DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 171: Completion of n=1 RMP ELM control studies | ||
| Name: | Richard Buttery
( |
Affiliation: | UKAEA |
| Research Area: | ELM Control & Pedestal Physics | Presentation time: | Requested | Co-Author(s): | Max Fenstermacher, T Evans, D Howell, M Schaffer, O Schmidt, R Moyer, T Osbourne, G Jackson |
| Description: | This is a small half day proposal to complete a key element of the 2007 experiment exploring n=1 control of ELMs. Although these experiments did not complete a number the field configurations planned, this follow up proposal is targeted on a particular aspect using one further field configuration that should allow us to go further in addressing the key limiting issue and evaluating the technique.
The 2007 experiments revealed a more severe limitation than expected in the n=1 error field technique due to the inducing of 2/1 error field locked modes. However these experiments used C coils to correct intrinsic error in order to allow I coil operation with other configurations. The C coils are known to correct intrinsic error on DIII-D less effectively that I coils in 240 phasing, and to do so in a way that is not well understood theoretically, potentially even enhancing some non-resonant perturbations. It is therefore proposed that, we explore use of the I coil in 240 degree phasing configurations. This will allow edge resonant I fields to be applied, while simultaneously phasing the coils for correction of core resonant intrinsic error fields. In this way better correction of intrinsic errors should be possible compared to 2007, enabling us to apply increased edge resonant fields without encountering error field locked mode limits. If successful the effects can be further explored with q95 and amplitude optimisation. | ||
| Experimental Approach/Plan: | As previously, experiments will apply maximum available fields within locked mode limits, and utilise q95 scans to determine the resonance spot and optimal effect. No RMP references will be taken. At optimal resonance repeats using RMP field ramps will be taken and compared with no-RMP cases. We would use a previously optimised plasma configuration and ramp-up in the ITER-like single null shape. I coils would be in the usual 240 phasing configuration (but key element is to phase them in the direction used for intrinsic error correction). C coils might be used to explore extension of I fields to higher amplitudes by using them to (ie use C coil to cancel core resonances generated by I fields as they go beyond intrinsic error correction levels). About half a day is needed. | ||
| Background: | Recent experiments on JET have shown that the application of broadly resonant n=1 perturbations can substantially (~x10) reduce ELM sizes, while increasing their frequency and having modest impact on confinement [Liang PRL 2007]. The DIII-D tokamak has unique capabilities to study the physics and design requirements of this process, through its configurable I coils, as well as its C coil systems. It ought to be able to improve on the results obtained at JET by applying more edge resonant fields.
Experiments were undertaken on DIII-D using n=1 RMP s to control ELMs in 2007. Significant change to ELM behaviour was observed with substantial rises in type I ELM frequencies. Thus the experiment was partially successful, but with locked mode difficulties and control system failures on the day, it did not complete most of the phase scans envisaged. The 2007 experiments were limited by the occurrence of locked modes, even when the C coils were deployed to help cancel out core resonant 2/1 error fields. This is attributed to the uncertainties introduced by the C coil correction process, which is calculated to *enhance* fields, and hypothesised to act through non-resonant field effects on plasma viscosity. Therefore the C correction may not be the appropriate technique to use in experiments where additional fields are ramped up to control ELMs. | ||
| Resource Requirements: | I coils 240, C Coils, 4 beams | ||
| Diagnostic Requirements: | Usual ELM control diagnostics, with particular emphasis on pedestal measurement. Esp. CO2, TS, CER, magnetics, filterscopes. | ||
| Analysis Requirements: | Results should be self evident, but detailed diagnostic analysis will be required to quantify effects. | ||
| Other Requirements: | -- | ||
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