D.S. Gray

Fast scanning probe for tokamak plasmas

We describe a fast reciprocating probe drive, which has three main new features: (1) a detachable and modular probe head for easy maintenance, (2) a combination of high heat flux capability, high bandwidth, and low-Z materials construction, and (3) low weight, compact, inexpensive construction. The probe is mounted in a fast pneumatic drive in order to reach plasma regions of interest and remain inserted long enough to obtain good statistics while minimizing the heat flux to the tips and head. The drive is pneumatic and has been designed to be compact and reliable to comply with space and maintenance requirements of tokamaks. The probe described here has five tips which obtain a full spectrum of plasma parameters: electron temperature profile Te(r), electron density profile ne(r), floating potential profile Vf(r), poloidal electric field profile Eθ(r), saturation current profile Isat(r), and their fluctuations up to 3 MHz. We describe the probe show radial profiles of various parameters. We compare the de...

Gas jet disruption mitigation studies on Alcator C-Mod and DIII-D

High-pressure noble gas jet injection is a mitigation technique which potentially satisfies the requirements of fast response time and reliability, without degrading subsequent discharges. Previously reported gas jet experiments on DIII-D showed good success at reducing deleterious disruption effects. In this paper, results of recent gas jet disruption mitigation experiments on Alcator C-Mod and DIII-D are reported. Jointly, these experiments have greatly improved the understanding of gas jet dynamics and the processes involved in mitigating disruption effects. In both machines, the sequence of events following gas injection is observed to be quite similar: the jet neutrals stop near the plasma edge, the edge temperature collapses and large MHD modes are quickly destabilized, mixing the hot plasma core with the edge impurity ions and radiating away the plasma thermal energy. High radiated power fractions are achieved, thus reducing the conducted heat loads to the chamber walls and divertor. A significant (2 × or more) reduction in halo current is also observed. Runaway electron generation is small or absent. These similar results in two quite different tokamaks are encouraging for the applicability of this disruption mitigation technique to ITER.

Scaling of plasma turbulence suppression with velocity shear

The scaling of plasma turbulence, turbulent particle flux and cross-phase with shear in the edge of the TEXTOR-94 tokamak is obtained from measurements from fast scanning probes and compared with various existing analytical theories. It is found that the scaling can be expressed as a second order polynomial and that the cross-phase plays a key role in the suppression of particle flux. The variable rate of shear, kept below the value required to produce a low to high particle confinement transition, was obtained by changing, on a shot to shot basis, the voltage applied to an electrode introduced 4 cm into the plasma.

Turbulent transport and turbulence in radiative I?mode plasmas in TEXTOR-94

First measurements of turbulence levels and turbulence induced transport in the outer edge of the plasma of TEXTOR-94 during radiative improved mode discharges show a reduction by a factor of 4-7 of the radial particle turbulent transport. The quenching is most evident on the normalized potential fluctuations and is strongest above 100?kHz. Non-linear gyrokinetic particle-in-cell simulations of these discharges show impurity induced suppression of the electrostatic fluctuations associated with the ion temperature gradient driven mode over most of the cross-section, including the edge. Such a mechanism is proposed as the explanation for the improved confinement and turbulence reduction. The reduction in the edge turbulent transport levels is consistent with increased particle confinement time and the reduction of the SOL thickness. Particle and energy fluxes to the limiter are reduced by an order of magnitude. A concomitant increase of the measured energy and particle confinement times ?E and ?p versus radiated fraction suggests a common underlying suppression mechanism.

Self-consistent plasma-neutral modeling in tokamak plasmas with a large-area toroidal belt limiter

Plasma-neutral phenomena in the edge plasma and scrape-off layer of the Torus Experiment for Technology Oriented Research [G.H. Wolf and the TEXTOR Team, J. Nucl. Mater. 122&123, 1124 (1984)] with the toroidal belt Advanced Limiter Test (ALT-II) [D.M. Goebel et al., J. Nucl. Mater. 162–164, 115 (1989)] are simulated using the code package B2-EIRENE [D. Reiter et al., Plasma Phys. Controlled Fusion 33, 1579 (1991)]. Spatially-constant, anomalous radial transport coefficients (D,V,χ) are used for fitting measured electron temperature and density profiles. Primary neutral fluxes are determined by plasma fluxes to material surfaces, and Dα emissions are predicted from them. Comparison of the predicted Dα emission with measurements indicates a critical need, in predictive modeling, for a self-consistent model of fluxes to material surfaces that are parallel to the magnetic field. Appropriate factors are calculated for deducing D+ source rates from Dα emissions measured in various locations, taking into account...

