Impurity sources during lower hybrid heating on Alcator

Lipschultz, B.; LaBombard, B.; Manning, Herbert L.; Terry, J. L.; Knowlton, S.; Marmar, E.S.; Porkoláb, M.; Rice, J.E.; Takase, Y.; Texter, S.; Wan, A.

doi:10.1088/0029-5515/26/11/002

Cited by 10 publications

(5 citation statements)

References 15 publications

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“…However few, if any, experimental measurements have been performed to determine the energy reflection coefficients. Most of the low energy data are obtained using computational models 2 4 , 25 ,2 6 . The actual energy reflection coefficient is a function of the incident particle energy, incident angle, and material…”

Section: Resultsmentioning

confidence: 99%

“…The slit opening at the plasma side must be less than twice the sheath thickness, 6 ,heath, in order to assure the continuity of the sheath potential surface across the front of the probe. This design constraint assures that the charged particle distribution functions are not perturbed by a uneven distribution of the sheath potential.…”

Section: Design Considerationmentioning

confidence: 99%

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Janus, a bidirectional, multifunctional plasma diagnostic

Wan

Yang

Lipschultz

et al. 1986

Review of Scientific Instruments

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Janus is a multiple function edge probe used to diagnose the Alcator C limiter shadow plasma whose components include two sets of identical diagnostics; each set facing a different direction. Included within each set of diagnostics are a retarding-field energy analyzer (RFEA), Langmuir probe, and calorimeter. Janus is constructed to make measurements both parallel and antiparallel to a magnetic field. It can withstand high heat fluxes and can be scanned perpendicular to the magnetic field in the limiter shadow. The RFEA can alternatively sample both the ion and electron parallel energy distribution functions during a tokamak discharge. From the Langmuir probe, one can infer electon temperature, density, and the plasma floating potential. The calorimeter independently detects the total parallel heat flux incident on an electrically floating plate. Together these three diagnostics enable detailed, localized edge plasma studies on Alcator C. This paper presents the design considerations for each of the diagnostics along with a brief summary of the analysis techniques. Some experimental results obtained using Janus will also be presented.

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Section: Resultsmentioning

confidence: 99%

Section: Design Considerationmentioning

confidence: 99%

Janus, a bidirectional, multifunctional plasma diagnostic

Wan

Yang

Lipschultz

et al. 1986

Review of Scientific Instruments

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“…For a qualitative check of the validity of the above conclusions, an edge erosion code was used to calculate heating, evaporation/sublimation rates, and thermal sputtering rates. Using edge plasma parameters and the physical properties of the exposed structural materials as input, the code yields calculated impurity source rates from both the limiter and the Faraday shield [47]. The overall sputtering yield includes thermal sputtering by bulk ions (H or D) and self- consistent contributions from impurity ions.…”

Section: Erosion Code Comparisonmentioning

confidence: 99%

Impurity generation during ICRF heating experiments on Alcator C

et al. 1986

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Observations of impurity behavior are presented from ICRF heating experiments at 180 MHz performed over a variety of conditions on the Alcator C tokamak, using graphite limiters and stainless steel antenna Faraday shields. Spectroscopic observations revealed significant increases in metal impurity concentrations during the RF pulse, with iron levels increasing by as much as a factor of 12 at the highest RF powers (-350-400 kW). Analysis of the inferred iron source rates shows an approximately linear dependence on RF power up to 400 kW. with no clear dependence on resonance conditions or bulk plasma parameters.However, a sharp increase in the temperature in the limiter shadow region was observed during the ICRF pulse, which was well correlated with the iron influx rate. It is concluded from this and other evidence that physical sputtering of the Faraday shield due to an elevated sheath potential is the primary source of metal impurities during ICRF heating on Alcator C. The same process, occurring at the graphite limiter, is believed to be the dominant source of carbon and oxygen. Calculated sputtering yields obtained from an edge erosion code demonstrate the plausibility of this model.

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“…Research using the predecessor tokamak, Alcator C, concentrated on the study of poloidal asymmetries 1 , the physics of MARFEs 2,3 , as well as impurity generation 4 and transport 5,6 . This work was expanded to divertor studies with the conceptual development of the vertical-plate divertor for…”

Section: Introductionmentioning

confidence: 99%