DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 61: Direct Measurement of ECCD Width from Modulated ECCD | ||
| Name: | C. Craig Petty ( |
Affiliation: | General Atomics |
| Research Area: | Heating & Current Drive | Presentation time: | Not requested | Co-Author(s): | -- |
| Description: | Directly measure the ECCD profile width using the MSE diagnostic by modulating the ECCD power at a low frequency (10 Hz) and observing the back EMF effect. | ||
| Experimental Approach/Plan: | For off-axis ECCD, modulate the gyrotron power at 5-10 Hz with the beams on continuously to obtain MSE data. Stationary plasma conditions are preferred so that the time dependence of the MSE signals are dominated by the effects of the modulated back EMF. Unwanted electron temperature oscillations can be avoided by operating the ECCD in a push-pull manner, that is, alternating between co and counter ECCD at the same location. However, in this case an accurate determination of the ECCD profile width requires that the co and counter ECCD be well aligned at the same location. The MSE data can be analyzed between shots using HPA (Heat Pulse Analysis) to determine the amplitude and phase of the pitch angle response. | ||
| Background: | The modulation in the current density and back EMF effect can be detected on the MSE diagnostic by applying FFT to the measured pitch angles. The phase difference between the ECCD power modulation and the measured change in the pitch angle allows one to separate the back EMF effect from changes in the current profile. Despite the simplicity of this technique, we have never attempted it on DIII-D before 2003 since we have always been more concerned about the magnitude of the current drive rather than the profile width. However, recently the issue of ECCD profile broadening has gained importance, so the time is opportune to attempt to directly measure the ECCD profile width using the modulation technique. The optimal modulation frequency is as slow as possible, but one needs on order ~10 pulses to do FFT analysis. | ||
| Resource Requirements: | NBI: Co and counter MSE beams are needed, and one additional source.
EC: Minimum 5 gyrotrons. | ||
| Diagnostic Requirements: | -- | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | -- | ||
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