Plasma recovery after various events in HT-7 superconducting tokamak

doi:10.1016/j.fusengdes.2008.03.001

Fusion Engineering and Design

Volume 83, Issues 5–6, October 2008, Pages 689-694

https://doi.org/10.1016/j.fusengdes.2008.03.001 Get rights and content

Abstract

Normal plasma recoveries after various events, such as after shutdown, various boronization, oxidation and large air leak, were investigated in the 2007 campaign of HT-7. Plasma recoveries, including disruptive plasmas, would depend on the wall status, such as impurities content and hydrogen retention. After shutdown or air leak, impurities made plasma recovery very difficult. After boronization, plasma recoveries would depend on the procedures of the boronization (C₂B₁₀H₁₂). After oxidation, boronization would effectively suppress impurities and would be beneficial for plasma recovery. ICRF cleanings in various working gases, such as He and D₂, would be useful for impurities and hydrogen removal. This research is important for effective operation of HT-7 and would be useful for EAST and ITER operations.

Introduction

Oxidation experiments on hot walls, which admitted abundant oxygen into inner-vessel, were proposed for removal of a long-term accumulation of high fraction tritium fuel in the surface or bulk material of plasma facing components in fusion devices, such as ITER [1], [2], [3], [4]. Plasma recovery after cleanup is one of the key issues for oxidation of wall conditioning. Besides oxidation experiments, plasma recoveries after shutdown and air leak are also important for effective operation in a tokamak. By suppression of metallic and oxygen impurities, boronization had been testified as an effective way for obtaining high performance plasmas [5], [6], [7], [8]. Plasma recovery after boronization should be investigated for optimizing the procedures of boronization.

The permanent presence of toroidal field in future tokamak will preclude GDC cleaning; therefore, ICR conditionings are envisioned for in-between pulse cleaning. High hydrogen removal rates have been reported in ICR experiments in Tore Supra with He and D [9] and Textor with He [10]. Disruptive discharge cleaning capable of operating in the presence of magnetic fields was used in TFTR for limiter conditioning [11]. Planned disruptions (i.e. without impurity injection) have utilized in order to provide localized surface heating in an attempt to promote H/D release from the C-Mod walls [12].

In the last HT-7 campaign in the spring of 2007, after various events, such as after shutdown, various boronization, oxidation and large air leak, different cleanings, especially ICRF cleaning techniques, were done for wall recovery. However, disruptive plasmas up to a few tens shots happened during plasma recovery due to the remained oxygen after leak or oxidation experiments, or due to hydrogen recycling after boroniztion. Those disruptive plasma discharges, taken a few to 10 h, strongly influenced on the operation efficiency of HT-7.

In this paper, different plasma recovery in HT-7 after various events was introduced. The main missions were that (1) to analyze plasma recovery procedures under various wall status; (2) to analyze how impurities or hydrogen recycling influenced on the following plasma operations; (3) to discuss how to optimize wall conditioning procedures, including cleanup, for easily plasma recovery. Those studies would be important for effective operation of HT-7, and would be useful for future tokamaks, such as EAST and ITER.

Section snippets

Experiments

HT-7 is a medium-sized superconducting tokamak (R = 1.22 m, a = 27 cm). Normal techniques were investigated for the impurities and hydrogen removal, such as baking, GDC and ICRF cleanings. Especially, He(D₂)-ICRF cleanings and boronization-ICRF with C₂B₁₀H₁₂ becomes routine methods for impurity removal or impurities suppression [13]. The oxidation experiments, including O-ICRF, O-GDC and thermal oxidation with neutral oxygen, have been performed in HT-7 in the last 2 years [14], [15], [16]. Besides

Plasma recovery after shutdown

While no leak could be detected at the initial stage of the last HT-7 campaign, long baking (∼150 °C) and long He-GDC cleaning were used for impurities removal. Total duration of baking was about 400 h and that of He-GDC was about 37 h. While the superconducting coils were cooled down and base vacuum reach ∼3.3 × 10⁻⁵ Pa, plasma discharge was started. However, totally 128 disruptive plasma were required before obtaining normal ohmic plasma. Including the interval of plasma discharges, totally 600 min

Discussion

Plasma recoveries after some special events are very important issue for effective and economic operation of a tokamak machines. Plasma recoveries would depend on the wall status, such as impurities content and hydrogen retention. Wall status would influence on the procedures of disruptive plasma discharges, but also influence on the following recovered plasmas. After shutdown, air leak and oxidation experiment, impurities were the main possible factor influenced on the plasma recoveries. After

Summary

Plasma recovery under various events, such as after shutdown, boronization, oxidation and air leak, were systematically studies in the 2007 campaign of HT-7. This research is important for effective operation of HT-7 and would be useful for EAST and ITER operation.

Plasma recoveries would depend on the wall status, such as impurities content and hydrogen retention. After shutdown, air leak and oxidation experiment, impurities were the main possible factor influenced on the plasma recoveries.

Acknowledgments

This work was funded by the National Nature Science Foundation of China under contract No. 10705030. This work was partly supported by the JSPS-CAS Core-University Program on Plasma and Nuclear Fusion.

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Plasma recovery after various events in HT-7 superconducting tokamak

Abstract

Introduction

Section snippets

Experiments

Plasma recovery after shutdown

Discussion

Summary

Acknowledgments

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

Fusion Eng. Des.

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J. Nucl. Mater.

J. Nucl. Mater.

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Fusion Eng. Des.