2024 – Parametric Decay Instability Physics in Helicon Injection Without Neutral Beams

Parametric Decay Instability Physics in Helicon Injection Without Neutral Beams

2024 Research Campaign, Heating and Current Drive

Purpose of Experiment

This experiment is to increase our quantitative understanding of a phenomenon known as parametric decay instabilities (PDI). PDI occurs when electromagnetic waves at high power levels propagate in plasmas, and as such occurs in laser fusion experiments as well as in experiments in tokamaks in which high-power radiofrequency waves are launched into the plasma for the purpose of heating and driving non-inductive current in the core of the plasma. In PDI, some fraction of the electromagnetic wave power applied to the plasma excites waves at frequencies other than the applied frequency, and as such can constitute a loss mechanism for the intended heating and current drive. In this particular experiment, the intention is to study plasmas in which only rf heating is applied, and none of the neutral beams are used. This is done partly because the neutral beams themselves can create instabilities of a similar nature (where they are called Ion Cyclotron Emission or ICE), so in plasmas with both rf and neutral beams, the situation can get confusing.

Experimental Approach

This experiment is intended as a “standby experiment” that can be usefully carried out when some set of DIII-D’s capabilities are not available for some reason. The experimental proposal consists of a list of three short experiments, each with a different reduced set of available resources.1. Compare plasmas in which the edge of the discharge is in contact with a solid surface (‘limited plasmas) with ones with the more usual ‘divertor’ shape is employed. We know from previous work that the experimental signatures of PDI during rf heating using the helicon wave appear to be significantly different in the limiter condition than they are in the divertor condition, due to the very different plasma properties in the region near the wall, the so-called “scrape-off layer”. Experiment (1) can be done in plasmas without any other rf heating than helicon, as well as without neutral beams.2. Compare plasmas with the two possible polarities of the main magnetic field in the tokamak, the toroidal field. This is enabled by DIII-D’s toroidal field reversing switch, which allows the field to be reversed in polarity from one discharge to the next one. We expect that the helicon power level at which PDI sets in may be significantly different from one polarity to the other, but this has not been tested yet. This also can be performed with no other auxiliary heating besides the helicon power.3. Compare PDI in plasmas in L-mode to those in H-mode, in which the transition from one confinement regime to the other is obtained with high-power electron cyclotron heating (ECH) (no neutral beams). Our previous work indicates that the PDI can be very different in the two confinement regimes, due to the different scrape-off layer parameters associated with each. This part of the experiment of course requires the use of the ECH system in addition to the helicon system. The intention is to do whichever part or parts of the experiment are consistent with the available resources, so flexibility on the part of the experimental team is necessary.

Interested in a behind-the-scenes look at DIII-D? Join us for a virtual OR in-person tour during Fusion Energy Week (May 5-9)! Sign up for a tour here.

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