Speaker
Dr
Tsuyoshi Akiyama
(National Institute for Fusion Sceience)
Description
Interferometers which measure plasma induced phase shifts are a common approach for probing line-integrated electron density. However, since changes in the beam path length caused by mechanical vibrations can often cause comparable or larger phase shifts, one of the main challenges of this measurement is reduction of the vibration contribution either through an optical solution or simply a stabilizing structure. One solution, a dispersion interferometer (DI), where the probe beam is a mixture of fundamental and second harmonic components, measures the phase shift which arises from dispersion alone. The conventional DI is a homodyne interferometer, hence variations of the detected intensity lead to measurement errors. In order to resolve this problem, phase modulation was introduced and its feasibility was demonstrated on LHD [1] and is planned on ITER [2]. While the phase-modulated DI approach can achieve high density resolution, sufficient for density control, the temporal resolution (< 50 kHz) is not adequate for density fluctuation measurements. By implementing an acousto-optic cell based heterodyne technique, developed as part of a US-Japan collaboration [3], the DI bandwidth can be expanded into the MHz range and eliminate unwanted sensitivity to intensity variations.
Following successful proof-of-principle bench tests, a CO2 laser based heterodyne DI was installed on DIII-D. The round trip path length from the laser room to DIII-D is approximately 100 m and requires active feedback alignment [4]. The return fundamental beam power after the 100 m round trip is reduced to 0.3 W from 9 W injection due to transmission losses (primarily absorption in the BaF2 vacuum windows). Because the DIII-D DI is equipped with high efficiency nonlinear OP-GaAs crystals, the 0.3 W return power generates sufficient 2nd harmonic power and beat signal amplitude (400 mVpp) for phase shift extraction. The DIII-D heterodyne DI is capable of measurements of the electron density even in a disruption phase and shows good agreement with the density measured by the existing two-color laser interferometer [5]. The line-integrated density resolution, which is likely determined by offset drifts due to interaction with water vapor, is $\sim 3$ ×10$^{18}$ m$^{-2}$ for 1 s. This density resolution is achieved despite the presence of centimeter level motion during a discharge, the perpendicular component of which is cancelled at the nonlinear crystal by feedback alignment and the parallel component induced phase shifts by the dispersion interferometer scheme itself. Additionally, the 40MHz heterodyne beat frequency, determined by the acousto-optic cell drive allows low-noise measurements of density fluctuations into the MHz range.
This work was supported by US-Japan Fusion Collaboration Program FP5-3 (2015) and FP5-8 (2016), as well as U.S. DOE under DE-FC02-04ER54698 and DE-AC02-09CH11466.
[1] T. Akiyama et. al., Rev. Sci. Instrum. 85, 11D301 (2014).
[2] T. Akiyama et. al., Rev. Sci. Instrum. 87, 11E133 (2016).
[3] T. Akiyama et. al., Rev. Sci. Instrum. 87, 123502 (2016).
[4] M.A. Van Zeeland et. al., PPCF, in submission (2017)
[5] T.N. Carlstrom et. al., Rev. Sci. Instrum. 59 1063 (1988).
Primary author
Dr
Tsuyoshi Akiyama
(National Institute for Fusion Sceience)
Co-authors
Dr
A. Chavez
(General Atomics)
Dr
A. Colio
(Palomar College)
Dr
C.M. Muscatello
(General Atomics)
Dr
D.K. Finkenthal
(Palomar College)
Dr
D.L. Brower
(University of California Los Angeles)
Dr
J. Chen
(University of California Los Angeles)
Mr
J. Vasque
(General Atomics)
Mr
M Watkins
(General Atomics)
Mr
M. Perry
(Cal State University, San Marcos)
Dr
M.A. Van Zeeland
(General Atomics)
Dr
R. O'neill
(General Atomics)
Dr
R.L. Boivin
(General Atomics)
Dr
T.N. Carlstrom
(General Atomics)
Dr
W.X. Ding
(University of California Los Angeles)
