[Show abstract][Hide abstract] ABSTRACT: The progress in the understanding of the physics and the concurrent parameter extension in the large helical device since the last IAEA-FEC, in 2012 (Kaneko O et al 2013 Nucl. Fusion 53 095024), is reviewed. Plasma with high ion and electron temperatures ( T i(0) ∼ T e (0) ∼ 6 keV) with simultaneous ion and electron internal transport barriers is obtained by controlling recycling and heating deposition. A sign flip of the nondiffusive term of impurity/momentum transport (residual stress and convection flow) is observed, which is associated with the formation of a transport barrier. The impact of the topology of three-dimensional magnetic fields (stochastic magnetic fields and magnetic islands) on heat momentum, particle/impurity transport and magnetohydrodynamic stability is also discussed. In the steady state operation, a 48 min discharge with a line-averaged electron density of 1 × 10 19 m −3 and with high electron and ion temperatures ( T i(0) ∼ T e (0) ∼ 2 keV), resulting in 3.36 GJ of input energy, is achieved.
[Show abstract][Hide abstract] ABSTRACT: Successful production of high βp plasmas (βp 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.
[Show abstract][Hide abstract] ABSTRACT: A Rogowski probe consisting of a small multi-layer Rogowski coil, five magnetic pick-up coils, and a Langmuir probe was developed to measure the local current density and its direction. It can be moved along the major radius and can be turned around its axis. This probe was used to measure the current density profile near the last closed flux surface of Ohmic plasmas in Tokyo Spherical Tokamak-2. The current density profile was measured successfully with a signal to noise ratio of greater than 20.
[Show abstract][Hide abstract] ABSTRACT: The multi-pass Thomson scattering (TS) scheme enables obtaining many photons by accumulating multiple TS signals. The signal-to-noise ratio (SNR) depends on the accumulation number. In this study, we performed multi-pass TS measurements for ohmically heated plasmas, and the relationship between SNR and the accumulation number was investigated. As a result, improvement of SNR in this experiment indicated similar tendency to that calculated for the background noise dominant situation. (C) 2014 AIP Publishing LLC.
[Show abstract][Hide abstract] ABSTRACT: 28 GHz Electron Cyclotron Current Drive (ECCD) effect was clearly observed in Ohmically
heated plasmas with feedback regulation of Center Solenoid (CS) coil current in 2nd har-
monic inboard off-axis heating scenario. In non-inductive current drive experiments only
by the 28 GHz injection, the 54 kA plasma current Ip was sustained for 0.9 sec. Higher
Ip of 66 kA was non-inductively obtained by slow ramp-up of vertical �eld Bv using the
28 GHz ECH/ECCD. Non-inductive high-density/ current plasma start-up, which is a key
issue for fusion reactor design has been demonstrated using 2nd harmonic ECH/ECCD.
Density jump across 8.2 GHz cutoff density was observed in superposed 28 GHz / 8.2 GHz
injections. The 50 kA plasmas were sustained by the 8.2 GHz injection into the 28 GHz
target plasma if the stable plasma shaping was obtained.
25th IAEA Fusion Energy Conference, Saint Petersburg, Russia; 10/2014
[Show abstract][Hide abstract] ABSTRACT: The global gas balance in the various types of steady discharges has been re-examined by measuring
the partial pressures of hydrogen and helium, and plasma induced permeated flux at various positions. A new
approach to study the dynamics of particle circulation in steady state tokamak operation SSTO has been
proposed and demonstrated in QUEST with all metal walls baked at 100°C. Using perturbations of particle
source H2 and plasma-wall interaction PWI the system functions of processes of retention and release into/from the wall are determined both in time and frequency domains. The time dependent probability function of the transition between high and low recycling states has been derived.
