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The plasma current start-up experiment is conducted through electron cyclotron (EC) heating in the QUEST spherical tokamak. During the EC heating, the application of a toroidal electric field in the opposite direction to the plasma current effectively inhibits the growth of energetic electrons. Observations show rapid increases in plasma current and hard x-ray count immediately following the cancellation of the retarding electric field. When a compact tokamak configuration maintains equilibrium on the high field side, along with the retarding field, it leads to effective bulk electron heating. This heating achieved an electron temperature of T _e ≈ 1 keV at electron density n _e > 1.0 × 10 ^18 m ^−3 . Ray tracing of the EC wave verifies that more power absorption into plasma through a single-pass occurs around the second resonance layer with higher values of electron density and temperature.

Results from the successful solenoid-free plasma startup using the method of transient coaxial helicity injection (transient CHI) in the QUEST spherical tokamak (ST) are reported. Unlike previous applications of CHI on HIT-II and on NSTX which required two toroidal insulating breaks to the vacuum vessel, QUEST uses a first of its kind, floating single biased electrode configuration, which does not use such a vacuum break. Instead, the CHI electrode is simply insulated from the outer lower divertor plate support structure. This configuration is much more suitable for implementation in a fusion reactor than the previous configurations. Transient CHI generated toroidal currents of 135 kA were obtained. The toroidal current during the formation of a closed flux configuration was over 50 kA. These results bode well for the application of transient CHI in a new generation of compact high-field STs and tokamaks in which the space for the central solenoid is very restricted.

Spatially resolved rotational temperature of ground state hydrogen molecules desorbed from plasma-facing surface was measured in QUEST, LTX-β, and DIII-D tokamaks, and the increases of the rotational temperature with the surface temperature and due to collisional-radiative processes in the plasmas were evaluated. The increase due to collisional-radiative processes was calculated by solving rate equations considering electron and proton collisional excitation and deexcitation and spontaneous emission. The calculation results suggest a high sensitivity for the rotational temperature to electron and proton densities, but a negligible sensitivity to the electron, proton, and surface temperatures. In the three tokamaks with different plasma parameters and plasma-facing surface materials, the spatial profile of the rotational temperature was estimated using Fulcher-α emission lines (600–608 nm). In QUEST, the spatial profile of the rotational temperature was estimated from spatially resolved spectra. In the other tokamaks, the rotational temperature was evaluated assuming a single point emission with a location determined from the Fulcher-α emission profile as measured with a filtered camera. In metal-walled devices QUEST and LTX-β, the rotational temperature increased with the surface temperature, and the calculated collisional-radiative increase is consistent with measured increase assuming that the rotational temperature at the surface is approximately 500–600 K higher than the surface temperature. In DIII-D with carbon walls, a larger collisional-radiative increase than the other tokamaks was observed because of the higher density leading to a large difference from the calculated increase compared to the other smaller tokamaks. Measurement of the Fulcher-α emission profile with higher spatial resolution in DIII-D may reduce the difference and reveal the effect of the surface temperature on the rotational temperature. These results show the increases in the rotational temperature with the surface temperature and due to the collisional-radiative processes.

A 28 GHz system with a high-power gyrotron tube has been used for the QUEST spherical tokamak to form an over-dense plasma for electron Bernstein wave (EBW) heating and current drive with an 8.2 GHz-wave. Non-inductive high-density plasma ramp-up experiments with dual-frequency (dual-f ) electron cyclotron (EC) (8.2 GHz and 28 GHz) waves were conducted. A spontaneous density jump (SDJ) to an over-dense state was first observed as a bifurcation phenomenon in the dual-f  wave experiment. The over-dense plasma on the 8.2 GHz wave was non-inductively ramped up to 25 kA, and was maintained for 0.4 s under stable plasma equilibrium after two such jumps in one shot. Heating to mildly energetic electrons and bulk electrons was observed even in the over-dense region. The electrostatic EBW heating effect on the mildly energetic electrons in the over-dense region is assessed following a dispersion analysis of the 8.2 GHz wave. The bulk electron heating effect observed is explained as heat exchange from mildly energetic electrons heated by the electrostatic EBW. Remarkably, a high hard-x-ray-radiation temperature of  ∼500 keV was also observed in tangential viewing for current-carrying electrons in the over-dense core region. Synergetic heating from the overlap of different 28 GHz EC harmonic resonances as well as higher harmonic heating is discussed for maintaining the highly energetic electrons in the over-dense core region. In addition, the SDJ process and mechanism are considered based on the discussion of the electron heating effects with the 8.2 GHz wave. [ABSTRACT FROM AUTHOR]

