Thomson scattering system using ruby lasers

[Objectives]
Spatial profile measurement of electron temperature (T_e) and density (n_e) of JT-60U plasma with high precision, resolution (8~22 mm), repetition (time interval: 0.002 to 2 second) and reliability.

[Detectors, Diagnostic Method]
Thomson scattering light of multiple ruby lasers obtained through collection optics, fiber optics and spectrometers is detected by 20x12 photodiode array (PDA) for high temperature core plasma and 144 photomultiplier tubes (PMTs) for low temperature edge plasma. T_e and n_e can be estimated through a non-linear least square fit of fully relativistic Thomson scattering spectrum to measured one reconstructed with the detector signals and calibration data.

[Specification]

(1) Parameters of ruby lasers, beam combiner, and beam optics

-	Ruby Laser1 (PDS4, Lumonics)	Ruby Laser 2 (PDS2s, Lumonics)
Ruby Lasers
energy (J)	2.5, 5, 10, 20	10
pulse interval (sec)	1, 2, 4, single	4
pulse width (nsec)	30	30
divergence (mrad)	less than 300	less than 300
polarization	P	S
diameter (mm)	26	22
Beam Combiner
polarizer plate	transmission	reflection
Faraday rotation angle	90 degree	none
total energy loss	less than 5%	less than 2%
Faraday rod
dimension (mm)	45^f x 40^length	-
Verdet const. (min./Oe/cm)	-0.242 at 632.8 nm	-
solenoid coil
dimension (mm)	64^f x 197^length	-
coil current (A)	~750	-
number of turns	150	-
minimum time delay against PDS4 (ms)	-	2
Beam Optics
number of bending mirrors	8, including three computer-controlled mirror gimbals for adjusting a single beam path line
focusing lens	f/9.75m (beam diameter in vessel : 3~7 mm)
beam dump	colored filter glass
energy monitor
components	integrating sphere with ND filter and Fresnel lens, optical fiber PIN photodiode, and gated ADC/memory
location	just after beam combiner (input side) and just after beam dump (output side)

(2) Parameters of vacuum hardware, collection optics, fiber optics, and alignment system.

-	Edge Plasma Measurement	Core Plasma Measurement
Vacuum Hardware
window size (mm)	260^f x 20^t	260^f x 30^t
remote controlled shutter	yes	yes
viewing dump	fine ditched graphite	fine ditched graphite
Collection Optics
type	double Gaussian type lens	Cassegrain type reflection mirror
effective field of view	18 degree	46 degree
span of measurement (m)	~0.7	~1.4
spatial resolution (mm)	8, 16	22
F number	2.1	-
focal length (mm)	340	225.57
magnification	-0.22	-0.16
transmission	90%	75%
solid angle (sr)	0.0065	0.0055
Fiber Optics
core/clad diameter ( mm)	180/200, quartz	180/200, quartz
I/O bundle size (mm)	1.73^hx1.59^w and 0.80^h x3.50^w 3.50^hx1.42^w and 1.50^h x3.40^w	3.50^hx1.07^w and 1.60^h x2.40^w
N_A	~0.3	~0.3
length (m)	104	104
transmission (He-Ne)	45~50%	45~50%
measurement bundles	30	30
alignment bundles	4(2x2)	4(2x2)
Alignment System
for object field of collection fiber optics
parallel correction (Bt direction)	-6.5 mm to +6.5 mm (0.001mm step)	-5.5 mm to +5.5 mm (0.001mm step)
rotation correction on optical axis	-6.5 degree to +6.5 degree (0.001 degree step)	-3.5 degree to +3.5 degree (0.001 degree step)
function of regular monitor and auto-recovery through direct and quantitative procedure	yes	yes
for beam optics remote correction through the following procedure: (1) monitoring beam position on target screen detected by CCD cameras (2) adjusting beam line to prescribed path through three computer-controled mirror gimbals

(3) Parameters of spectrometers and coupling optics

-	Edge Plasma Measurement	Core Plasma Measurement
Spectrometers
type	Littrow x 1 (for PMT system)	Littrow x 2 (for PMT & PDA systems)
F number	2.9	2.9
focal length (mm)	275	275
grating (1/mm)	1200	600
grating size (mm)	97x100	97x100
output image size (mm)	60.0^hx51.3^w	60.0^hx51.3^w
spectral resolution of PMT	4~5	6
spectral resolution of PDA	-	12
spectral range (nm)	550~688	403~683
Fiber Bundles from Spectrometer to PMTs
core	glass	glass
clad diameter ( mm)	50	50
N_A	0.57	0.57
length ( m)	~2.3	~2.5
transmission	50~60%	50~60%
number of bundles	130 ch [spatially 30 and spectrally (4~5) ]	60 ch [spatially 10 and spectrally 6 ]
Wavelength Cutoff Glass for Core Plasma Measurement by PDA System
width in dispersion direction	-	4.0 mm for Ruby (694.3 nm) 4.0 mm for H_a /D_a emission 3.8 mm for H_b /D_b emission
Coupling Lens for Core Plasma Measurement by PDA System
field lens & collimator lens	-	achromat lens (made to order)
focusing lens	-	Noct Nikkor 58mm/F1.2S

(4) Parameters of PDA/PMT detector systems.

