J. Plasma Fusion Res. SERIES, Vol.5 (2002) 193-196

Design of the Gollective Thomson Scattering Diagnostics
for Large Helical Device Using a Ouasi-opticalFrequency Tunable Gyrotron as a Radiation Source
PAVLICHENKO Rostyslav, IDEHARA Toshitaka, OGAWA Isamu',
SATO Motoyasu2 and KAWAHATA Kazuo2
FIR Center , Fukui University, Fukui 910-8507, JapanI Faculty of Engineering, Fukui University, Fukui 910-8507, Japan
2

National Institute for Fusion Science, Toki 509-5292, Japan(Received: 11 December 2001 / Accepted: 28 August 2002)

Abstract
Development of the collective Thomson scattering (CTS) diagnostic system for LHD is presented.
High frequency, tunable (87-91 GHz), medium power (-100 kW) quasi-optical gyrotron will be used asaradiation source. We will show the detailed description of the system as well as initial calculations of
the scattered microwave power from the LHD plasma. Using the quasi-optical gyrotron as a radiation
source was inspired by the fact that this particular evice has the ability of tuning of the operationalfrequency. Which is made it very attractive for using under the various scenarios for LHD plasmas.

Keywords:
collective Thomson scattering, sma diagnostics, ion temperature, LHD, quasi-optical gyrotron

1. Introduction

frequency spectrum at the selected wave vector.

Collective Thomson scattering (CTS) has been
traditionally used for measuring the electron and ion
temperature in laboratory plasmas. The detected signal
is the one thal originate due to the scattering of theelectromagnetic radiation by the Debye cloud ofelectrons, which effectively surround each ion. Theseclouds of electrons move with the ions and impart a

For a plasma containing several ion species thescattering form factor can be written in terms of G" andG1, the dielectric susceptibilities of the of the electronsand of the ions of species i.
S(k, a) = It

-D c,l'r.+ n;'lc"l'22? x,r,(2)lr-o"->o,l'Doppler shift to the scattered radiation.
Theory shows that the net scattered power isdependent on the amplitude of fluctuations in electron
density [1-3]. The scattered radiation power P"(ar) per
frequency interval dal into solid angle interval dQ is:F" and F1 are the velocity Maxwellian distribution, ofelectrons and ions of species i, Ni the number density ofions of type i, and 21 is their charge. The dielectric susceptibilities G" and G1 are related to the plasma dispersion function W(E) byP"(a)dQda = P,Yr2on"LrrS(k, a)dQdot , (1)G"=-W(6)u2, and G,=-2,+WG)ar. (3)where P1 is source power, 162 is classical electron radius,

Y and Lor are geometrical factors. S(/c, al) is the scattering form factor, containing the information about theC orre spondin g autho r' s e -mai I : s lav ik @fir.fukui- u. a c@2002 by The Japan Society of Plasma
Science and Nuclear Fusion Researchjpr93

Pavlichenko R. et al., Des\gn of the Collective Thomson Scattering Diagnostics for Large Helical Device

io'uw(€)=

r-2(exp(-fl,t " rfo,)- i tn(expt2'IO -(4)-with { = (alkv), v =',l2KTl* is the thermal velocity andSalpeter parameter a = llkho [4] - the usual scale

1o'-Fglength to Debye length ratio.
The form of S(k, cr) depends on a. For small scale

d9.^fluctuations (a << 1) the electrons become uncorrelated.
This is the case of incoherent Thomson scattering, forwhich S(ft, at) is simple Gaussian. In opposite case (a 2lot1), collective scattering fluctuations occur. S(ft, ar)consist of a low frequency part, called ionfeature at c4= kvTi, and high frequency part near electron plasma

t

loto

10

1oolooo

1onlouplsna t€mpeEtu€, levl

wave frequency.

Fig. 1 Plot of condition a= llkh = 1.5 for 92 GHz quasioptical gyrotron and CO2 laser radiation as function of density and temperature for LHD plasmas.

