2024 – Assess the helicon heating and current drive efficiency with core absorption in L-mode plasmas

Assess the helicon heating and current drive efficiency with core absorption in L-mode plasmas

2024 Research Campaign, Heating and Current Drive

Purpose of Experiment

In order to keep the hot plasma confined within the fusion reactor, a tokamak uses nested magnetic field lines that spiral around doughnut-shaped surfaces. The twist in the magnetic field lines comes from an electrical current that is created in the plasma. One way to generate this current is by injecting electromagnetic waves into the plasma. Just like a microwave oven can be used to heat food with electromagnetic waves with frequencies that are chosen to match the vibrations of water molecules and drive them like a tuning fork, electromagnetic waves can transfer energy and momentum to the plasma by matching the natural motion of the electrons, which carry the current in the plasma. The purpose of this experiment is to demonstrate that a specific type of electromagnetic wave, called helicon waves, can generate a current in the plasma, which has been predicted theoretically but never before measured in a fusion experiment. If successful, helicon waves will provide a new method for sustaining the plasma current in a future commercial fusion power plant that may be more efficient than other possible techniques, potentially reducing the amount of input energy needed to generate the same amount of fusion power.

Experimental Approach

In order to isolate the current generated by the helicon waves from all other potential sources of current, this experiment will use a very simple plasma in order to achieve the best possible signal to noise ratio. Successive shots will be taken with 1) the helicon waves injected clockwise into the tokamak, 2) the helicon waves injected counter-clockwise, and 3) without any helicon waves at all. Measurements of how much the magnetic field lines are twisted — which is proportional to the amount of plasma current — will be compared between the three different helicon configurations in order to determine if the helicon waves generated current in the plasma, and if so, how much. In other shots, the injected helicon power will be rapidly modulated at a fixed frequency, which will in turn create an oscillation in the electron temperature at the same frequency as the helicon heats the plasma. This electron temperature oscillation will then be analyzed to determine how much of the helicon power was actually absorbed by the plasma. The ratio of the current generated in the plasma by the waves divided by the wave power absorbed by the plasma is a measure of how efficiently helicon waves can produce a current in the plasma. After the experiment, this efficiency will be compared to theoretical calculations and computer simulations in order to determine whether the observations agree or disagree with the theoretical predictions, and whether the existing models can be considered reliable for making predictions for helicon current drive in the design of future fusion power plants.