25th IAEA Fusion Energy Conference, Saint Petersburg, Russia; 10/2014
[Show abstract][Hide abstract] ABSTRACT: Successful production of high βp plasma (βp 1) and its long pulse sustainment by fully non-inductive (NI) current drive with the help of a modest power (< 100 kW) Electron Cyclotron Waves is demonstrated. We found that (i) high βp plasma is naturally self organized to form a stable natural Inboard Poloidal field Null (IPN) equilibrium, (ii) a critical βp, which defines the transition boundary from Inboard Limiter (IL) to IPN equilibria and (iii) a new feature of plasma self organization to enhance its negative triangular shape to sustain high βp. This result shows 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. Furthermore, spontaneous toroidal rotation is observed in IPN configurations as well as open magnetic field line under high magnetic curvature, which is sustained in steady state for the entire plasma duration.
25th Fusion Energy Conference (FEC 2014) 13 -18 October 2014, Saint Petersburg, Russia; 10/2014
[Show abstract][Hide abstract] ABSTRACT: Plasma current start-up and ramp-up using the lower hybrid wave (LHW) were investigated on the TST-2 spherical tokamak. The LHW was launched by a dielectric-loaded waveguide array (grill) antenna. The antenna–plasma coupling of this antenna deteriorates as the input power exceeds several kW. This deterioration is believed to be caused by the density depletion due to the ponderomotive force. This conjecture was confirmed by the measurement of density reduction and the result of a non-linear full wave numerical calculation based on the finite element method (FEM).
[Show abstract][Hide abstract] ABSTRACT: In multi-pass Thomson scattering (TS) scheme, a laser pulse makes multiple round trips through the plasma, and the effective laser energy is enhanced, and we can increase the signal-to-noise ratio as a result. We have developed a coaxial optical cavity in which a laser pulse is confined, and we performed TS measurements using the coaxial cavity in tokamak plasmas for the first time. In the optical cavity, the laser energy attenuation was approximately 30% in each round trip, and we achieved a photon number gain of about 3 compared with that obtained in the first round trip. In addition, the temperature measurement accuracy was improved by accumulating the first three round trip waveforms.
[Show abstract][Hide abstract] ABSTRACT: A new type of traveling wave antenna which excites the lower hybrid wave directly was developed. This antenna is similar to the inductively-coupled combline antenna in that only the first element of the antenna array is excited externally, and subsequent elements are excited passively by mutual coupling between adjacent elements. The main difference is that whereas the inductively-coupled combline antenna makes use of mutual inductance, the presently proposed antenna makes use of mutual capacitance. The radiating elements are located at the voltage maximum, and the electric field induced in the plasma is in the toroidal direction rather than the poloidal direction, matching the polarization of the lower hybrid wave. Optimization studies were carried out to obtain a band-pass characteristic centered around 200 MHz, and a unidirectional wavenumber spectrum with the parallel index of refraction corresponding to approximately 5. Plasma current ramp-up to 2 kA has been achieved on the TST-2 spherical tokamak with 12 kW of RF power at 200 MHz during the initial experimental period using this antenna. Further optimization studies are being performed.
[Show abstract][Hide abstract] ABSTRACT: Lower hybrid current drive experiments were performed on the TST-2 spherical tokamak (R = 0.38 m, a = 0.25 m, Bt = 0.3 T, Ip = 0.1 MA). A waveguide array antenna consisting of four dielectric (alumina, ∊r = 10.0) loaded waveguides was used. The coupling characteristics were investigated over a wide range of input power (0.1 W - 40 kW). The reflection coefficient of this antenna increased when the input power exceeded approximately 1 kW. This result was compared with a numerical simulation based on the finite element method (FEM). The ponderomotive effect was calculated for the wave field calculated by COMSOL . This calculation also showed variation of the reflection coefficient with the input power. Non-inductive plasma current start-up to 10 kA was demonstrated using 40 kW of lower hybrid wave (LHW) power. The current drive figure of merit (ηCD = IpneR/PRF) of this antenna was higher than that obtained using the combline antenna, which is designed to excite a travelling fast wave. The best current drive efficiency was obtained in the case in which the n∥ (= ck∥/ω) spectrum of the excited LHW was peaked around 9 and the toroidal field was higher than in previous experiments.