2. Determination of the Scattered AnglesDensity and temperature for collective type ofUsing of 90 GHz range radiation allow to achieve
larger u values. For the typical LHD plasma parameters(Bo= 2.75 T, ft" = ni= 5 x 1013 cm-3, and Z" = Ti = 3keV) we conduct the calculation of the possible
scattered angles. Those estimation shows that almost
any kind of the scattering experiment is possible. The
scattered angles vary from near very small angles - 0*
5' (quasi-forward scattering) to almost backward

scattering are located over each line.

frequency tunability and the geometric separation between the spent electron beam and the microwave output. Above mentioned characteristics offers flexibility
for experiments on fusion devices, where it may be advantageous to change the localization of power deposition zone (even for diagnostic usage) without changing
the magnetic field and therefore affecting the plasma

scattering values - 180'. From the expression ofSalpeter parameter a= llkLo:.

properties.

^,^ 0", l.o8x lo4li /r"1.,nt1.\''s'ni'=

a t-i."t i

For coupling the output gyrotron radiation to a(5)corrugated circular waveguide matched optical unit
(MOU) is used. MOU mirrors (mirrors A and B at theFig. 2) are designed to reshape beam profile fromgyrotron output so that the radiation smoothly couples
with HE11 waveguide transmission mode.
Transmission line: At present time for the delivery microwave radiation to the plasma we are planned to use
one of the present ECH transmission line2 [5]. CTS
transmission line (Fig. 2) is using 31.75 mm evacuatedcomrgated waveguides. The full path will have l0 miter
bends and about 100 m in length. This transmission system was optimized for delivering of 7O-90 GHz microwave radiation. The probing microwave beam, accord-

one can see that utilization of lasers (for instance CO2laser) is possible for a very small angles only. This fact
will be make separation of insident and scattered beams
more difficult. A plot of condition d = 1.5 for 92 GHz
quasi-optical gyrotron and CO2 laser radiation as function of density and temperature for LHD plasmas (seeFig. 1).

3. Description of the System
Gyrotron and MOU: The proposed system for LHD
will utilize the quasi-optical (Q. O.) gyrotron as a power

ing present plans, will be injected through the downside
1.5 L port. The detection part will be placed at corresponding upper side port. The investigation ofthe possi-source. The Q. O. gyrotron designed for operation at thefundamental frequency (a = d)c) in the range of 92 + 5
GHz and in the range of 2 x (92 + 5) GHz at the second
harmonic (a = 2d)"")t In Q. O. gyrotron the resonant
structure is a Fabry-P6rot resonator placed transverselyto electron beam and operating in the pure GaussianTEMgoq mode. The advantages of Q. O. concept are therAt present time the gyrotron under conditioning atFIR, Fukui University.
2Desiened at NIFS
194

Pavlichenko R. et al., Design of the Collective Thomson Scattering Diagnostics for Large Helical Device.9 1.0E

SNR=5
Av=4GHz.9Fo

0.1:!(!

Av=2GHz

3 o.or-16

Fig. 2 Schematic diagram of CTS transmission line.

-14-12

loglo Ps [w/Hz]

Fig. 4 Estimated SNR of the heterodyne detection sys-tem.

Local Osc.

75GHzNolch filterHeterodyn€fIn the real plasma experiment, measurable spectral

re@iver

width is constrained to lie within the bandwidth
capability of heterodyne detector and conventional

Preamplifi6r andmulti- channel

electronics, typically about of several GHz. In the casefiltor bankwhen both temperatures, for electrons and ions, arepractically the same (7" = T) the spectral width could begiven by kvil2n = ",l2otoil2na. This condition can be

Data
Acquisition

easily met by using a sufficiently high a. If heterodyne
detector noise dominates, the parameters of frequency
analyzing circuit can be derived from the signal-to-noiselevel at the detector s = Plu\NEP (NEP = 8.6 x 10-17
WIJz-ttz) and the output of frequency analyzer S =fi 1 avss41 + s), where P,^u is power of scatteredradiation delivered into heterodyne detector per Hz
bandwidth, Ay is the bandwidth of resolution interval

Fig. 3 Schematic diagram of the heterodyne detection
system.

and r is integration time (which equal to gyrotron pulse
length). Estimated SNR is shown on the Fig. 4.

bility of using horisontal port for scattered radiation isunder way now.

