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Electron Bernstein wave emission based diagnostic on National Spherical Torus Experiment (invited)
S J Diem1, G Taylor, J B Caughman
1Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, New Jersey 08543, USA.
The Review of Scientific Instruments
|December 3, 2008
Summary
Electron Bernstein wave emission offers potential for electron temperature measurements in spherical tokamaks. However, transmission efficiency is sensitive to plasma conditions, particularly during H-modes, but lithium conditioning shows promise for improvement.
Area of Science:
- Plasma Physics
- Fusion Energy Research
- Diagnostic Techniques
Background:
- Spherical tokamaks (STs) like the National Spherical Torus Experiment (NSTX) operate at high densities and low magnetic fields, limiting conventional electron cyclotron emission diagnostics.
- Electron Bernstein waves (EBWs) can propagate and are emitted at electron cyclotron harmonics in ST plasmas, suggesting their potential for electron temperature (Te) measurements.
- A robust EBW emission (EBE) diagnostic requires high electron Bernstein wave to O-mode (B-X-O) transmission efficiency (>90%) across diverse plasma conditions.
Purpose of the Study:
- To investigate the feasibility of using thermal EBW emission for local Te measurements in NSTX.
- To study the physics of EBW emission and coupling, focusing on B-X-O transmission efficiency.
- To identify and address challenges affecting the reliability of EBE diagnostics in ST plasmas.
Main Methods:
- Utilized an obliquely viewing EBW to O-mode (B-X-O) diagnostic with steerable antennas on NSTX.
- Employed absolutely calibrated radiometers to measure EBW emission.
- Conducted Electron Bernstein Wave Emission (EBE) simulations to understand transmission behavior.
- Performed initial edge lithium conditioning experiments during H-modes.
Main Results:
- Observed significant (>20%) fluctuations in B-X-O transmission efficiency due to edge density scale length variations.
- Documented a substantial decay (5-10x) in B-X-O transmission efficiency during H-modes, dropping to <3%.
- Simulations indicated that EBW collisional damping significantly reduces emission when Te < 30 eV within the B-X-O mode conversion layer.
- Lithium conditioning experiments successfully increased Te within the B-X-O mode conversion layer, thereby enhancing B-X-O transmission.
Conclusions:
- While EBE is a viable diagnostic for Te in NSTX, its application is hindered by transmission efficiency issues, especially during H-modes.
- Collisional damping in the mode conversion layer is a key factor limiting EBW transmission in low-temperature regions.
- Edge lithium conditioning presents a promising method to improve EBW transmission efficiency, enhancing the potential of EBE diagnostics in STs.
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