A new visible Bremsstrahlung system was developed for the VEST Tokamak to monitor impurity content in its plasmas. This system will contribute to a better understanding of impurity behavior and its impacts on fusion plasma performance.

Have you even seen a plasma ball or the striking blue or red glow from a plasma discharge at a science exhibition? These visible lights are not only fascinating to look at. They also carry valuable information that plays an important role in the fusion energy development.

For commercial fusion energy production, it is required to confine the high-temperature plasma (≥100,000,000K) for a sufficiently long duration in a fusion reactor. Achieving this requires minimizing the interaction between the high-temperature plasma and the reactor wall, as such interactions generate impurities that degrade plasma performance through fuel dilution and increased radiative power loss. Since the interaction with the wall is unavoidable, effective impurity control strategies must be developed based on understanding of impurity behavior in fusion plasmas. The first step toward this goal is to measure the spatial and temporal distribution of impurities in the plasma.

Another type of the light emitted from the plasma is Bremsstrahlung radiation, which originates primarily from free electrons in the plasma during collisions with ions, including impurity ions. The intensity of the Bremsstrahlung radiation depends on the impurity content, allowing the overall impurity level in the plasma, represented by effective charge (Z_eff), to be estimated from its measurements. Since Bremsstrahlung radiation exhibits a continuous wavelength spectrum, its intensity can be measured in the visible range using relatively simple and cost-effective optical diagnostics.

A new system for measuring visible Bremsstrahlung (VB) radiation intensity was developed to study impurity behavior in the VEST Tokamak, a university-scale fusion experimental device. This seven-channel system enables measurements of the spatial distribution of impurity content in the plasma. The development process began with identifying the appropriate wavelength range for VB detection, followed by determining the line of sight for each channel in the optical system. For the hardware implementation, commercial lenses and plastic optical fibers were used to build a cost-effective but reliable system. Furthermore, a tomographic reconstruction code was developed to infer the distribution of impurity content from the line-integrated VB measurements of each channel. Following installation on the VEST Tokamak, the system was calibrated for absolute intensity levels, allowing for successful measurements of impurity content based on the absolute intensity level of the measured VB radiation.

This system will be used to study impurity behavior in fusion plasmas, serving as a foundation for future development of impurity monitoring and control systems in fusion reactors. In addition to measuring the absolute intensity level of VB radiation, the system is capable of capturing its fluctuations. These fluctuation measurements will provide insight into the effects of impurities on plasma instabilities and transport, including interactions between impurities and instabilities. As noted earlier, impurity is one of the critical factors influencing fusion plasma performance. The physics relevant to impurities in the fusion plasma, which will be investigated by using the developed visible Bremsstrahlung system, will enhance our physical understanding of impurity dynamics and their impact on plasma performance. This will support both the advancement of fusion science and the development of key technologies essential for future fusion reactors.