[Show abstract][Hide abstract] ABSTRACT: The progress of physical understanding as well as parameter improvement of net-current-free helical plasma is reported for the Large Helical Device since the last Fusion Energy Conference in Daejeon in 2010. The second low-energy neutral beam line was installed, and the central ion temperature has exceeded 7 keV, which was obtained by carbon pellet injection. Transport analysis of the high-Ti plasmas shows that the ion-thermal conductivity and viscosity decreased after the pellet injection although the improvement does not last long. The effort has been focused on the optimization of plasma edge conditions to extend the operation regime towards higher ion temperature and more stable high density and high beta. For this purpose a portion of the open helical divertors are being modified to the baffle-structured closed ones aimed at active control of the edge plasma. It is compared with the open case that the neutral pressure in the closed helical divertor increased by ten times as predicted by modelling. Studies of physics in a three-dimensional geometry are highlighted in the topics related to the response to a resonant magnetic perturbation at the plasma periphery such as edge-localized-mode mitigation and divertor detachment. Novel approaches of non-local and non-diffusive transport have also been advanced.
[Show abstract][Hide abstract] ABSTRACT: Plasma current (Ip) start-up in a spherical tokamak (ST) by waves in the lower-hybrid (LH) frequency range was investigated on TST-2. A low current (~1 kA) ST configuration can be formed by waves over a broad frequency range (21 MHz–8.2 GHz in TST-2), but further Ip ramp-up (to ~10 kA) is most efficient with waves in the LH frequency range. Ip ramp-up to 15 kA was achieved with 60 kW of net RF power PRF in the fast wave (FW) polarization at 200 MHz excited by the inductively coupled combline antenna. X-ray measurements showed that the photon flux and temperature are higher in the direction opposite to Ip, consistent with acceleration of electrons by a uni-directional RF wave. There is evidence that the LH wave is excited nonlinearly by the FW, based on the frequency spectra measured by magnetic probes. Similar efficiencies of Ip ramp-up were obtained with the inductive combline antenna and the dielectric-loaded waveguide array ('grill') antenna, and tendencies for the current drive efficiency to increase with plasma current and toroidal field were observed. During operation of the grill antenna, wavevector components were measured by an array of magnetic probes. Results were qualitatively consistent with expectations based on dispersion relations for the FW and the LH wave. A capacitively coupled combline antenna has been developed to improve coupling to the plasma and the wavenumber spectrum of the excited LH wave, and will be tested in 2013.
[Show abstract][Hide abstract] ABSTRACT: Non-inductive current start-up via relativistic electron cyclotron resonance interaction is investigated for the high ratio (~10 %) of vertical Bv to toroidal Bt fields and the concave field lines in the QUEST spherical tokamak. In the start-up scenario with an internal poloidal field null (IPN), the fast current start-up rate of 0.3-0.5 MA/sec and correlation with mildly relativistic electrons accelerated due to multiple ECR interaction are observed. In steady state high p equilibrium characterized by the inboard null (Rs ~ 0.7×R0) and p of 1.5 is achieved, where p are the inverse aspect ratio and poloidal beta, respectively. Relaxation oscillations in this equilibrium and confinement of the energetic electrons are discussed.
[Show abstract][Hide abstract] ABSTRACT: Plasma current start-up experiments were performed on the TST-2
spherical tokamak (R= 0.38 m, a = 0.25 m, Bt = 0.3 T,
Ip = 0.1 MA) using the lower hybrid wave (LHW) at f = 200
MHz. A waveguide array antenna consisting of four dielectric (alumina,
ɛr = 10.0) loaded waveguides was used. The coupling
characteristics of this antenna were investigated by low power
experiments (PFWD< 5 kW). The measured characteristics
were qualitatively consistent with those predicted by calculations using
a finite element method solver package (COMSOL). The experimentally
observed reflection coefficient is large (greater than 36 % averaged
over four waveguides), and there are large differences in reflectivities
in neighboring waveguides. It was necessary to take into account of the
private limiter surrounding the antenna in order to reproduce these
features. Non-inductive plasma current start-up to 6 kA has been
demonstrated using 20 kW of LHW power. In this experiment, the
reflection coefficient was very high because the initial plasma density
was much lower than the predicted optimum plasma density.