Device could help to harness plasma for nuclear fusion

A new instrument that could help scientists developing nuclear-fusion reactors has been used to analyse experiments in China.

The X-ray imaging crystal spectrometer (XICS), developed by the US Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), allows researchers to observe in detail how radio-frequency (RF) waves affect the way hot ionised gas (plasma) moves.

A team at the Experimental Advanced Superconducting Tokamak (EAST) at the Institute of Plasma Physics in Hefei, China, is hoping that the device will enable it to understand how plasma can be better controlled and contained for use in nuclear fusion.

‘With plasmas, you are dealing with very high temperatures and flow velocities,’ said PPPL’s Manfred Bitter, one of the designers of the XICS. ‘Those must be determined from the radiation emitted by the plasma.’

The XICS enables researchers to measure the frequencies and intensities of X-rays emitted by plasma impurities and so calculate the plasma’s temperature and rotational speed.

The scientists already knew that the RF wave, known as a lower-hybrid wave, drives current in the plasma, but they also found that it caused the plasma to flow as a whole at high velocities through the vacuum vessel — a property referred to as toroidal rotation.

This could be important for projects hoping to produce energy from fusion, such as ITER in France, that rely on self-generated or RF-driven movement of plasma in a reactor.

The new spectrometer allows researchers to study self-generated and RF-driven flow with the goal of controlling it in future reactors so that plasmas can be more carefully contained.

The spectrometer, designed by Bitter and his colleague Kenneth Hill, is made up of several components, including a tiny piece of quartz that has been moulded into a sphere, a two-dimensionally imaging X-ray detector and a beryllium window.

The project is supported through through the US-China Fusion Co-operation Program and includes researchers from EAST, PPPL and the National Fusion Research Institute in Korea.

Similar spectrometers designed by Bitter and Hill have been installed on experimental fusion machines in Korea and Japan, and at the Plasma and Fusion Science Center at the Massachusetts Institute of Technology.