in which diamagnetism persists several milliseconds after NB termination due to accumulated fast ions. Under the optimum C-2U operating conditions, plasma sustainment for ~5+ ms as well as long-lived plasma discharge of up to 10+ ms were successfully achieved [6], in which the performance was mostly limited by hardware and stored energy constraints such as the NB's pulse duration and the current sourcing capability of the end-on plasma guns. Furthermore, with careful 0-D global power-balance analysis [10,11], there appeared to be a strong positive correlation between electron temperature Te and energy confinement time; i.e., the electron energy confinement time tE,e in C-2U FRC discharges scales strongly with a positive power of Te [6,11], which is basically the same characteristics/trend as observed in C-2 [4].…”

“…Under the optimum C-2U operating conditions, plasma sustainment for ~5+ ms as well as long-lived plasma discharge of up to 10+ ms were successfully achieved [6], in which the performance was mostly limited by hardware and stored energy constraints such as the NB's pulse duration and the current sourcing capability of the end-on plasma guns. Furthermore, with careful 0-D global power-balance analysis [10,11], there appeared to be a strong positive correlation between electron temperature Te and energy confinement time; i.e., the electron energy confinement time tE,e in C-2U FRC discharges scales strongly with a positive power of Te [6,11], which is basically the same characteristics/trend as observed in C-2 [4]. This positive confinement scaling is very attractive, and similar features of temperature dependence have also been observed in other high-beta devices such as NSTX and MAST, whereby the energy confinement time scales nearly inversely with collisionality [12,13].…”

Combination Doppler backscattering/cross-polarization scattering diagnostic for the C-2W field-reversed configuration

“…Typically, in fusion plasmas, radial particle and heat transport in excess of classical or neoclassical transport is observed, which is caused by plasma turbulence. In the C-2U FRC device at TAE Technologies, 1,2 Doppler Backscattering (DBS) measurements 3,4 with a diagnostic described earlier 5 clearly show that fluctuations at low toroidal wavenumber are absent/stable in the FRC core. 6 Gyrokinetic stability analysis has attributed core stability to the combined effect of large ion Larmor radius, short field-line connection length restricting the parallel wavenumber spectrum, and favorable magnetic field gradient.…”

MHD Simulation of Merging Fueling Method Used for ST Plasma

“…Historically, merging experiments of spheromak have been carried out at TS-3/4 [2,3], SSX [4], and others. Additionally, Tri-Alpha Energy Inc. has succeeded in generating field-reversed configuration (FRC), with good confinement performance, by merging two FRC plasmas [5,6]. In recent years, merging experiments of spherical torus (ST) plasmas have also been carried out at UTST, and reports have been submitted on the generation of high energy electrons during the merging process [7].…”