Parameter Dependencies of the Configuration-Dependent 1-2 kHz Fluctuation in W7-X

Sean Ballinger


Tuesday, November 5, 2019



PSFC Student Seminars

A 1–2 kHz fluctuation is present in a large fraction of discharges in the W7-X stellarator's standard magnetic field configuration with edge iota=0.97 and 5/5 island structure. The fluctuation is present in edge electric and magnetic fields, as well as plasma density, temperature, and radiated light. The surface temperature of divertor tiles can fluctuate by 2.5 ºC at 1–2 kHz, indicating that the fluctuation also has an impact on power handling in W7-X. Its cause and precise location in the plasma are still unknown, and a variety of factors determine its peak frequency and power. In this work, a survey using magnetic pick-up coil data finds that the toroidal bootstrap current is positively correlated with the fluctuation's peak frequency and power. The fluctuation power is highest at high electron cyclotron heating power and decreases with plasma density. A stellarator power plant with higher heating power might therefore need to contend with a stronger fluctuation in divertor heat flux. Finally, shedding some light on the location of the fluctuation in the plasma, electron cyclotron temperature measurements show that the electron temperature fluctuates throughout the plasma minor radius, but with gaps where no fluctuation is observed.