Development of Electron Temperature Diagnostics Using Soft X-ray Absorber Foil Method in Versatile Experiment Spherical Torus (VEST)

Abstract

Electron temperature diagnostics using SXR (soft X-ray) absorber foil method has been developed to measure line averaged electron temperature of the VEST (Versatile Experiment Spherical Torus) at Seoul National University. SXR absorber foil method is relatively simple method to diagnose line averaged electron temperature, which is measured by the ratio of transmitted radiation intensity between two different thickness foils. It has very good time resolutions so it can be useful to detect various phenomena. Also it can be a complementary diagnostics of other electron temperature diagnostics such as Thomson scattering system. The expected electron temperature of the VEST core plasmas is about one hundred eV. In order to measure temperatures in this range, aluminum foils with thicknesses of 0.8 μm and 1.5 μm aluminum foils are chosen by calculating the variations of intensity ratio and signal strength according to electron temperature. Each foil is located in front of detectors which are installed in the midplane of the VEST chamber to diagnose core plasmas. AXUV-16ELG photodiode is employed as a detector in favor of its good responsivity in the SXR region. Each signal lines are twisted and carefully shielded with copper braided wires which can reject electromagnetic noises. This diagnostic method can be affected by impurity line radiation and non-thermal radiations generated by runaway electrons, so it is important to know the amount and causes of these radiations. Calculations are performed to estimate these radiation effects. From this calculation if there is oxygen line radiation which is considered as a major impurity in VEST overestimates or underestimates in measured electron temperature is expected according to its value. If there is non-thermal high energy radiations underestimated electron temperature is expected. To estimate intensity of these radiations two experimental components are prepared. Bandpass filter around 780 nm is used for monitoring the evolution of oxygen atom and an additional thick aluminum foil is used to see the effect of the non-thermal high energy photons. With these calculation and experimental setup H-alpha line signal and triple Langmuir probe are also used for the interpretation of experimental results. To see the performance of the diagnostics test experiments are performed in ECH (Electron Cyclotron Heating) pre-ionized ohmic plasmas in VEST. In the first test experiments the electron temperature at the plasma current peak is about 110 eV and this value sustained almost the same during the plasma ramp down phase. In the latter part of the discharge large uncertainties are expected in measured electron temperature due to the high oxygen impurity amount. Second test experiments are performed with two test cases that have different operating pressures and oxygen impurity amounts while other parameters are fixed. In this comparison two discharges with different electron temperatures expected. Difference in measured electron temperature is observed as expected. However, transmitted emissions with 2.3 μm thickness aluminum foil are too low compared with 0.8 μm and 1.5 μm thickness aluminum foil. With this result measured electron temperature is below 20 eV. There is large difference in two test experiments. Low transmitted emission level of 2.3 μm can be explained by assuming the presence of non-thermal high energy radiations. And combined effect of non-thermal high energy radiation and oxygen impurity line radiation can be explaining the difference in measured electron temperature. When more precise diagnostics for these radiations is prepared, the potion of these radiation compare with continuum radiation can be determined then it is possible to diagnose the electron temperature more precisely.