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An experiment to investigate the source of plasma equilibrium of the residual plasma at zero cross over of plasma current is presented. The role of limiter in providing this equilibrium by short circuiting the electric field developed due to the toroidal drifts is examined by measuring the current flowing through the limiter. The role of a fast poloidal rotation observed by a set of Mach probes, during zero cross over is also examined in providing the equilibrium. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Plasma current start-up to 100 kA was achieved successfully in the JT-60U tokamak without the use of the centre solenoid (completely solenoidless tokamak operation). Only poloidal field coils located on the outboard side of the torus were used, in combination with strong ionization by electron cyclotron (EC) power. The presence of a field null was not necessary for plasma current start-up, but the flux conversion efficiency was low in such a case. In a nearly solenoidless start-up, low neutral pressures were favoured, and the optimum location of the EC resonance was slightly to the high field side of the vacuum vessel centre. The required EC power for efficient utilization of flux swing in JT-60U was about 1 MW. A plasma current of 260 kA was maintained for 1 s by NB only, and plasma current ramp-up from 215 to 310 kA was achieved by EC and neutral beam (NB) only (without lower hybrid current drive (LHCD)). However, the ramp-up efficiency was much lower compared with LHCD. Recharging of the centre solenoid was observed with only counter and perpendicular NB injection, indicating bootstrap overdrive. Integration of these elements can lead to the achievement of a completely solenoidless tokamak operation.

One cycle alternating current (AC) plasma operation without a dwell time has been achieved in the STOR-M tokamak with good reproducibility using a newly developed ohmic heating circuit. The plasma current of +24 kA is smoothly ramped down in 10 ms with a rampdown rate of around 2.0 kA/ms and then ramped up to between -20 and -24 kA directly without a dwell time. The plasma density of up to (3.7+or-0.6)*1018 m-3 remains at the current reversal as observed in recent soft landing experiments. The key to a successful, reproducible and direct transition in AC tokamak operations on STOR-M is to control both the total vertical field by a feedback control system and the plasma density by careful gas puffing during the current reversal phase. This experiment has demonstrated that the initial loop voltage for the second negative current is minimized when the dwell time approaches zero, and the AC operation without dwelling is possible whenever the plasma current can be softly terminated with a finite residual plasma density

We propose inductively operated D-3He tokamak reactors (R = 9.5 m, a = 2.8 and 3.2 m, Bt = 10 and 7.5 T, and Ip = 53.6 and 52.5 MA respectively) with an a.c. or unidirectional current (UDC) mode, operated in the first stability regime, with a small current drive power of about 500 kW in the plasma side, and about 2 MW in the transformer side. The high electron temperature Te (0) ∼ 62.5 keV together with a high bootstrap current fraction greater than 70% decreases the plasma resistance (to less than 1 nΩ) and the loop voltage (less than 10 mV range) and then prolongs the pulse length to more than 15 h. The ignition characteristics of a D-3He tokamak reactor are analyzed by the operation path method on the Pht τE2 −T plane and POPCON for different confinement scalings.

The phased‐array antenna system for Electron Cyclotron/Bernstein Wave Heating and Current Drive experiments has been developed in the QUEST. The antenna was designed to excite a pure O‐mode wave in the oblique injection for the O‐X‐B mode conversion experiments, and its good performances were confirmed at a low power level. The plasma current (

The differential-phase reflectometry has been developed to measure the density profile, concerning electron Bernstein Wave (EBW) heating and current drive in the QUEST. The Phased Array Antenna (PAA) system was proposed for the reflectometry to improve the ratio of signal to noise. The PAA was confirmed to be effective, as a launching antenna to receive the larger reflected-wave signal.

The plasma density behaviour in the STOR-1M tokamak with alternating current (AC) operation is described using the Murakami-Hugill diagram (1/qa, nR/Bt). At the current reversal, Ip = 0 (1/qa = 0), the plasma density remains finite and the Murakami parameter is nR/Bt = (0.66 ± 0.22) × 1018m-2.T-1. Gas puffing before the current reversal does not noticeably increase the plasma density at the current reversal, but allows AC operation with larger currents and improves its reproducibility. A qualitative explanation for the finite plasma density at the current reversal is given on the basis of a short circuit effect by the limiter

An improved Ohmic confinement phase has been observed in the STOR‐M tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 1, p. 323] after application of a turbulent heating (TH) pulse. This improved Ohmic confinement phase is characterized by (a) increased ne, (b) reduced Hα radiation from the edge, (c) reduced density and magnetic fluctuations at the edge, (d) a steeper density profile at the edge, and (e) a more negative radial electric field. Almost complete suppression of sawtooth oscillations during the improved confinement phase has also been observed. A linear dispersion relation describes the density and magnetic fluctuations in the frequency range up to 350 kHz. In the region r

