The improved energy band alignment of Pt/TiO2/Ga2O3/Cu2O structure results in a positive onset potential of ∼1 Vvs.RHE and a stable cathodic photocurrent under appropriate TiO2deposition temperature.

A THz gyrotron with a pulse magnet has been designed, constructed and operated in FIR FU. It is developed as one of high frequency gyrotrons included in Gyrotron FU Series. The gyrotron has already achieved the first experimental result for high frequency operations whose radiation frequency exceeds 1 THz. In this paper, the design detail and the operation test results for sub-terahertz to terahertz range are described. The second harmonic operation is confirmed experimentally at the expected frequency of 1.005 THz due to TE 6,11 cavity mode at the magnetic field intensity of 19.0 T.

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.

Non-inductive plasma current start-up by EC and RF power was carried out on the TST-2 device.Low frequency RF (21 MHz) sustainment was demonstrated, and the obtained high β p spherical tokamak configuration has similar equilibrium values as the EC (2.45 GHz) sustained plasma. Equilibrium analysis revealed the detailed information on three discharge phases: (i) In the initial current formation phase, linearity between the plasma current and the stored energy was confirmed. (ii) In the current jump phase, the initial closed flux surfaces cause a change in the current increasing rate, but the stored energy does not show such a change. (iii) The current sustained plasma is characterised by the fraction of the current inside the last closed flux surface to the total current, and the fraction seems to determine the ratio of the plasma current to the external vertical field. MHD instabilities often terminate the RF sustained plasma, but no such phenomenon was observed in the EC sustained plasma. IntroductionKey issues in spherical tokamak (ST) research are plasma current I p start-up and formation of the ST configuration without the use of a central solenoid (ohmic coil). Successful current generation, ST formation and sustainment have been achieved by injecting RF power (usually in the EC frequency range) to a configuration with a toroidal field and a weak vertical field. This scenario was developed in CDX-U using EC waves [1], and similar experiments were performed in the ST devices: LATE [2,3], TST-2@K [4], TST-2 [5], CPD [6], MAST [7]. A clear transition from open field line configuration to ST configuration, accompanied by a rapid increase in I p (so called current jump), was found in LATE. This phenomenon was also observed in TST-2 and in CPD. Experiments in these devices suggest that a higher vertical field B z and a higher EC power are preferable to achieve a higher I p as long as a current jump occurs. However, the current formation mechanism and the ST formation mechanism are still not clearly understood. Especially, equilibrium reconstruction was performed for only one case [4], and the time evolution and the variation in different operations remain unknown. The mechanisms for ST formation and the features of the plasma should be identified to extrapolate present results to next step ST devices. In TST-2, effects of various operational parameters are studied in [8]. It was found that the sustained current is roughly proportional to the vertical field strength B z , but dependences on other parameters are very weak. On the other hand, the initial current ramp-up rate depends on several parameters, and a scaling law was obtained. A current jump occurs when the initial current reaches a value proportional to B z , and the value is consistent with the condition to form closed flux surfaces. Recently, a low frequency RF source (21MHz) was used, and ST configuration was sustained by a non-EC heating method for the first time [9]. There is a threshold in the RF power, and the threshold for deuterium plasma was lower...

Coating n-type buffer and protective layers on Cu2O may be an effective means to improve the photoelectrochemical (PEC) water-splitting performance of Cu2O-based photocathodes. In this letter, the functions of the buffer layer and protective layer on Cu2O are examined. It is found that a Ga2O3 buffer layer can form a buried junction with Cu2O, which inhibits Cu2O self-reduction as well as increases the photovoltage through a small conduction band offset between the two semiconductors. The introduction of a TiO2 thin protective layer not only improves the stability of the photocathode but also enhances the electron transfer from the photocathode surface into the electrolyte, thus resulting in an increase in photocurrent at positive potentials. These results show that the selection of overlayers with appropriate conduction band positions provides an effective strategy for obtaining a high photovoltage and high photocurrent in PEC systems.

Experiments were performed to study non-inductive current generation by electron cyclotron heating (ECH) in the TST-2 spherical tokamak. A magnetron (2.45 GHz/5 kW) and a horn antenna were used to inject either the O-or X-mode. The maximum plasma current does not depend on the injected wave polarization; however, it has a weak dependence on the vertical field configuration and is proportional to the vertical field. The initial current ramp-up rate depends on various operational parameters. The ramp-up rate increases with the injected EC wave power, and decreases with the filling pressure, resonance position (i.e., the toroidal field strength), and vertical field strength. It also depends on the magnetic field configuration. Conversely, the ramp-up rate does not depend on wave polarization, suggesting that multiple pass absorption of the EC wave is important.

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.

To realize a compact spherical tokamak (ST) reactor, operation without the central solenoid (CS) must be demonstrated. In particular plasma current (I p) ramp-up from zero to a level required for fusion burn is crucial. Plasma initiation and I p ramp-up in ST by waves in the lower-hybrid (LH) frequency range were demonstrated for the first time on TST-2. A combline antenna was used to inject RF power of ~ 100 kW at 200 MHz. Formation of a low current (~ 1kA, mainly driven by pressure gradient) ST configuration can be accomplished by waves over a broad frequency range (21 MHz to 8.2 GHz in TST-2), but further I p ramp-up (to ~ 10 kA, mainly driven by RF) is most efficient with uni-directional traveling waves in the LH frequency range. I p ramp-up to 15 kA was achieved with 60 kW of net RF power. Soft X-ray emission in the direction of electron acceleration by RF wave was enhanced more strongly in the co-drive case (acceleration in the direction to increase I p) compared to the counter-drive case. Hard X-ray spectral measurements showed that the photon flux is an order of magnitude higher and the photon temperature is higher in the co-current-drive direction than in the counter-current-drive direction. These observations are consistent with acceleration of electrons by a unidirectional RF wave. The combline antenna excites vertical electric fields which match the polarization of the fast wave (FW). There is evidence that the LH wave (or the slow wave, SW) is excited nonlinearly, based on the frequency spectra measured by magnetic probes in the plasma edge region. The time evolution indicates the tendency of the pump wave to weaken when the sideband waves intensify. It is expected that the effectiveness of current drive would improve if the LH wave could be excited directly by the antenna. Two types of travelingwave LH antennas will be tested on TST-2, a dielectric-loaded waveguide array ("grill") antenna, and an array of capacitively coupled elements with the electric field polarized in the toroidal direction. During initial operation of the grill antenna, wavenumber components were measured by an array of magnetic probes. Results were qualitatively consistent with expectations based on dispersion relations for the FW and the SW.