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.
Spherical tokamaks (STs) have the advantage of superior stability at high beta, but to realize a compact fusion reactor, the central solenoid (CS) must be eliminated. The plasma current could be maintained mainly by the self-driven current during the steady-state burning phase, but there is no established method of non-inductive current ramp-up from zero to a high enough level required for fusion burn. The lower-hybrid wave (LHW) is commonly used in tokamaks, but this technique is believed to be unusable in ST plasmas with very high dielectric constant. Wave excitation, propagation and damping were evaluated by numerical modeling for the TST-2 spherical tokamak (R = 0.68 m, a = 0.25 m, B = 0.3 T, I = 100 kA). Wave excitation is calculated by the RF antenna simulation tool based on the finite element method (COMSOL). The plasma is modeled as a medium with cold plasma dielectric response and artificially enhanced loss. Excitation of a traveling fast wave (FW) by the TST-2 combline antenna was confirmed by COMSOL calculation. However, the FW must be mode converted to the LHW to achieve efficient current drive. The TORLH full-wave solver, which can treat diffraction effects properly, was applied to the TST-2 LH current drive experiment. It is shown that core current drive by LHW is possible in the low density, low current plasma formed by ECH. It is important to keep the density low during current ramp-up, and the wavenumber must be reduced as the current increases in order to maintain core current drive. The results of these calculations and the initial experiment on TST-2 will be used to design an optimized LHW antenna with appropriate polarization and wavenumber spectrum controllability for the current ramp-up experiment. Taking a conservative value for the current drive figure of merit, the steadystate driven current by 200 kW of RF power is estimated to be 150 kA, which should be adequate for evaluating the usefulness of this technique up to a current level of 100 kA in TST-2. This technique should be usable during the low-density, low-current, non-burning start-up phase of a reactor to reach a sufficient current level needed for further heating. Initial results from the TST-2 experiment demonstrated that initial current formation and ST equilibrium formation can be achieved by RF power in the LH frequency range. Further current ramp-up by directly excited LHW will be investigated on TST-2. The success of the TST-2 experiment would provide a scientific basis for quantitatively evaluating the required CS capability for a low aspect ratio reactor.
Non-inductive plasma current start-up and sustainment by waves in the lower-hybrid frequency range (200 MHz) have been studied on the TST-2 spherical tokamak (R 0 0.38 m, a 0.25 m, B t0 0.3 T, I p 0.14 MA) using three types of antenna: the 11-element inductively-coupled combline antenna, the dielectric loaded 4-waveguide array antenna, and the 13-element capacitively-coupled combline (CCC) antenna. The maximum plasma currents of 15 kA, 10 kA and 16 kA were achieved, respectively. The highest current drive figure of merit η CD ≡ n e I p R/P RF was achieved by the CCC antenna. The efficiency of current drive should improve by reducing prompt orbit losses of high energy electrons by operating at higher plasma current (to improve orbit confinement) and higher toroidal magnetic field (to improve wave accessibility to the plasma core), while keeping the density high enough (to avoid excessive acceleration of electrons), but under the 'density limit'.
A double-pass Thomson scattering system, in which a laser pulse makes a round trip through the plasma, was constructed. Using the same optics and a fast detection unit, we can resolve backward and forward scattering pulses in the signal. Because these scatterings reflect velocity distribution along different directions, electron temperature anisotropy can be estimated from the double-pass Thomson scattering system. Multi-pass Thomson scattering scheme is attractive for low-density plasma and electron temperature anisotropy measurements. A significant improvement in the signal-to-noise ratio was achieved using a multi-pass Thomson scattering scheme on TEXTOR [1]. However, it is difficult to apply a similar multi-pass system to other devices because of the use of the intra-cavity configuration. In contrast, the configuration using a confocal spherical mirror is simple. The theoretical performance of this system was analyzed and confirmed experimentally [2]. When the angle between − → k s − − → k i and the toroidal field is far from 45• , the multi-pass configuration provides an opportunity for temperature anisotropy measurements. Bowden et al. measured temperature anisotropy using a single-pass Thomson scattering for electron-cyclotron-heated lowdensity plasmas (T e ∼ 1 eV and n e ∼ 10 17 m −3 ) [3]. They switched the direction of the incident laser and measured reproducible plasmas. They detected slight temperature anisotropy. As a pilot experiment for the temperature anisotropy measurement, a double-pass Thomson scattering system was constructed in the Tokyo Spherical Tokamak 2 (TST-2) device. Combining the double-pass configuration with a fast detection system, we can measure the forward and backward scattering pulses almost simultaneously (i.e., within a time separation of 20-30 ns). Therefore, we can measure electron temperature anisotropy even if plasma reproducibility is poor. This is important when we study phenomena related to instabilities. In this paper, the double-pass Thomson scattering scheme is described and the experimental results are presented. author's e-mail: hiratsuka@fusion.k.u-tokyo.ac.jpIn the Thomson scattering system in TST-2, the pulse energy, repetition rate, and pulse width of the injection Nd:YAG laser are 1.6 J, 10 Hz, and 10 ns, respectively. Newtonian collection optics allows for a large solid angle [4]. The polychromator has six wavelength channels, and each channel has a filter and an avalanche photodiode. Applying a long, large numerical-aperture fiber and a high-power injection laser, the signal intensity was enhanced significantly; maximum signal-to-noise ratio was more than 10 [5]. Only the data at the center of plasma are presented in this paper, because the stray light is extremely bright in other regions in the double-pass configuration. In order to distinguish pulses from each laser transit through the plasma, a fast low-noise detection system was developed, and the measured full width at half maximum of the laser pulse was approximately 10 ns [6]. A schematic diagram of ...
The Thomson scattering system in the TST-2 has been upgraded to improve the reliability and accuracy of measurements. The signal intensity increased because of a new high-energy (1.6 J) laser. A large-numericalaperture (N.A.) fiber was tested, and it was found that a 6-m-long fiber can be used without significant transmission loss. With the upgraded system, the typical central electron temperature and the electron density for ohmic discharges (with a plasma current of 60 kA) are 150 eV and 1.5 × 10 19 m −3 , respectively. The temperature profile has a maximum near the center of the plasma.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.