P.K. Mioduszewski

ALPS–advanced limiter-divertor plasma-facing systems

The advanced limiter-divertor plasma-facing systems (ALPS) program was initiated in order to evaluate the potential for improved performance and lifetime for plasma-facing systems. The main goal of the program is to demonstrate the advantages of advanced limiter:divertor systems over conventional systems in terms of power density capability, component lifetime, and power conversion efficiency, while providing for safe operation and minimizing impurity concerns for the plasma. Most of the work to date has been applied to free surface liquids. A multi-disciplinary team from several institutions has been organized to address the key issues associated with these systems. The main performance goals for advanced limiters and divertors are a peak heat flux of \ 50 MW:m 2 , elimination of a lifetime limit for erosion, and the ability to extract useful heat at high power conversion efficiency (40%). The evaluation of various options is being conducted through a combination of laboratory experiments, www.elsevier.com:locate:fusengdes

Experimental studies on pump limiters

Abstract Pump limiters are mechanical devices for He-ash removal, fuel particle control, and possibly impurity control. Different designs have been suggested by various authors over the past decade. However, the magnetic divertor concepts seemed to be more promising, mainly because of their remote plasma-material interactions. All of the characteristics of magnetic divertors have been proven experimentally, but the overall performance and complexity cause concern about their application to tokamak reactors. Consequently, it is now time to explore the potential of mechanical particle control devices, i.e. pump limiters. Because of the high recycling at the limiter, it is sufficient to exhaust only a small fraction, about 1–10%, of the limiter particle flux to remove e.g. He at its rate of production. Pump limiter experiments have been conducted so far on Alcator, PDX, Macrotor, and ISX. Depending on the experimental design, a pressure build-up of between 1 mTorr and 50 mTorr has been reported. The closed configuration pump limiters provide high collection efficiencies, but have to accomodate high power fluxes at the leading edge. An open configuration, on the other hand, avoids leading edges but provides only fairly low collection efficiencies. The pump limiter development program now calls for a full pump limiter to be implemented in a major tokamak device. Presently, full-size pump limiter experiments on PDX, ISX, and TEXTOR are in preparation.

Control of wall particle inventory with divertor pumping on DIII-D

Reduction of the net wall particle inventory has been achieved in the DIII-D tokamak by the use of a divertor cryopump for the particle exhaust. A sequence of 12 discharges was conducted without helium glow discharge cleaning (HeGDC) and without active cryopump exhaust, resulting in a net wall loading of 1250 torr.L (8.8*1022 atoms) by the end of the sequence. The cryopump was activated for the subsequent set of 10 discharges. At the end of this set of discharges, the net wall loading was reduced to its initial value or less, i.e. the wall loading state for the reference discharges with HeGDC. Plasma performance did not suffer without the HeGDC in that the stored energy during the ELMy phase was restored to the reference discharge level during the discharges with the active cryopump. Hence, with continuous particle exhaust (provided, for example, by a pumped divertor), next generation, long pulse devices, such as ITER, will not require interdischarge HeGDC for particle control, which would require turning off superconducting coils between discharges

Confinement improvement in beam heated ISX-B discharges with low-z impurity injection

Abstract Results are reported on improved confinement in the Impurity Study Experiment (ISX-B) neutral beam heated plasmas when a small amount of neon is injected shortly after the start of beam heating. The scaling of energy confinement is modified by the introduction of a dependence on line-averaged density. Calculations show the improvement is primarily caused by a reduction in electron heat conduction.

Effect of edge neutrals on the low-to-high confinement transition threshold in the DIII-D tokamak

To study the effect of edge neutrals on the low-to-high confinement transition threshold, a broad range of plasma discharges has been analyzed. These discharges vary by gas puffing and pumping rates, position of the X point, and line-averaged density. It is shown that the determination of the neutral density (or neutral pressure) in the scrape-off layer (SOL) can give a misleading indication of the neutral population inside the separatrix. An increase of neutral density in the SOL creates an increase of plasma density that, in turn, increases the opacity to the neutrals and results in reduced neutral penetration. At a constant magnetic field, the transition power divided by the density appears to be a function of a single parameter measuring the neutrals effect. From this analysis, this parameter cannot be uniquely identified. For instance, it may be the radial decay length of the neutral profile or the charge-exchange damping rate at about r/a≈0.95. A similar correlation exists between these neutral para...

