Study of plasma start-up initiated by second harmonic electron cyclotron resonance heating on WEGA experiment

M. Preynas, H. P. Laqua, M. Otte, T. Stange, T. Wauters, D. Aßmus

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

Abstract

Although both 1st harmonic ordinary mode (O1) and 2nd harmonic extra-ordinary mode (X2) have been successfully used to initiate pre-ionization and breakdown in many devices, a complete theoretical model is still missing to explain the success of this method. Moreover, some experimental observations are not completely understood, such as what occurs during the delay time between the turn-on of ECRH power and first signals of density or light measurements. Since during this free period the ECRH power has to be absorbed by in-vessel components, it is of prime importance to know what governs this delay time. Recently, dedicated start-up experiments have been performed on WEGA, using a 28 GHz ECRH system in X2-mode. This machine has the interesting capability to be run also as a tokamak allowing comparative experiments between stellarator (ι/2π > 0) and tokamak (ι/2π = 0) configurations. Different scans in heating power, neutral gas pressure, and rotational transform (ι) show clearly that the start-up is a two step process. A first step following the turn-on of the ECRH power during which no measurable electron density (or just above the noise level in some cases), ECE and radiated power is detected. Its duration depends strongly on the level of injected power. The second step corresponds to the gas ionization and plasma expansion phase, with a velocity of density build-up and filling-up of the vessel volume depending mainly on pressure, gas and rotational transform. Moreover, an interesting scenario of ECRH pre-ionization without loop voltage in tokamak configuration by applying a small optimal vertical field is relevant for start-up assistance on future experiments like ITER. The results from this experimental parametric study are useful for the modeling of the start-up assisted by the second harmonic electron cyclotron resonance heating. The aim of this work is to establish predictive scenarios for both ITER and W7-X operation.

Original languageEnglish
Title of host publicationRadiofrequency Power in Plasmas - Proceedings of the 20th Topical Conference
PublisherAmerican Institute of Physics Inc.
Pages498-501
Number of pages4
ISBN (Print)9780735412101
DOIs
Publication statusPublished - 2014
Event20th Topical Conference on Radiofrequency Power in Plasmas - Sorrento, Italy
Duration: 25 Jun 201328 Jun 2013

Publication series

NameAIP Conference Proceedings
Volume1580
ISSN (Print)0094-243X
ISSN (Electronic)1551-7616

Conference

Conference20th Topical Conference on Radiofrequency Power in Plasmas
Country/TerritoryItaly
CitySorrento
Period25/06/1328/06/13

Keywords

  • ECRH
  • Plasma Start-up
  • second extraordinary harmonic
  • stellarator
  • tokamak

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