L. I. Krupnik

Improved confinement regimes induced by limiter biasing in the TJ-II stellarator

The influence of limiter biasing on plasma confinement, turbulence and plasma flows is investigated in the TJ-II stellarator. Experimental results show that it is possible to modify global confinement and edge plasma parameters with both positive and negative biasing. Significant and minor modifications in the structure of plasma fluctuations have been observed during the transition to improved confinement regimes induced by limiter biasing. These results show evidence of electric field induced improved confinement via multiple mechanisms. The investigation of the relaxation of plasma potential and electric fields shows evidence of two different characteristic decay times.

Development of heavy ion beam probe diagnostics

The results of the activity of the Kharkov and Moscow HIBP groups during recent years are presented in this paper. This is a review of papers published in the USSR and also contains extracts of some unpublished reports. The mathematical aspects of the diagnostics including the problems of the planning and design of an experiment along with the problems of the interpretation of the experimental data for plasma density and current determination are reviewed. The probing apparatus used in the experiment for plasma potential and density measurements are described. Some results of measurements are discussed and the trends to spread the use of HIBP in modern research are noted. >

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

An advanced 200 keV heavy ion beam diagnostic has been developed for the TJ-II stellarator based on the simultaneous utilization of a 30° Proca–Green electrostatic energy analyzer and a multiple cell array detector. This innovative design allows instantaneous measurements of plasma potential and electron density profiles together with their respective fluctuations. In this article we present a description of the main parts of a heavy ion beam diagnostic (injection system, detectors, and control and data acquisition system) and the results obtained during the first operation on TJ-II. The problems of plasma loading of the detectors and hard x-ray generation associated with the probing beam are reported.

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.

Heavy ion beam probe diagnostics on TJ-1 tokamak and the measurements of the plasma potential and density profiles

For the investigation of the plasma density and potential in the TJ-1 tokamak, we applied the heavy ion beam probe (HIBP) diagnostics with the beam energy up to 100 keV. The geometry of the experiment allows us to observe the whole plasma cross section and to catch the primary beam for direct calibration. HIBP on TJ-1 operates in two modes: shot-by-shot measurements which allow us to obtain the time evolution of the plasma parameters at every desired point of the detector grid, and scanning along the detector line (generally passing through the center) in the single shot which allows us to get a few profiles of the plasma parameters. When scanning we varied the injection angle with a period less than 2 ms. The spatial resolution of measurements was 2–5 mm. The bandwidth used (≈20 kHz) allows us to see the slow oscillations of the potential and density. The measurements were performed in the regimes with the magnetic fieldB=1.1–1.4 T, current I=26–32 kA, and the shot duration up to 25 ms. HIBP on TJ-1 enables us to measure simultaneously the time-spatial evolution of the potential and density; that makes HIBP a powerful tool for direct investigation of the transport processes in the plasma driven by the radial electric field.

Improved 20 keV injection system for the heavy-ion-beam diagnostic of the tokamak ISTTOK

The initial injection system of the heavy-ion-beam diagnostic (HIBD) of the tokamak ISTTOK provides a 2 mm diameter and 3.6 mrad divergence beam of Cs+ or Xe+ ions, after extraction from a plasma ion source and acceleration up to 20 keV by a gap lens and diameter definition by a set of apertures. However, the beam current is significantly reduced to 1.5 μA, therefore, complicating the HIBD measurements of the plasma parameters. Creation of the beam with a higher current and the same, or close, geometrical parameters requires a significant improvement of the ion optics of the injection system. This article describes an injection system built in the acceleration tube, and presents results of test experiments with a solid-state thermionic source (SSTS) and a monocusp plasma ion source (MPIS). The improved injection system is characterized by a good match to both ion sources. The use of the long focus ion optics of the acceleration tube allows one to obtain an output beam current up to 11 μA of Cs+ ions with ...

Heavy ion beam injection systems for fusion plasma diagnostics

This article presents long-focus ion beam injection systems developed for investigation of plasma parameters by heavy ion beam probing diagnostics in fusion devices. Ion beam parameters vary in a wide range of both beam intensity of alkali probing ions (1–100 μA) and initial energy (20–300 keV), determined by the experimental claims on each plasma device under study. This article describes the characteristics of beam extraction systems with transport ion optics and control, and accelerator power supplies with high stability (not worse than 10−5). Presented systems allow us a fine tuning of the beam focusing along the beam trajectory in plasmas studied. They provide an accurate spatial resolution of measurements in plasma of modern fusion devices up to 5 mm.

Operation of a multiple cell array detector in plasma experiments with a heavy ion beam diagnostic

A multiple cell array detector (MCAD) has been developed to investigate the spatial structure of plasma turbulence in fusion plasmas. This system is expected to provide simultaneous measurements of edge and core density fluctuations with both temporal and spatial resolution, extending the range and number of the sample volumes simultaneously recorded by a heavy ion beam diagnostic (HIBD). Since the detector (usually located close to the vessel wall of a plasma device) operates in a strong plasma radiation environment, the effective shielding of the detector presents a special problem. This article describes and compares the MCAD operation conditions on ISTTOK tokamak and TJ-II stellarator. Experimental results of the detector performance are presented together with the first measurements of neσeff in the TJ-II plasmas.

Modified biased split detectors for the HIBP electrostatic energy analyzer

In electrostatic energy analyzers of heavy ion beam probes the required accuracy of plasma potential measurements is provided by differential detection of the ion beam on the split detector. Secondary electrons created on the detector surface by the analyzed ions and ultraviolet plasma radiation can strongly disturb the measurements. In this article we consider the influence on analyzer operation of secondary electrons emitted from the detector, describe the relation for corresponding errors in the plasma potential measurements, and present two modified biased split detectors which avoid these errors.

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.