A. Field

First results from MAST

MAST is one of the new generation of large, purpose-built spherical tokamaks (STs) now becoming operational, designed to investigate the properties of the ST in large, collisionless plasmas. The first six months of MAST operations have been remarkably successful. Operationally, both merging-compression and the more usual solenoid induction schemes have been demonstrated, the former providing over 400 kA of plasma current with no demand on solenoid flux. Good vacuum conditions and operational conditions, particularly after boronization in trimethylated boron, have provided plasma current of over 1 MA with central plasma temperatures (ohmic) of order 1 keV. The Hugill and Greenwald limits can be exceeded and H mode achieved at modest additional NBI power. Moreover, particle and energy confinement show an immediate increase at the L-H transition, unlike the case of START, where this became apparent only at the highest plasma currents. Halo currents are small, with low toroidal peaking factors, in accordance with theoretical predictions, and there is evidence of a resilience to the major disruption.

H mode discharges with feedback controlled radiative boundary in the ASDEX Upgrade tokamak

Puffing of impurities (neon, argon) and deuterium gas in the main chamber is used to feedback control the total radiated power fraction and the divertor neutral particle density simultaneously in the ASDEX Upgrade tokamak. The variation of Psep=Pheat-Prad(core) by impurity radiation during H mode shows a similar effect on the ELM behaviour as that obtained by a change of the heating power. For radiated power fractions above 90%, the ELM amplitude becomes very small and detachment from the divertor plates occurs, whilst no degradation of the global energy confinement is observed (completely detached high confinement mode). Additional deuterium gas puffing is found to increase the radiated power per impurity ion in the plasma core owing to the combined effect of a higher particle recycling rate and a lower core penetration probability. The outer divertor chamber, which is closed for deuterium neutrals, builds up a high neutral pressure, the magnitude of which is determined by the balance of particle sources and pumping. For this particular situation, the effective pumping time of neon and argon is considerably reduced, to less than 0.3 s, mainly owing to an improved divertor retention capability. The radiation characteristics of discharges with a neon driven radiative mantle are modelled using a 1-D radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The results of simulations are in good agreement with experiment

Ideal magnetohydrodynamic stability of the tokamak high-confinement-mode edge region

The ideal magnetohydrodynamic (MHD) stability of the tokamak edge is analyzed, with particular emphasis on radially localized instabilities; it is proposed that these are responsible for edge pressure gradient limits and edge localized modes (ELMS). Data and stability calculations from DIII-D [to appear in Proceedings of the 16th International Conference on Fusion Energy, Yokohama (International Atomic Energy Agency, Vienna, 1998), Paper No. IAEA-F1-CN-69/EX8/1] tokamak equilibria indicate that two types of instability are important: the ballooning mode (driven by pressure gradient) and the peeling mode (driven by current density). The characteristics of these instabilities, and their coupling, are described based on a circular cross-section, large aspect ratio model of the tokamak equilibrium. In addition, preliminary results are presented from an edge MHD stability code which is being developed to analyze general geometry tokamak equilibria; an interpretation of the density threshold to access the high-...

Overview of recent experimental results on MAST

Note: Proc. 19th IAEA Fusion Energy Conference, Lyon, France, October 2002, IAEA-CN-94/EX/OV2-3 Reference CRPP-CONF-2002-068 Record created on 2008-05-13, modified on 2017-05-12

Molecular impurities in ASDEX UPGRADE plasma discharges

Abstract The introduction presents a historical review of the role of molecules in tokamak research starting from the first installations at the Kurchatov Institute. Molecular impurities were mostly considered as a transient conditioning problem, but with the use of carbon for wall and limiter elements, it was perpetuated. New results about the elementary processes involved in hydrogenic carbon erosion are reported and the existing data base is briefly discussed. Results from mass spectrometry are presented as well as data from optical spectroscopy including determination of CD 4 and CD fluxes from molecular band intensities. A typical yield of about 5% for hydrogenic chemical erosion is obtained. In combination, all these results show the impact of hydrogenic carbon erosion. They strongly suggest that with boronized walls it remains as the dominating process for the carbon fluxes in the SOL and even dominates the carbon concentration in the central plasma in spite of a high SOL screening action for hydrocarbons.