This paper reports on two recent experiments carried out on the STOR‐M tokamak. The first experiment studied the nature of MHD activities based on singular value decomposition algorithm during the improved confinement phase induced by compact torus injection. The typical MHD modes with mode numbers m = 2, 3, and 4 are suppressed during the improved confinement phase. Shortly before the termination of the improved confinement phase, MHD activities reemerge, starting with a gong‐mode‐like burst followed by oscillations of a rotating m = 2. The second experiment was successful current start‐up with a simulated spherical tokamak configuration where the inner Ohmic heating coils surrounding the iron core are deactivated in STOR‐M. Current start‐up was also achieved with all the vertical equilibrium field coils deactivated. In the latter case, the vertical equilibrium field was provided solely by the image vertical field produced by the magnetization current in the iron core and compensated for by the current through the feedback control vertical field windings. The observed waveforms agree well with numerical simulations.

Stable and clean AC tokamak discharges have been achieved in the STOR-1M device. The plasma current is reversed from +4.1 kA to −4.0 kA within 1.9 ms. During the reversal, no disruptive behaviour is observed, the loop voltage changes smoothly from +1 V to −5 V without any spike, and impurities are not released. An electron density of (2–4) × 1012 cm−3 is maintained during current reversal. The possibility of continuous tokamak operation with a low frequency alternating plasma current is discussed.

A neutral Li beam probing combined with a spectroscopic technique to measure local electron‐density fluctuations is experimentally demonstrated in an ECR test plasma. The density fluctuations of up to 180 kHz are excited and controlled by applying periodic bursts of sinusoidal waves to a grid. The electron‐density fluctuation levels thus obtained are found to agree well with those measured with a Langmuir probe. It is also suggested that this method can be applied to plasmas whose densities are in a wide range of 1010–1013 cm−3.

Density fluctuations in the outer edge of the plasma, behind the limiter, have been investigated in the TEXTOR tokamak by a new technique using a probing beam of thermal neutral lithium. With this method, radial profiles of the electron density and its fluctuations are obtained and thus also the relationship between turbulence and plasma parameters, especially the density gradient scale lengths. The turbulences are found to consist of coherent fluctuations in the frequency range of sawtooth oscillations (

The transition from the pulsed state to the steady state in a tokamak is numerically studied using a model circuit for the current drive operations. The problem of shutting off the primary circuit without reducing plasma current is analysed for the cases of air core and iron core transformers in a medium-sized tokamak. It is found that the air core transformer is more suitable for the current drive operations because of smaller primary-plasma coupling and the absence of saturation problems. Iron core transformers require careful adjustment in the vertical field in order to compensate for the change due to core saturation. The iron core transformer may be shut off only when plasma equilibrium in the negative saturation phase is maintained by a supplementary vertical field.

In this paper the STOR-1M tokamak and its diagnostics are described. The plasma behavior before, during, and after turbulent heating is also discussed. Anomalously rapid current penetration has been observed during both the ohmic current-rise phase and the turbulent heating phase. The critical streaming parameter for the onset of current driven turbulence has been measured. Efficient electron and ion heating during turbulent heating has been observed. Plasma confinement properties show significant improvement, following the turbulent heating pulse, for a period much longer than the duration of the turbulent heating pulse.

Detailed measurements of the poloidal image fields due to the iron core transformer have been made in the STOR-1M Tokamak (R0=22 cm, a0=3.5 cm, Bt approximately 10 kG). The measured fields are in close agreement with theoretical predictions. Approximately one half of the required vertical magnetic field is provided by the plasma current itself due to image effects.

Three modes of current drive operation in a tokamak — continuous, cyclic, and rfinitiated-are studied for air core and iron core transformer. It is found that the air core transformer is in general more flexible than the iron core transformer for current drive operation. For continuous operation, the shutoff time of the Ohmic heating circuit of the air core transformer can be reduced to zero by using a bias current. On the other hand, the shutoff time of the iron core transformer remains finite even if the bias current is used, because of hysteresis. For cyclic operation, methods of shortening the recharging time are investigated for both types of transformer. The effects of the transformer on rf-initiated operation are investigated. A model design of a saturable iron core tokamak for current drive experiments is also presented.

The paper presents a method for determining the potential of a reactor design to reach ignition conditions. The method makes use of a 'generalized' contour map with a three-dimensional representation. On this map, the height of (with Pht the total heating power density and τE the energy confinement time) at the saddle point provides a criterion for reaching ignition. By a combination of this map with Goldston's L-mode scaling law, it is found that the operating point of the tokamak moves along a line of constant ; the value of for H-mode scaling can be increased with auxiliary heating power so that the saddle point can be reached. Confinement degradation effects due to alpha particle heating can be treated by replacing by (fα is the fraction of the alpha particle heating Pα causing confinement degradation, ηα is the fraction of the alpha heating power trapped in the plasma). The saddle point becomes higher and shifts to a larger nτE regime by this alpha degradation effect.