-	PMT System for Edge & Core Plasma Measurement	PDA System for Core Plasma Measurement
Detectors	photomultiplier tube (R943/R1387, Hamamatsu Photonics)	2-D photodiode array (PDA) with proximity focused type image intensifier
photocathode size(mm)	10 x 10 (GaAs) 34^f (S-20) partially used for core measurement	25^f (S-20, multialkali)
photodiode size (mm)	-	17.42 x 13.84
quantum efficiency
photocathode	0.13(400~700 nm) for GaAs 0.16 (400 nm), 0.09 (540 nm) for S-20	0.16 at 400 nm, 0.03 at 694.3 nm
photodiode	-	0.93 at 400 nm (peak wavelength of P-47)
phosphor decay time (nsec)	-	210 (P-47), up to 5% residual intensity
ADC gate width (nsec)	125 (ADC)	125 (Photocathode - MCP)
A/D conversion bits	( 11 in LeCroy 4301B)	12
A/D conversion time ( msec)	(~10 in LeCroy 4301B)	max. 3
data transfer time ( msec)	-	480 (signal 240 ch + background 240ch)
number of elements
for edge measurement	130 PMTs = spatially 30 x spectrally (4~5)	-
for core measurement	60 PMTs = spatially 10 x spectrally 6	240 ch /unit = spatially 20 x spectrally 12
dynamic range	more than 10⁵	~10³

[References]
[1] H. Yoshida, et al., "JT-60U Thomson scattering system with multiple ruby lasers and high spatial resolution for high electron temperature plasma measurement ", JAERI-Research 96-061; 1996, 20p.
[2] H. Yoshida, et al., "Beam Combiner for Transient Phenomena Measurement in the JT-60 Thomson Scattering Diagnostic", Rev. Sci. Instrum. Vol. 66, 143~147 (1995).
[3] H. Yoshida, et al., "Quantitative method for precise, quick and reliable alignment of collection object fields in the JT-60U Thomson scattering diagnostic", Rev. Sci. Instrum. Vol. 68, 1152~1161 (1997).
[4] H. Yoshida, et al., "Approach to a window coating problem by in-situ transmission monitoring and laser blow-off cleaning developed in the JT-60U Thomson scattering system", Rev. Sci. Instrum. Vol. 68, 256~257 (1997).
[5] H. Yoshida, et al., "Solution for a window coating problem developed in the JT-60U Thomson scattering system", JAERI-Research 96-062; 1996, 25p.
[6] O. Naito, et al., "Analytic formulation for fully relativistic Thomson scattering spectrum", Phys. Fluids B Vol. 5, 4256~4258 (1993).
[7] O. Naito, et al., "Relativistic incoherent Thomson scattering spectrum for generalized Lorentzian distributions", Phys. Plasmas Vol. 3, 1474~1476 (1995).
[8] O. Naito, et al., "A formula for reconstructing fully relativistic electron distributions from incoherent Thomson scattering data", Phys. Plasmas Vol. 4, 1171-1172 (1997).
[9] H. Yoshida, O. Naito, O. Yamashita, S. Kitamura, T. Sakuma, Y. Onose, H. Nemoto, T. Hamano, T. Hatae, A. Nagashima, and T. Matoba, "Multilaser and high spatially resolved multipoint Thomson scattering system for the JT-60U tokamak", Rev. Sci. Instrum. Vol. 70, 751~754 (1999).
[10] H. Yoshida, O. Naito, T. Sakuma, S. Kitamura, T. Hatae, and A. Nagashima, "A compact and high repetitive photodiode array detector for the JT-60U Thomson scattering diagnostic", Rev. Sci. Instrum. Vol. 70, 747~750 (1999).
[11] O. Naito, H. Yoshida, S. Kitamura, T. Sakuma, and Y. Onose, "How many wavelength channels do we need in Thomson scattering diagnostics?", Rev. Sci. Instrum. Vol. 70, 3780~3781 (1999).