Title:“Enhanced plasma edge characterization for investigation of helicon wave coupling and neutral compression with the small angle slot divertor in the DIII-D U.S. national fusion facility”

Public Abstract:

The U.S. National Fusion Facility DIII-D is implementing two new exciting upgrades during the ongoing long-torus opening. They deal with improving the capacity of the high performance plasma core confined by the strong magnetic cage of DIII-D with the surrounding wall elements. This interaction with the material surfaces defines the longevity of the plasma material interface but also defines the impurity release and hence the amount of impurities which can enter the plasma. Because these impurities irradiate confined plasma energy and hence cool the plasma, the electrical resistivity changes impacting the plasma current which provides one component of the confining magnetic field. A specific method of radio-frequency wave, similar to microwaves known from household use, are being explored to control the plasma current and supply parts of it by means of the so-called helicon wave. The first new capability being a main focus of this project at DIII-D is the realigned Small Angle Slot (SAS) divertor. This heat and particle exhaust device within the DIII-D tokamak is geared towards leveraging increased divertor closure to obtain plasma detachment at lower pedestal densities. In this detached plasma state surface material heat fluxes vanish promising a largely extended life time of the armor in front of sensitive device components. The second one is a new radio-frequency (RF) antenna, emitting RF waves in the whistler regime,  so-called helicon waves, to test if this type of RF wave can be used for sustained current drive for non-inductive plasma operation and fine control the plasma current profiles.

The success of these device upgrades is critically dependent on good diagnostics of the nearby plasma boundary region. This includes the plasma edge density and temperature radial profiles, as well as the neutral gas pressure and species composition of the nearby neutral reservoir. This project addresses this diagnostic need with two innovative techniques. A thermal helium beam diagnostic at the helicon antenna is proposed for the continuous measurement of electron density and temperature radial profiles in the scrape-off layer (SOL) and the outer pedestal region. Small amounts of deliberately injected helium atoms are used as atomic min-probes and the light emitted during penetrating the plasma edge is analyzed by dedicated atomic models. This diagnostic will be accompanied by installation of Wisconsin In-Situ Penning (WISP) gauges for simultaneous, in-situ measurement of fractional neutral pressures of the main plasma species and impurities. These proposed diagnostics will at the same time also deliver new and important experimental data to the general plasma boundary and pedestal program at DIII-D,  as the measurement quantities provided are presently not available at DIII-D as continuous measurements with a comparable spatial resolution in the relevant plasma parameter range.

Funding Information: funded by the Department of Energy, Office of Fusion Energy Science, DE-SC00020284, started in 08/2019