The fuel particle balance during long duration discharges in the Q-shu University Experiment with steady state spherical tokamak (QUEST) was investigated. QUEST has all-metal plasma facing walls (PFWs) that were temperature controlled during the experiments. The presence of a transport barrier for hydrogen (H) at the interface between a plasma-induced deposition layer and metallic substrate was confirmed by nuclear reaction analysis with exposing deuterium plasma. An effective method to evaluate global hydrogen flux to PFWs is proposed, taking advantage of the nature of wall saturation. The outgoing flux of fuel particles from the PFWs just after the plasma termination was proportional to the square of wall-stored H, which indicates that enhanced recombination of solved hydrogen played an essential role in dynamic retention and was in agreement with predictions from the H-barrier model. A simple calculation based on the combination of wall modelling and rate equations of the H states denoted a significant impact of wall modelling on the time response of the plasma density. Hence, a proper wall model including the effects of the deposition layer creating the H barrier is required to be developed, even for all-metal PFW devices. [ABSTRACT FROM AUTHOR]

Megawatt (MW) gyrotrons, with a wide frequency range from 14 to 300 GHz, are being developed as part of a collaborative electron cyclotron heating (ECH) study for advanced fusion devices and a demonstration power plant (DEMO). (1) Detailed designs for a 14 GHz 1 MW gyrotron are being developed for fabrication. For a 14 GHz radio frequency (RF) beam with high divergence, a calculated transmission efficiency of 94% to the corrugated waveguide coupling position was obtained initially by introducing the design concept (direct RF beam coupling by built-in waveguide) to minimize the RF transmission path. Installing a double-disk sapphire window will make it possible to develop a 1 MW gyrotron with a continuous wave (CW) at 14 GHz. (2) In experimental tests of a new 28/35 GHz dual-frequency gyrotron, the cooling characteristics of an optimal-structure double-disk sapphire window were evaluated. We confirmed that operating at 0.4 MW with a CW at 28 GHz is feasible, reaching twice the output power reported in previous studies. In a 2 ms short-pulse experimental test, maximum powers of 1.65 MW at 28.04 GHz and 1.21 MW at 34.83 GHz were achieved. (3) A design study of a 77/51 GHz dual-frequency gyrotron was performed. Oscillations above 1.5 MW for 77 GHz and 1.3 MW for 51.88 GHz are expected for a beam voltage Vk  =  80 kV and beam current Ik  =  60 A. (4) In an experiment with a 300 GHz gyrotron, the influence of the wave reflected from the window was reduced by tilting the output window, and mode competition in the cavity was suppressed. An output power of 0.62 MW with a pulse width of 1 ms, which is the new record for this frequency, was obtained. (5) We also performed a trial design study of a 240 GHz gyrotron for DEMO. [ABSTRACT FROM AUTHOR]

Fully non-inductive second (2nd) harmonic electron cyclotron (EC) plasma current ramp-up was demonstrated with a newlly developed 28 GHz system in the QUEST spherical tokamak. A high plasma current of 54 kA was non-inductively ramped up and sustained stably for 0.9 s with a 270 kW 28 GHz wave. A higher plasma current of 66 kA was also non-inductively achieved with a slow ramp-up of the vertical field. We have achieved a significantly higher plasma current than those achieved previously with the 2nd harmonic EC waves. This fully non-inductive 2nd harmonic EC plasma ramp-up method might be useful for future burning plasma devices and fusion reactors, in particular for operations at half magnetic field with the same EC heating equipment. [ABSTRACT FROM AUTHOR]

Fully non-inductive plasma maintenance was achieved by a microwave of 8.2 GHz and 40 kW for more than 1 h 55 min with a well-controlled plasma-facing wall (PFW) temperature of 393 K, using a hot wall in the middle-sized spherical tokamak QUEST, until the discharge was finally terminated by the uncontrollability of the density. The PFW was composed of atmospheric plasma-sprayed tungsten and stainless steel. The hot wall plays an essential role in reducing the amount of wall-stored hydrogen and facilitates hydrogen recycling. The behaviour of fuel hydrogen in the PFW was investigated by monitoring the injection and evacuation of hydrogen into and from the plasma-producing vessel. A fuel particle balance equation based on the presence of a hydrogen transport barrier between the deposited layer and the substrate was applied to the long-duration discharges. It was found that the model could readily predict the observed behaviour in which a higher wall temperature likely gives rise to faster wall saturation. [ABSTRACT FROM AUTHOR]

Successful production of high βp plasmas (εβp ⩾ 1) fully non-inductively (NI) and their long pulse sustainment with the help of modest power (<100 kW) of electron cyclotron waves is demonstrated. High βp plasmas are found for the first time to be naturally self organized to form a stable natural inboard poloidal field null (IPN) equilibrium. A critical βp value is identified, which defines the transition boundary from inboard limiter (IL) to IPN equilibrium. A new feature of plasma self organization is evidenced, which enhances its negative triangular shape to sustain high βp. These results show a relatively simple method to produce and sustain high βp plasma close to the equilibrium limit in a stable configuration exploiting its self organization property. [ABSTRACT FROM AUTHOR]