Experiments on steady state particle control in Tore Supra and DIII-D

Particle control is playing an increasingly important role in tokamak plasma performance. The present paper discusses particle control of hydrogen/deuterium by wall pumping on graphite or carbonized surfaces, as well as by external exhaust with pumped limiters and pumped divertors. Wall pumping is ultimately a transient effect and by itself not suitable for steady state particle exhaust. Therefore, external exhaust techniques with pumped divertors and limiters are being developed. How wall pumping phenomena interact and correlate with these inherently steady state, external exhaust techniques, is not well known to date. In the present paper, the processes involved in wall pumping and in external pumping are investigated in an attempt to evaluate the effect of external exhaust on wall pumping. Some of the key elements of this analysis are: (1) charge-exchange fluxes to the wall play a crucial role in the core-wall particle dynamics, (2) the recycling fluxes of thermal molecules have a high probability of ionization in the scrape-off layer, (3) thermal particles originating from the wall, which are ionized within the scrape-off layer, can be directly exhausted, thus providing a direct path between wall and exhaust which can be used to control the wall inventory. This way, the wall can be kept in a continuous pumping state in the sense that it continuously absorbs energetic particles and releases thermal molecules which are then removed by the external exhaust mechanism. While most of the ingredients of this analysis have been observed individually before, the present evaluation is an attempt to correlate effects of wall recycling and external exhaust.

Plasma edge control in TORE SUPRA

TORE SUPRA is a large superconducting tokamak designed for sustaining long inductive pulses (t approximately 30 s). In particular, all the first wall components have been designed for steady-state heat and particle exhaust, particle injection, and additional heating. In addition to these technological assets, a strict control of the plasma-wall interactions is required. This has been done at low power: experiments with ohmic heating have been mainly devoted to the pump limiter, ergodic divertor and pellet injection experiments. Some specific problems arising in large tokamaks are encountered; the pump limiter and the ergodic divertor yield the expected effects on the plasma edge. The effects on the bulk are discussed.

Particle removal with pump limiters in ISX-B

Abstract The first pump limiter experiments were performed on ISX-B. Two pump limiter modules were installed in the top and bottom of one toroidal sector of the tokamak. The modules consist of inertia cooled, TiC-coated graphite heads and ZrAl getter pumps each with a pumping speed of 1000–2000 l/s. The objective of the initial experiments was the demonstration of plasma particle control with pump limiters. The first set of experiments were performed in ohmic discharges (OH) in which the effect of the pump limiters on the plasma density was clearly demonstrated. In discharges characterized by Ip = 110 kA, B T = 15 kG , n e = 1−5 × 10 13 cm −3 and t = 0.3 s, the pressure rise in the pump limiters was typically 2 mTorr with the pumps off and 0.7 mTorr after activating the pumps. When the pumps were activated, the line-average plasma density decreased by up to a factor 2 at identical gas flow rates. The second set of measurements were performed in neutral beam heated discharges (NBI) with injected powers between 0.6 MW and 1.0 MW. Due to a cooling problem on one of the ZrAl pumps, the NBI experiments were carried out with one limiter only. The maximum pressure observed in NBI-discharges was 5 mTorr without activating the pumps, i.e., approximately twice as high as in OH-discharges. The exhaust efficiency, which is defined as the removed particle flux divided by the total particle flux in the scrape-off layer, is estimated to be 5%.

Observations of arcing in the ISX tokamak

Abstract Arcing has been proposed as a major source of metal impurities in tokamak plasmas. Arc tracks have been observed in the ISX tokamak on the limiter, the inner-wall surface, and on the samples from the surface analysis station. Linear as well as fern-like arc tracks have been observed. From optical and SEM analysis of the tracks, it was estimated that about 1016–1017 atoms were released per arc. To study the influence of arcing on the tokamak discharge, an experiment was set up to measure electrical and optical signals of arcing in situ. In well controlled tokamak discharges, arcing was observed only during the initial breakdown of the plasma and during the quenching phase at the end of the discharge. In disrupted discharges, each plasma disruption was accompanied by arcing. The pulse-length of one single unipolar arc was measured to be about 50 μs and the current amplitude was typically about 20 A.

Plasma boundary experiments on DIII-D tokamak

Abstract A survey of the boundary physics research on the DIII-D tokamak and an outline of the DIII-D Advanced Divertor Program (ADP) is presented. We will present results of experiments on impurity control, impurity transport, neutral particle transport, and particle effects on core confinement over a wide range of plasma parameters, Ip ≲ 3 MA, βT ≲ 10.7%, P(auxiliary)≲ 20 MW. Based on the understanding gained in these studies, we in collaboration with a number of other laboratories have devised a series of experiments (ADP) to modify the core plasma conditions through changes in the edge electric field, neutral recycling, and plasma-surface interactions.