Plasma Exhaust and Density Control in Tokamak Fusion Experiments with Neutral Beam or ICRF Auxiliary Heating

Particle exhaust studies have been carried out with the pump limiter ALT-II in the TEXTOR tokamak, under ohmic conditions as well as with NBI and with ICRF auxiliary heating, and the pumping effectiveness is shown to meet the requirements for a fusion reactor. Quantitative measurements of Dα emission, made with a CCD camera, have been used to determine the particle efflux from the plasma. Roughly one third of the Dα emission occurs in a diffuse `halo that surrounds the limiter belt. The particle confinement time is less than the energy confinement time by a factor of typically 4. Modelling in 2-D of plasma and neutral flows in the TEXTOR boundary has been performed. The source of D+ ions can be related to the Dα emission by a factor that is found to depend on the location of the emission and on the discharge density. The predicted total Dα emission agrees with the measurements within a factor of about 2. Pumping of ALT-II allows for density control; with NBI, the density can be increased well beyond the ohmic limit without the discharge ending in disruption. The plasma particle efflux and the pumped flux both increase with density as well as with heating power. The exhaust efficiency is typically ~2%, with the highest values observed in high density NBI discharges. Higher exhaust rates are observed with NBI than with ICRF. Plasma and neutral flows in the ALT-II scoops have been simulated, making use of a simple plasma model. The scoop may be viewed as a non-linear amplifier of the plasma particle flux; the amplification is found to range from about 2 to 3 for most cases. Flow reversal in the scoop is found in some of the NBI cases and particularly in the highest density case.

Robust Langmuir probe circuitry for fusion research

Langmuir probes attached to the plasma facing components of fusion experiments are biased with constant or swept voltages to obtain measurements of plasma parameters such as electron temperature and density. The circuitry used must be rugged and protect the power supplies and electronics against generally harsh conditions and sudden discharge terminations, or disruptions. Modularity, ease of repair and expandability are important because short-lived radiation from neutron activation is often present after the discharges, preventing access to the circuitry. We report the implementation of modular probe circuitry featuring robust protection, remote testing and reset and easy maintenance and expandability, achieved by using DIN-rail modules.

PLASMA EXHAUST REQUIREMENT FOR SUSTAINED IGNITION: RELAXATION DUE TO PROFILE CONSIDERATIONS

The problem of helium ash removal from a fusion reactor is treated with spatially varying plasma parameters, and the allowance of variations is found to strongly influence the necessary conditions for sustained deuterium-tritium (DT) ignition. In particular, a one dimensional (1-D) particle transport model is used to calculate the core helium concentration on the basis of a suitably parametrized source profile. It is estimated that the necessary exhaust efficiency for sustained DT ignition is of the order of 2%, rather than the familiar 10% figure deduced from uniform plasma calculations. This is due to: (1) the fact that tau P and tau E are not equal; and (2) the fact that fusion alphas are produced only in a fraction of the plasma volume. It is also shown that, in order for the ash concentration to be effectively controllable by pumping, it is necessary that the particle diffusion coefficient be sufficiently high and that the reactors power density not be excessive. In order to determine the exact pumping requirements for a machine such as ITER, a more detailed and self-consistent model is needed

Exposure of CFC-materials to high transient heat loads in the TEXTOR tokamak

Transient high heat flux events like ELMs, vertical displacement events and disruptions can cause the thermal ablation of plasma facing material. Until now experimental work in this field had been carried out by exposing material specimens to heat loads by electron or laser beam or by tests in pulsed plasma accelerators. In the present work carbon specimens were directly exposed to intense plasma fluxes in the TEXTOR tokamak. The exposure was performed with a fast probe allowing the insertion of the material over a distance of 9 cm into the edge plasma for a duration of 80 ms. The results of in-situ diagnostic measurements and of the post-experiment examination of the specimens are compared with a reference experiment by electron beam and with numerical analyses. Results indicated that the heat flux to the probe surfaces and the probe erosion is much lower than expected.

Effects of an RF limiter on TEXTOR's edge plasmas

Abstract Studies directed towards the reduction of particle and heat fluxes to plasma facing components by the application of ponderomotive forces generated by radio frequency (RF) are being conducted in TEXTOR. A modified poloidal limiter is used as an antenna with up to 3 kW of RF power; the data obtained show that the plasma is repelled by the RF ponderomotive potential. The density is reduced by a factor of 2–4 and the radial decay length is substantially altered. The density near the limiter decays exponentially with RF power. The electron temperature profile changes, with the decay length becoming longer (almost flat) during the RF. The temperature in the scrape off layer (SOL) increases and its increase is roughly proportional to the RF power until it saturates, suggesting that the heating efficiency drops with power, and that improved performance is to be expected at higher powers.