7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Interferometer Systems on KSTAR
Yong Un Nam1, June-Woo Juhn1, Kwan Chul Lee1, Dong-Geun Lee2 and Dong-Jae Lee1
1
Korea Institute of Fusion Energy
2
Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science andTechnology
e-mail (speaker): yunam@kfe.re.kr
Interferometry is based on the phase measurements,
which means that its values should be tracked from zerodensity but are free from the intensity modulation error.
It guarantees reliable absolute density measurementswith a proper fringe-jump compensation scheme.
Depending on the electron density, frequency of thebeam source should be carefully selected. Low frequency
system makes too many fringe-jump on the high density
plasma, while high frequency system suffers from themechanicalvibration.
Well-designed
vibrationcompensation system is crucial for the high density
fusion plasma measurements. On Korea Superconducting
Tokamak Advanced Research (KSTAR), several types ofinterferometer systems were developed through its
operation stage, including a radial millimeter-wave
interferometer (MMWI)[1], a vertical far infra-red laserinterferometer (FIRI)[2], a multi-channel tangential
two-color interferometer (TCI)[3,4] and a vertical single
crystal dispersion interferometer (SCDI)[5].
The MMWI system uses two 280GHz voltage controlled
Gunn oscillators that differ by 60MHz for the heterodyne
measurements. The microwave is radially injected onto
plasma via oversized waveguides, rectangular hornantennas and concave mirrors to minimize the beam
power loss. The beam is reflected on a plasma facing
carbon tile, which is specially shaped to focus the beamon the plasma center. Although the MMWI was designed
for the first plasma of KSTAR that was expected to reachthe peak electron density of several 1019 m-3, it wasoperated on the high density H-mode condition ofKSTAR with support of in-situ fringe jumpcompensation techniques[6] up to 10 years. Now thesystem was dismounted, and a new 1 THz interferometersystem (THI) using microwave sources based on an
amplifier-multiplier chains (AMC) technique wasdesigned[7]. The THI will use quasi optics dielectric lens
and will share the optics with KSTAR ECEI system via
specially designed wire grid beam splitter. The THI will
be used to measure radial line-integrated density withhigh precision and to analyze high frequency fluctuation
components on the line density.
The FIRI is 119μm CH3OH laser interferometer thatmeasures vertical line density. A vibration compensatedcorner-cube mirror was installed on the top end ofvertical beam line. The measured density showed
excellent agreement with the MMWI via comparison
using rtEFIT reconstructed flux surface. An H-mode
real-time density feedback control system was developed
based on the FIRI signal. The system successfully
controlled electron density up to 4.5 x 1019 m-3 during
H-mode condition and up to 55 s of long pulse H-mode
plasma. The FIRI was replaced to a tangential TCI,

which has better stability and multi-channel
measurements capability. To compensate phase error
comes from mechanical vibrations, the TCI uses two
frequency sources: one is a 10.6 μm CO2 laser andanother is a 660nm diode-pumped solid-state (DPSS)
laser. Relatively short source wavelength removes a risk
from fringe-jumps during fast density changing events
such as edge localized modes (ELMs) during H-mode
operation. Increased vibration effect is compensated by atwo-color method. In addition, supported by improved
wavelength stability of CO2 laser and a phasecomparison algorithm using field programmable gatearray (FPGA), the TCI shows extremely robust operation
during entire KSTAR campaign without issues from thefringe-jump and the signal drift. The TCI has 5 tangential
channels, the system provides information on the density
distribution over core and edge region of plasma. All 5channels have real-time measurements capabilities andcombined on the plasma control system (PCS). The TCI
is used for average density feed-back control, and evenmore, could be used for the separated density control oncore and edge region with proper actuators. Second TCI
system that adds 5 channels is planned. With total 10
tangential channels, the density profile can bereconstructed from the TCI data.
The SCDI is a 1064nm Nd:YAG laser interferometerbased on dispersion technique using a nonlinear crystal
that makes second harmonic beam sharing same beampath. With this technique, severe vibration error on shortwavelength interferometer could be effectively
compensated. While even TCI with the short-wavelengthof 10 um suffer from a pellet injection experiment thatmakes extremely high density change over several 1021
m-3 within milliseconds range, the SCDI successfully
followed these events. The SCDI therefore became an
essential diagnostic for one of the most important topics
for the next generation tokamaks such as ITER on
disruption mitigation technique using shattered pellet
injections.
References[1] Y.U. Nam et al, Rev. Sci. Instrum. 79, 10E705 (2008)[2] J.-W. Juhn et al, Rev. Sci. Instrum. 87, 11E1313(2016)[3] K.C. Lee et al, Fus. Eng. Des. 113, 87 (2016)[4] J.-W. Juhn et al, Rev. Sci. Instrum. 92, 043559 (2021)[5] D.-G. Lee et al, 92, 033536 (2021)[6] Y.U. Nam et al, Rev. Sci. Instrum. 89, 10B111 (2018)[7] D.-J. Lee et al, “Development of a THz
interferometer system for Korea Superconducting
Tokamak Advanced Research”, 1st International Fusion
and Plasma Conference (2022)