B. Goncalves

Influence of electrode and limiter biasing on the ISTTOK plasma

In this contribution, limiter and electrode biasing experiments are compared, in particular with regard to their effects on the edge plasma parameters. For a positive electrode bias (EB), the plasma potential profile is strongly modified in the region between the electrode and the limiter. Both the edge Er and its shear increase significantly (values of Er larger than 10 kV m−1 have been measured). The modification in the edge parameters is accompanied by a substantial increase in the average plasma density and an improvement in the gross particle confinement. For moderate/high plasma densities no steady-state discharges have been achieved with EB since a degradation in confinement is observed a few milliseconds after the bias is applied. For negative bias (−250 < Vbias < 0 V), no significant modification in either the global or the edge parameters is observed since the electrode current drawn is too low to modify the plasma parameters. The general discharge behaviour for positive limiter biasing is similar to that observed with EB. However, the larger increase in radiation losses and the smaller modification of the edge electric field observed when bias is applied leads to almost no modification in particle confinement and to a stronger and faster degradation in confinement a few milliseconds later.

Reciprocating probe measurements of ELM filaments on JET

The ELM filamentary structure in the JET scrape-off layer (SOL) has been investigated using a multi-pin reciprocating Langmuir probe system inserted at the top, low field side of the poloidal cross-section. These measurements clearly demonstrate that both Type I and III edge localized modes (ELMs) are composed of a number of filaments. The filament propagation in the SOL is investigated and the radial velocity estimated using different methods. The large radial velocities observed during Type I ELMs (up to 6 km s−1) imply that filaments have no time to drain significant energy along the field lines, reaching the wall with a reasonable fraction of the pedestal density and temperature. In spite of the time between Type I ELMs being much longer than the ELM duration, most of the particle flux to the main chamber plasma-facing components occurs in the latter period. Furthermore, filaments in Type I ELMs are significantly larger and faster than those observed in Type III ELMs. During Type I ELMs a strong parallel flow (M ~ 0.4) is measured towards the inner divertor, which increases during filaments. These observations are in agreement with the ballooning nature of the ELM losses.

Plasma Potential Measurements by the Heavy Ion Beam Probe Diagnostic in Fusion Plasmas: Biasing Experiments in the TJ-II Stellarator and T-10 Tokamak

Abstract The effect of edge biasing on plasma potential was investigated in the TJ-II stellarator and the T-10 tokamak. The Heavy Ion Beam Probe (HIBP) diagnostic, a unique tool for studying the core potential directly, was used in both machines. Experiments in TJ-II show that it is possible to modify the global confinement and edge plasma parameters with limiter biasing, illustrating the direct impact of radial electric fields on TJ-II confinement properties. For the first time it was shown that the plasma column in a stellarator can be charged as a whole for a hot, near-reactor-relevant plasma. The plasma potential and electric fields evolve on two different characteristic time scales. Although the experimental conditions in the two machines have many important differences, the basic features of plasma potential behavior have some similarities: The potential response has the same polarity and scale as the biasing voltage, and the fluctuations are suppressed near the electrode/limiter region. However, whereas both edge and core plasma potential are affected by biasing in TJ-II, the potential changes mainly near the biased electrode in T-10.

Control of the edge turbulent transport by emissive electrode biasing on the tokamak ISTTOK

In this paper results are presented on the changes induced by emissive electrode biasing in the ISTTOK edge transport. The boundary plasma is characterized with focus on the relation between E × B sheared flows and particle transport. We suggest that the distinct behaviour of the particle confinement for positive and negative bias observed in ISTTOK is related to the low E × B shear induced by positive bias in the core periphery region associated with the appearance of large amplitude fluctuations. In addition, the effect of the electrode bias on the edge turbulent transport has been investigated identifying the changes induced in the fluctuations frequency spectrum and probability density function. We have shown that negative electrode bias reduces the propagation of large-scale events, making the fluctuations distribution more Gaussian and resulting in low amplitude fluctuations across most of the edge plasma region. For positive bias, large amplitude broad spectrum fluctuations appear in the core periphery, which increase the cross-field diffusion and contribute to the observed asymmetry in particle transport with the bias polarity.

A multichannel reflectometer for edge density profile measurements at the ICRF antenna in ASDEX Upgrade

A multichannel reflectometer will be built for the new three-straps ICRF antenna of ASDEX Upgrade (AUG), to study the density behavior in front of it. Ten different accesses to the plasma are available for the three reflectometer channels that can be interchanged without breaking the machine vacuum. Frequency is scanned from 40 GHz to 68 GHz, in 10μs, which corresponds to a cut-off density ranging from 1018÷1019m−3 in the Right cut-off of the X-mode propagation, for standard toroidal magnetic field values of AUG.

Implementation of the new multichannel X-mode edge density profile reflectometer for the ICRF antenna on ASDEX Upgrade

A new multichannel frequency modulated continuous-wave reflectometry diagnostic has been successfully installed and commissioned on ASDEX Upgrade to measure the plasma edge electron density profile evolution in front of the Ion Cyclotron Range of Frequencies (ICRF) antenna. The design of the new three-strap ICRF antenna integrates ten pairs (sending and receiving) of microwave reflectometry antennas. The multichannel reflectometer can use three of these to measure the edge electron density profiles up to 2 × 1019 m-3, at different poloidal locations, allowing the direct study of the local plasma layers in front of the ICRF antenna. ICRF power coupling, operational effects, and poloidal variations of the plasma density profile can be consistently studied for the first time. In this work the diagnostic hardware architecture is described and the obtained density profile measurements were used to track outer radial plasma position and plasma shape.

Comparative study design of a heavy ion and neutral beam diagnostic for the International Tokamak Experiment Reactor

In this article, we address the major physics principles and the engineering issues associated with the implementation of a heavy ion beam diagnostic for the International Tokamak Experiment Reactor (ITER) for the edge measurements of the plasma electric and poloidal magnetic fields. Two different diagnostic configurations are discussed. The ions’ trajectories and attenuations were calculated by dedicated simulation codes using the relevant ITER equilibrium parameters. Estimations of measurement accuracy are presented for a chosen configuration.

Installation of the advanced heavy ion beam probing diagnostic on the TJ-II stellarator

An advanced heavy ion beam diagnostic has been developed for the TJ-II stellarator based on the simultaneous utilisation of two different detection systems for the secondary ions: a multiple cell array detector and a 30° Proca-Green electrostatic energy analyser. This innovative design aims at enlarging the HIBD capabilities to allow the instanteneous measurements of electron density and plasma potential profiles together with their respective fluctuations.This paper presents the detailed description of the main parts of HIBD and their characteristics obtained during the first operation on TJ-II. Special attention is paid to the control and data acquisition system built on two VME controllers. The results of the diagnostic beam propagating through the magnetic structure of TJ-II into electrostatic energy analyser are presented and compared with the trajectory calculations. The operation and calibration of a 30° electrostatic energy analyser free of guard rings and with a new biased split detector are described. High intensities of the caesium and thallium ions was obtained from thermionic source using new stable and long-time special operation regimes.