DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 483: Parasitic edge losses during Fast Wave Current Drive and Heating | ||
| Name: | Miklos Porkolab ( |
Affiliation: | Mass. Inst. of Technology |
| Research Area: | Heating & Current Drive | Presentation time: | Not requested | Co-Author(s): | R.I. Pinsker, C.C. Petty, R. Prater, F.W. Baity, R. Moyer, J. Watkins, J.C. Hosea |
| Description: | Study the edge losses due to RF sheaths at the wall and/or Parametric Decay Instabilities (PDI) in the sol/scrape-off layer plasma during FWCD and/or FW heating campaigns. | ||
| Experimental Approach/Plan: | We propose to measure the sheath potentials in the plasma near the walls and/or limiters during high power FW heating and current drive. Initially, for these studies we would use existing Langmuir probes in collaboration with the edge physics group. By varying plasma conditions, such as the outer gap and ELM frequency, as well as line integrated density, theory predicts that the cut-off for fast wave propagation moves toward or away from the walls, thus greatly affecting sheath formation and subsequent edge losses. As a consequence, central electron absorption will be modified. As part of the experimental scans, we would vary the injected FW power, change the antenna phasing, and study the dependence of the sheath potential on these parameters. The results would be compared with recent predictions of theory under the SCIDAC program. To monitor Parametric Decay Instabilities (PDI), we would initially use existing RF magnetic loop probes located on the outer wall and on the center post. The PDI measurements will be greatly facilitated by the new high speed (up to 1G sample/sec) data acquisition system that is on loan from ORNL for the coming campaign. In the future, it would be desirable to install emissive probes to directly measure the plasma potential in the presence of RF waves, thus making a comparison between theory and experiment easier. In addition, to improve the interpretation of PDI spectrum in terms of theory, it would be desirable to equip the reciprocating probe with a coax tip to measure the spatial variation of the amplitude and shape of the RF spectrum. This has consequences for predicting the RF pump wave depletion due to spatial growth of the decay waves. | ||
| Background: | It has been observed in nearly all Fast Wave experiments at DIII-D and elsewhere, that under conditions of weak single pass absorption, parasitic edge absorption may deteriorate the current drive or heating efficiency. This is in addition to absorption by energetic beam ions in the presence of intense NBI. Although at the present time the nature of such absorption is not well understood or documented, under some conditions it may dominate the desired electron heating or current drive by Fast Waves in the core of high performance DIII-D plasmas. Given the availability of 6 MW of source ICRF power at DIII-D, it is of great importance to understand the physics of edge loss mechanisms and set up conditions to avoid them. Such high performance plasmas are typically in H-mode, and/or at high density, aimed at the high beta regime. The previous paper by Petty et al (Nucl. Fusion 39, 1421(1999) explored various ELMy H-mode conditions where edge losses completely dominated as the ELM frequency was increased to 200-300 Hz. It was shown that the edge density rose to sufficiently high level that the right hand cut-off layer moved to the plasma edge, allowing the development of strong RF sheaths at the wall or limiter surfaces. However, these sheaths were never measured. Related observations have been made on NSTX very recently and presented by Hosea at the 2007 RF conference, as well as at the 07 November APS meeting. Another mechanism leading to edge losses is the Parametric Decay Instability (PDI) into ion cyclotron quasi modes and Ion Bernstein waves in the SOL (M. Porkolab, Fusion Eng. Design 12, 93(1990)). Such spectra was observed both in DIII-D (R. Pinsker et al, Nucl. Fusion 46, S416 (2006) ) and NSTX (R. Wilson et al, APS, Albuquerque, 2003) but its role was not assessed. By understanding the physics of these processes, we propose to explore the conditions under which these mechanisms can be avoided, thus optimizing central electron heating and current drive. Furthermore, both of these processes are under intense investigation by the SCIDAC RF theory programs. | ||
| Resource Requirements: | At least 1 MW total FW power, in combinations of 60 MHz and/or 90 MHz; up to 6 MW NBI power. | ||
| Diagnostic Requirements: | Probes as described above in addition to the usual profile diagnostics. Edge reflectometry. | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | -- | ||
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