S. Bozhenkov

Disruption mitigation by massive gas injection in JET

Disruption mitigation is mandatory for ITER in order to reduce forces, to mitigate heat loads during the thermal quench and to avoid runaway electrons (REs). A fast disruption mitigation valve has been installed at JET to study mitigation by massive gas injection. Different gas species and amounts have been investigated with respect to timescales and mitigation efficiency. We discuss the mitigation of halo currents as well as sideways forces during vertical displacement events, the mitigation of heat loads by increased energy dissipation through radiation, the heat loads which could arise by asymmetric radiation and the suppression of REs.

Generation and suppression of runaway electrons in disruption mitigation experiments in TEXTOR

Runaway electrons represent a serious problem for the reliable operation of the future experimental tokamak ITER. Due to the multiplication factor of exp(50) in the avalanche even a few seed runaway electrons will result in a beam of high energetic electrons that is able to damage the machine. Thus suppression of runaway electrons is a task of great importance, for which we present here a systematic study of runaway electrons following massive gas injection in TEXTOR.Argon injection can cause the generation of runaways carrying up to 30% of the initial plasma current, while disruptions triggered by injection of helium or of mixtures of argon (5%, 10%, 20%) with deuterium are runaway free. Disruptions caused by argon injection finally become runaway free for very large numbers of injected atoms.The appearance/absence of runaway electrons is related to the fraction of atoms delivered to the plasma centre. This so-called mixing efficiency is deduced from a 0D model of the current quench. The estimated mixing efficiency is 3% for argon, 15% for an argon/deuterium mixture and about 40% for helium.A low mixing efficiency of high-Z impurities can have a strong implication for the design of the disruption mitigation system for ITER. However, a quantitative prediction requires a better understanding of the mixing mechanism.

Plans for the first plasma operation of Wendelstein 7-X

Wendelstein 7-X (W7-X) is currently under commissioning in preparation for its initial plasma operation phase, operation phase 1.1 (OP1.1). This first phase serves primarily to provide an integral commissioning of all major systems needed for plasma operation, as well as systems, such as diagnostics, that need plasma operation to verify their foreseen functions. In OP1.1, W7-X will have a reduced set of in-vessel components. In particular, five graphite limiter stripes replace the later foreseen divertor. This paper describes the expected machine capabilities in OP1.1, as well as a selection of physics topics that can be addressed in OP1.1, despite the simplified configuration and the reduced machine capabilities. Physics topics include the verification and adjustment of the magnetic topology, the testing of the foreseen plasma start-up scenarios and the feed-forward control of plasma density and temperature evolution, as well as more advanced topics such as scrape-off layer (SOL) studies at short connection lengths and transport studies. Plasma operation in OP1.1 will primarily be performed in helium, with a hydrogen plasma phase at the end.

Main characteristics of the fast disruption mitigation valve.

The article presents a detailed investigation of the fast disruption mitigation valve developed at FZJ Juelich. The essence of this study is the novel direct observation of the piston motion by means of a fast framing camera. The piston stroke and the injection duration are shown to strongly depend on the operational pressure and the used gas. The same is true for the valve throughput. The analysis revealing the leading contribution of the injection duration in this modification is given. The knowledge of the injection duration is also used to reconstruct the characteristic pressure decay rates and the gas outflow rates. The means to increase the gas outflow are discussed. The main found valve characteristics are: (1) valve reaction time, i.e., the delay between the application of the trigger signal and the achievement of reliably observable opening 0.5 mm, is about 0.3 ms; (2) the maximum achieved throughput is 7.5 bar l for argon and 9.5 bar l for helium; (3) the maximum delivery rates are 500 bar l s(-1) for Ar and 1500 bar l s(-1) for He.

Influence of the dynamic ergodic divertor on transport properties in TEXTOR

Experiments to investigate transport properties under the influence of the dynamic ergodic divertor (DED) on TEXTOR are discussed. Relativistic runaway electrons are applied for studying transport properties of ergodization such as enhanced runaway loss. The ergodization causes an enhanced loss rate; this loss is higher for low relativistic electrons than for highly relativistic ones, in good agreement with particle orbit mapping. Edge transport can be controlled by the DED perturbation: in limiter H-mode plasmas ELM-like particle and heat bursts associated with the formation of enhanced edge pressure gradients are mitigated in the 6/2 configuration on the expense of a reduced pedestal height. Finally, the plasma is driven back to L-mode under the influence of the magnetic perturbation. In the 3/1 configuration the onset of tearing modes limits the possibility to affect edge transport. A mode of spontaneous density built-up has been found for the TEXTOR-DED as well. This mode is in particular strong for an inward shifted plasma; the built-up has a resonant character with respect to q(a). Langmuir probe measurements with two probe arrays show a strong influence of the magnetic ergodization on both the edge plasma equilibrium and fluctuation parameters. In particular, in the ergodic zone the turbulence properties and turbulence-driven flux are profoundly modified.