Measurement of the radial electric field in the ASDEX tokamak

Estimates of the radial electric field Er at the plasma periphery are obtained by measuring the drift velocities of low-Z impurity ions (He II, B IV, C III). The drift velocities are determined from the differential Doppler shift of visible line emission observed along opposite viewing directions. The principle of the measurement, including contributions from the diamagnetic drift, as well as radial gradients in the excitation rate and the effect of integrating along the line of sight are discussed in detail. The measured line of sight averaged drift velocities can be strongly influenced by the location and shape of the Er profile, especially if, as measured on other tokamaks, it is localized to a narrow region just within the separatrix. Values of Er estimated by assuming a constant radial profile underestimate maximum local values. During the H*-phase, however, high line of sight averaged perpendicular drift velocities of the B IV ions of a least 15 km/s in the electron diamagnetic drift direction are observed. From this, the presence of a strong negative radial electric field of at least 25 kV/m in the plasma edge region is inferred. Values of the B IV ion poloidal drift velocity calculated from an appropriate neoclassical theory are in the same direction as those measured. However, the calculated line of sight averaged values are much smaller than the measured ones. This reinforces the conclusion that a strong negative radial electric field is present just within the separatrix during the H-mode

Impurity transport and neoclassical predictions

The authors present a brief review of collisional (classical and neoclassical) and anomalous transport. Particular emphasis is devoted to the question of charge independence of the anomalous transport coefficients and the combined action of anomalous and collisional transport. In the light of these results the experimental facts are analysed and interpreted. It is found that impurity accumulation-characterized by peaked zeff-profiles-is caused by the combined effects of improved confinement (i.e. reduction of anomalous transport) and peaking of the electron density profile. For the cases of pellet refuelled plasmas and counter neutral injection heating quantitative comparisons are performed which show good agreement between the experimental measurements and simulations based upon neoclassical theory.

A spatially scanning vacuum ultraviolet and visible range spectrometer for spectroscopy of tokamak plasmas in ASDEX‐Upgrade

A spatially scanning, combined vacuum‐ultraviolet (VUV) and visible range spectrometer system for the spectroscopy of tokamak plasmas in the ASDEX‐Upgrade experiment is described. This system is designed to allow flexible observation of about 2/3 of the boundary plasma using VUV (30–200 nm) and visible range spectrometers viewing along a common line of sight which can be scanned during the plasma discharge by means of a rotatable mirror. From successive spectra recorded using intensified, multichannel photodiode detectors and the recorded position data, spatial profiles of the plasma emission can be reconstructed. Because radiation losses from the boundary plasma can largely be attributed to line emission in the VUV spectral region, this instrument finds application in quantitative studies of radiation loss processes as well as to studies of impurity production and transport. Simultaneous observation in the visible spectral range facilitates an in situ absolute calibration of the VUV instrument by means o...

2D modelling of the ASDEX-Upgrade scrape-off layer and divertor plasma☆

Abstract Due to the open field lines, the scrape-off layer and divertor region of tokamak plasmas is a complex, two-dimensional system, involving transport parallel and perpendicular to the magnetic field, as well as interaction of the plasma with surfaces and with the neutral gas. Therefore sophisticated two-dimensional codes are required to model the divertor and edge physics. In this paper, the B2-EIRENE code package is used to simulate the ASDEX-Upgrade scrape-off layer plasma and the neutral gas dynamics in a fully self-consistent way. Specific ASDEX-Upgrade discharges are modelled using the actual magnetic configuration and in-vessel components. Single fluid as well as multifluid calculations including self-consistent target and wall erosion of carbon are described. At given input power and bulk plasma radiation, typical divertor plasma profiles from Langmuir probes are fitted by varying the separatrix density and the transport coefficients. On the basis of such multifluid fits, spectroscopic divertor diagnostics are numerically modelled and compared with measured profiles, and reasonable agreement is found.

Comparison of MHD-induced rotation damping with NTV predictions on MAST

Plasma rotation in tokamaks is of special interest for its potential stabilizing effect on micro- and macro-instabilities, leading to increased confinement. In MAST, the torque from neutral beam injection can spin the plasma to a core velocity ~300 km s−1 (Alfven Mach number ~0.3). Low density plasmas often exhibit a weakly non-monotonic safety factor profile just above unity. Theory predicts that such equilibria are prone to magneto-hydro-dynamic (MHD) instabilities, which was confirmed by recent observations. The appearance of the mode is accompanied by strong damping of core rotation on a timescale much faster than the momentum confinement time.The modes saturated structure is estimated using the CASTOR code together with soft x-ray measurements, enabling the calculation of the plasma braking by the MHD mode according to neoclassical toroidal viscosity (NTV) theory. The latter exhibits strong similarities with the torque measured experimentally.