Multi-point measurement using two-channel reflectometer with antenna switching for study of high-frequency fluctuations in GAMMA 10

Abstract

A two-channel microwave reflectometer system with fast microwave antenna switching capability was developed and applied to the GAMMA 10 tandem mirror device to study high-frequency small-amplitude fluctuations in a hot mirror plasma. The fast switching of the antennas is controlled using PIN diode switches, which offers the significant advantage of reducing the number of high-cost microwave components and digitizers with high bandwidths and large memory that are required to measure the spatiotemporal behavior of the high-frequency fluctuations. The use of two channels rather than one adds the important function of a simultaneous two-point measurement in either the radial direction or the direction of the antenna array to measure the phase profile of the fluctuations along with the normal amplitude profile. The density fluctuations measured using this system clearly showed the high-frequency coherent fluctuations that are associated with Alfvén-ion-cyclotron (AIC) waves in GAMMA 10. A correlation analysis applied to simultaneously measured density fluctuations showed that the phase component that was included in a reflected microwave provided both high coherence and a clear phase difference for the AIC waves, while the amplitude component showed neither significant coherence nor clear phase difference. The axial phase differences of the AIC waves measured inside the hot plasma confirmed the formation of a standing wave structure. The axial variation of the radial profiles was evaluated and a clear difference was found among the AIC waves for the first time, which would be a key to clarify the unknown boundary conditions of the AIC waves.

Measurement of ICRF wave propagation using a microwave reflectometer with fast antenna switching on GAMMA 10

“…Their azimuthal mode numbers are the same, m = -1, and radial profiles are the same eigenmode-like structure. In a previous study using the reflectometer system with less antennas, the difference among the AIC waves other than those frequencies was firstly indicated along with an indication of axial standing wave structure from the 0 and π axial phase differences [3]. From figure 3, we can see that the relative strength among the visible four peaks is surely varied in the axial direction.…”

“…The latter operation was used in the experiment described in section 3. The more detailed information such as avoidance of the RF leak that is caused by the combination of two oscillators is described in the previous paper [3].…”

“…We recently introduced PIN diode switches to the microwave reflectometer system installed in the GAMMA 10 tandem mirror machine in University of Tsukuba [3,4]. The purposes are to enable a multipoint fluctuation detection with a minimum set of microwave components and data acquisition channels (low cost), and enable a dual operation function (multifunction).…”

“…Our reflectometer system was significantly modified in 2016; the details are described in ref. [3]. An initial study on internal measurement of slow wave propagation was performed using less microwave horn antennas [4].…”

Development of a Fast Visible Light Measurement System for the Study of Ion Cyclotron Range of Frequency Waves in GAMMA 10/PDX Plasmas

“…Such a measurement can be provided by two schemes: one is microwave reflectometry and the other is the plasma light measurement. The former measures the electron density fluctuations induced by RF electric fields [1,2], and the method was used in several fusion devices such as DIII-D [3,4], GAMMA 10 [5,6], TST-2 [7,8] and LHD [2,9]. The advantage of microwave reflectometry is its high sensitivity, and the relative density fluctuations down to about 10 −5 was measured in LHD [2].…”