A new disruption mitigation valve (DMV) and gas flow in guiding tubes of different diameter

A new disruption mitigation valve, the DMV-30, has been developed and tested. The orifice output area of the valve is a factor of 2.4 and 12.25 times larger than that of its predecessors, DMV-20 and DMV-10, and the gas reservoir amounts to 1.3 L while the older version used at JET had only 0.65 L. The coil which provides the magnetic field pulse for the activation of the piston by an eddy current is outside of the working gas volume such that all gas volumes are now made of stainless steel. The valve has the advantages of the previous developments: it is robust and reproducible, opens fully within 3 ms and releases 50% of the gas within about 5 ms (He) to 10 ms (Ar). The valve is attached subsequently to two different guiding tubes, one with an inner diameter of 38 mm as used presently at JET and one with 102 mm inner diameter; the aim of this paper is the analysis of the gas flows for different diameters. The front of the gas pulse propagates with a Mach number of about 2.5 through the tubes, independent of the two diameters. This high speed agrees with theoretical expectations of flow expansion of a half infinite tube in vacuum. In the quasi-stationary phase of the expansion, the gas flows with about sound speed in the 102 mm tube and with about half of the sound speed in the 38 mm tube.

Performance and properties of the first plasmas of Wendelstein 7-X

The optimized, superconducting stellarator Wendelstein 7-X went into operation and delivered first measurement data after 15 years of construction and one year commissioning. Errors in the magnet assembly were confirmend to be small. Plasma operation was started with 5 MW electron cyclotron resonance heating (ECRH) power and five inboard limiters. Core plasma values of keV, keV at line-integrated densities were achieved, exceeding the original expectations by about a factor of two. Indications for a core-electron-root were found. The energy confinement times are in line with the international stellarator scaling, despite unfavourable wall conditions, i.e. large areas of metal surfaces and particle sources from the limiter close to the plasma volume. Well controlled shorter hydrogen discharges at higher power (4 MW ECRH power for 1 s) and longer discharges at lower power (0.7 MW ECRH power for 6 s) could be routinely established after proper wall conditioning. The fairly large set of diagnostic systems running in the end of the 10 weeks operation campaign provided first insights into expected and unexpected physics of optimized stellarators.

Design and Analysis of Divertor Scraper Elements for the W7-X Stellarator

A set of new water-cooled divertor components is being designed for the Wendelstein 7-X stellarator to protect the edges of the primary plasma facing components during the bootstrap current evolution (~ 40 s). These new components, referred to as scraper elements (SEs), will intercept field lines and associated heat flux that would otherwise overload the main target edges in certain operational scenarios. The SEs are calculated to experience peak heat fluxes ~15-16 MW/m2 and will be constructed from carbon fiber reinforced composite monoblocks of a type that has been qualified for ITER. The heat flux distribution and magnitude is calculated from field line following in a 3-D magnetic field that includes the contribution from plasma currents. The heat flux calculations are coupled with an engineering design in an iterative process to generate SEs that meet the design criteria while reducing the geometric complexity of the elements.

Fuelling efficiency of massive gas injection in TEXTOR: mass scaling and importance of gas flow dynamics

Fuelling efficiency is an important parameter in designing a massive gas injection system for suppression of runaway electrons in ITER. In this work Z-dependence of fuelling efficiency is measured for TEXTOR. The dependence covers the following gases: He, Ne, Ar, Kr, Xe and a 10% Ar–D2 mixture. It is shown that the fuelling efficiency significantly decreases with the gas mass, from above 0.5 for He to below 0.03 for Xe.To explain the variation of the efficiency with the gas mass and pressure a simple model of gas flow from the valve to the plasma edge is developed. The flow model is validated using available laboratory flow measurements of a TEXTOR-like injection system. An unsteady gas flow and a premature plasma disruption are shown to explain the mass dependence of the efficiency.

Gas flow analysis of a disruption mitigation valve (DMV)

The gas flow of a disruption mitigation valve (DMV) has been analysed by using a Michelson interferometer. The gas flow follows the JET design where the gas is injected via a 4 m long and 4 cm inner diameter tube. The gas densities of helium, neon and argon are measured at three locations along the tube with high time resolution. At the valve exit, the gas pulse has a rise time of less than 1 ms; the gas pulse rise time is smeared out and delayed on its passage through the tube. For the favoured Ar gas, the rise time and the delay time become about 11 ms each while for He the delay amounts to 4 ms and the rise time 2 ms. The gas velocity depends on the gas species but only weakly on the filling pressure. The output density amplitude increases nearly linearly with the filling pressure of the DMVs.