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Title 241: Evaluation of effect of double-layer Faraday screen on rf voltage standoff in ELMing H-mode
Name:Robert I. Pinsker () Affiliation:General Atomics
Research Area:Heating & Current Drive Presentation time: Requested
Co-Author(s): F.W. Baity, M. Porkolab, E. Fredd, J.C. Hosea, J.-M. Noterdaeme, A. Horton, W.C. Martin
Description: This experiment aims to evaluate the effect of a double-layer, nearly-optically-opaque Faraday shield on an ICRF antenna. In particular, if the density burst in the antenna structure induced by large ELMs is responsible for transient depression of the rf standoff voltage in the antenna structure, the nearly optically-opaque Faraday shield installed on the 285/300 FW antenna on DIII-D should reduce the size of the decrement in standoff voltage due to ELMs.
Experimental Approach/Plan: This experiment primarily uses the 285/300 antenna at 60 MHz, since this is the one with a double-layer shield, but in all cases, we could also monitor the rf voltage standoff of one of the 0 deg or 180 deg antennas (with a single-layer tilted Faraday screen) for comparison. After establishing an antenna rf standoff voltage in vacuum of ~30 kV, then measure the reduction standoff voltage in 1) L-mode plasmas as a function of the outer gap, and 2) in H-mode plasmas using ECH (and 60 MHz FW) as the only heating sources, as a function of the outer gap and with different ELM types, controlled by shape changes (e.g. X-point height, etc.), and 3) substitute neutral beams for the EC power and repeat the scans of step 2, to investigate possible effects of fast ion losses on voltage standoff.
Background: In the late 1970s and early 1980s, ICRF antennas typically had a ceramic (MACOR) cover, on which the Faraday screen was placed, usually with two layers. First the ceramic cover was removed, with no apparent deleterious effect, then the second layer of the Faraday screen was removed, again without apparent deleterious effect. Since it is easier to make a single-layer shield that is thinner, hence place the strap closer to the plasma surface and increase the resistive loading (and thereby lower the peak voltage per MW coupled), single-layer Faraday screens quickly became nearly universally used. However, when antenna feed circuits that were able to cope with the rapidly changing loading due to ELMs were introduced (first at DIII-D then at ASDEX-U), it became apparent that the density burst at the antenna due to each large ELM tended to momentarily reduce the dielectric strength of the 'vacuum' near and in the antenna structure. Hence the double-layer shield for the DIII-D 285/300 antenna, which had been used in 1990-1991, was reinstalled during the LTOA in order to investigate this. Unfortunately, the antenna standoff in vacuum since the LTOA has been very poor, thus not permitting this experiment. During the present vent period, we hope to have remedied this problem by making a minor modification to the antenna vacuum coax, and if the antenna now returns to ~30 kV standoff in vacuum, we should be ready to carry out this experiment. This is of great significance to the ITER community as well as being of practical importance for the use of FW in ELMing H-mode discharges on DIII-D.
Resource Requirements: One day experiment. ~4 NB sources, at least 4 gyrotrons, the 285/300 antenna having been conditioned to ~30 kV in vacuum, and the 0 deg antenna similarly well conditioned.
Diagnostic Requirements: If the 0 deg antenna is used, the UCSD fast camera should be optimized for its view of the 0 deg antenna. If available, high time resolution loading diagnostics (~1 MHz digitizing rate) should be used on 285/300, each burst of data being triggered by the arc detector, with 50% pre-trigger samples.
Analysis Requirements: --
Other Requirements: --