The new single crystal dispersion interferometer installed on KSTAR and its first measurement

Abstract

Dispersion interferometers have been used to measure line integrated electron densities from many fusion devices. To optically suppress noise due to mechanical vibrations, a conventional dispersion interferometer typically uses two nonlinear crystals located before and after the plasma along the laser beam path. Due to the long beam path, it can be difficult to overlap the fundamental and second harmonic laser beams for a heterodyne dispersion interferometer and to focus the beams on the second nonlinear crystal located after the plasma, especially when the aperture of the nonlinear crystal is small, i.e., of the order of mm. To overcome such difficulties, a new concept of a heterodyne dispersion interferometer, a single crystal dispersion interferometer (SCDI), is developed and installed on KSTAR with the laser wavelength of 1064 nm. The concept and the optical setup of the KSTAR SCDI are discussed, as well as its first measurement during a shattered pellet injection that produces abrupt and large changes in the electron density. To demonstrate feasibility, the KSTAR SCDI measurements are also compared with those from the existing two-color interferometer.

Development of the upgraded single crystal dispersion interferometer (SCDI-U) and its first measurements of the line integrated electron densities in KSTAR during shattered pellet injections

“…In all cases, pure deuterium pellets are used and injected at 4.985 section The difference is that a pellet is injected from the G-port in shot #28922 (black line) or from the O-port in shot #28923 (blue line), while two pellets are simultaneously injected from the G-port and the Oport in shot #28912 (red line). Note that the vertical line of sight for the SCDI-U is located next to the G-port (see figure 5 in [12]); therefore, we expect to observe a stronger plasma response when a same pellet is injected from the G-port compared to an injection from the O-port.…”

“…These frequencyshifted beams act as the reference beams. Note that the SCDI used the first order diffraction of the second harmonic beam [12], i.e. 2ω + (∆ω) (blue dashed line in figure 1), causing a separation of the two reference beams.…”

“…The KSTAR SCDI-U uses Nd:YAG laser whose wavelength is 1064 nm, which is much shorter than using CO 2 laser with the wavelength of 10.6 µm or 9.57 µm used in many other TCI or DI systems [7][8][9][10][11]22]. We have intentionally chosen to use a shorter wavelength so that it can measure abrupt and large changes in electron density induced by shattered pellet injections [12,23] to investigate effects of the shattered pellets on mitigation of plasma disruptions [15]. From equation (4), it is clear that a shorter wavelength is favorable for measuring abrupt and large density changes, otherwise fringe jumps would complicate the data interpretation which is indeed the case we find from the KSTAR TCI system that uses 10.6 µm laser (see figure 6…”

“…Aligning and focusing both the fundamental and second harmonic beams to the second nonlinear crystal located after the plasmas requires a high level of precision because the beams are typically traveling more than a few tens of meters before being focused onto the nonlinear crystal whose aperture is less than a couple of mm [4]. To alleviate the required precision, a single crystal DI (SCDI) has been proposed [12] as well as its forward model for Bayesian analysis of the measured data [14]. The SCDI replaces the second nonlinear crystal with a frequency doubler, which is a simple electronic component, thereby removing the task of aligning and focusing the beams after the plasmas.…”

Overview and recent progress of KSTAR diagnostics

“…The former, TCI, provides 5-chord tangential line-densities succeeding to two legacy single-chord systems: 1.07 mm [8] and 119 μm [9,10]. The latter, SCDI, newly developed, is dedicated to the special experiments on disruption mitigation by injecting shattered pellets causing extremely high and transient electron densities up to 10 22 m −2 [11]. There will be another system to be installed in near future based on a Schottky-barrier-diode THz source that is expected to be very stable and rigid (figure 2).…”