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Title	97: Time-resolved measurement of plasma response spectrum
Name:	Jeremy Hanson hansonjm@fusion.gat.com	Affiliation:	Columbia University
Research Area:	Stability & Disruption Avoidance	Presentation time:	Requested
Co-Author(s):	M. J. Lanctot	ITPA Joint Experiment :	No
Description:	The proposed experiment would test an innovative technique to identify the frequency dependence of the n=1 plasma response, allowing for a time-dependent resolution of physical parameters in a single-mode plasma response model. In previous work, the spectrum has been identified by combining data from multiple discharges probed with single-frequency perturbations. The proposed method would obtain the same information using a perturbation with several superposed traveling waves. Since this technique would necessarily involve higher amplitude perturbing currents than are normally used in single-frequency active MHD spectroscopy, experimental time is required to assess the possible deleterious side effects for the plasma, such as mode-locking.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	Apply an n=1 spectroscopic waveform containing 5 frequency harmonics. Evaluate the beta dependence of the growth rate and coupling parameter from the 5-frequency response, and compare with the results from traditional, single-frequency spectroscopy. The measured dependencies will be compared with the predictions of plasma response codes such as VALEN and MARS. The measurements will be extended to n=2 if time allows.
Background:	Measurements of the plasma response to applied low-n magnetic perturbations can be used to assess the proximity to marginal RWM stability. The amplitude and toroidal phase of the plasma response can be related to the damping rate and mode rotation frequency of the stable RWM via a single-mode model [Reimerdes, et al, Phys. Rev. Lett. 93 (2004) 135002]. The link between the plasma response and stability can be understood in terms of the energy and torque required to perturb the plasma. As the plasma approaches marginal stability, less external energy is required to drive a fixed amplitude perturbation at the plasma surface. The plasma response is therefore a direct measurement of the proximity to marginal stability. A fit to multiple frequency components is needed to simultaneously determine both a complex coil-mode coupling parameter, wall eddy-current decay time, and the RWM growth rate in the single-mode model. The RWM growth rate can be calculated from single-frequency plasma response data by assuming the other parameters are fixed. However, the coupling parameter may vary with plasma equilibrium parameters such as shape and outer-gap. In addition, analysis of 2-frequency plasma response measurements in NSTX (assuming a fixed coupling parameter) revealed a strong beta-dependence of the wall eddy-current decay time [J.-K. Park, et al., Phys Plasmas 16 (2009) 082512]. The multi-frequency technique may provide a more reliable and direct comparison with RWM stability modes, compared to single-frequency plasma response measurements. Initial tests of this technique in startup plasmas show that the multi-frequency response is consistent with the single-mode plasma response model over a limited beta range well below the no-wall limit. However, additional investigations are needed to demonstrate the reliability of this technique above the no-wall limit and to address whether physics model parameters (other than the RWM growth rate) stay fixed as plasma parameters change.
Resource Requirements:	H-mode plasma, sufficient NBI power to vary normalized beta above the no-wall limit.
Diagnostic Requirements:	Magnetics, MSE, CER, Thomson scattering, ECE radiometer, density interferometer
Analysis Requirements:
Other Requirements: