||Evaluation of observable phase space with fast ion loss detector by calculating guiding centor orbits in consideration of plasma facing components and three-dimensional magnetic field
Shinohara, Kouji ,
Kim, Junghee ,
Young Kim, Jun ,
Rhee, TongnyeolIsobe, Mitsutaka
A fast ion loss detector (FILD) can obtain the information on their gyro radius and pitch angle of lost ions that reach the FILD. The information is obtained from a position on the scintillator screen in the FILD. From the information, the ion orbit that can reach the FILD can be calculated. The orbits suggest a source of the lost ions in a phase space. When a signal is not obtained by the FILD, we assume there is no source of lost ions. However, this is not obvious in an actual condition when we considered the following things: 1) plasma facing components (PFCs). 2) bending of magnetic field. These can block loss ions before the ions reach the FILD. The FILD head is placed just behind a limiter-like structure to avoid damage due to heat flux of bulk plasma. But the FILD can detect the fast ions due to their large gyro radius. In other word, loss ions can be easily interfered by PFCs. The situation is more complex when we considered the finite thickness of a PFC in the toroidal direction. The guiding center orbit of ions is unique when the detected gyro radius and pitch angle of them are identical on the FILD. However, it depends on the gyro-phase whether an ion with a particular guiding center orbit can hit the PFC even when the distance between the PFC and the guiding center is less than its gyro radius. An ion cannot hit the PFC if the ion is moving in the opposite side of a thin PFC against its guiding center. This fact suggests the statistical approach is required to determine whether a particular position on the screen can capture loss ions. In addition, the magnetic field bending can virtually behave as a bending wall. Thus, we need to take into account of three-dimensional magnetic field in this context. Such an approach to understand whether fast ions can reach a particular position on the screen corresponds to the approach to understand a phase space that the FILD can capture. We developed a numerical tool to evaluate the observable phase space by tracing ions backward from the detector position in consideration of the plasma facing components and three-dimensional magnetic field. We applied this tool to a case in a resonant magnetic field experiment in KSTAR. We have found that the FILD can capture only a limited phase space in the particular case and have also found that the observable phase space is largely changed by a different magnetic configuration.
KSTAR Conference 2017