4. Galculation of Expected Scattered
Spectrum from LHD Plasma

Detection system: Use of middle power gyrotron require good signal-to-noise ratio. Thus, a heterodyne typedetection system will be used (Fig. 3). The scattered radiation from LHD plasma, which consist of the incident
carrier and Doppler shifted parts, will be mixed with 75
GHz local oscillator (LO) signal. To suppress high radiation from the incident beam the tunable band-rejectfilter have to be inserted in front of detector. To obtainfrequency spectra at post detection stage the signal willbe split into several parts. To evaluate detailed spectrumof the scattered radiation each channel has its own intermediate frequency range that depends on corresponding
part of microwave spectrum.

Upon the inspection of the form factor it was foundthat a wide low part of the spectrum coming from thefirst term in Eq.(2), and tall narrow part coming fromthe second term of the same equation. The latter stems
from the collective motion of the electrons with the ions,which is what we are concern to measure. Our
calculations have been confined near the values ofangular frequency al for which second term makes asubstantial contribution to the whole spectrum, i.e. co S
3 ku,

The expected scattered spectrum from LHD neutral
195

Pavlichenko R. et al., Design of the Collective Thomson Scattering Diagnostics for Large Helical Device

there is sufficient scattered power, expected from bulk
ions, and the contribution from electrons is three orders
lower.

5. Summary
We have performed initial design of the LHD CTS
system based on the Q. O. gyrotron as a radiation
source. The main advantage of the using gyrotron as aradiation source instead CO2 laser is as follows. In thecase of lasers, the spectrum changes very rapidly withangle, only small-angle scattering can be used. This
complicates obtaining high spatial resolution Ar. InNIi ro'oo;
T10Yo6.Y-rn- 10----: : - - -;;;;;"- - - - - - -contrast, at gyrotron frequencies, much larger scattering
angles are available. (Arru, = 43 cm, Lrgy, = 2.7 cm).
The spectra change on slowly with angle, andreasonably large collection solid angles can be used limited only by coherence requirements for heterodyne05101520
trequency, [MHz]Fig.5 The expected scattered spectrum from LHD
plasma as a function of a Doppler shifted frequency from the gyrotron centered frequency.
Pfasma parameters : Bo=2.75T, n.= n, = 5 x 1013
cm-3, 7" = T=3 keV, E*", = 150 keV. We assume
that electrons and bulk ions velocity distribution

detection. The choice of the scattered angle will depend
primarily on effects of plasma refraction. For thesignificant part of the expected spectrum the fast ionscomponent is dominant.It will be encouraging fact forthe successful evaluation of the a-particles temperature
directly from the total scattering signal.

functions of the plasma are Maxwellian and that ahigh energetic ions have an isotropically 1/vp
slow-down distribution.

Preliminary estimates show that middle-power
gyrotron with power of up to hundred watts, and pulse
duration about l-2 msec is well suited for ion feature

heated plasma as a function of a Doppler shiftedfrequency from the gyrotron centered frequency isshown on the Fig. 5. For spectrum simulation thefollowing plasma parameters are chosen: Bo = 2.7 5 T, n"= r\= 5 x 1013 cm-3, andT.= 4 = 3 keV, E6"u- = 150
keV. We assume that electron and bulk ion velocity
distribution functions of the plasma are Maxwellian and(alphas.l have an isotropic l/v3 'slow-down' distribution.

evaluation.

References[1] J. Sheffield, Plasma scattering of electromagnetic

For evaluating of the scattered form factor, so-calledradiation, Academic Press, London, (1975).
[2] T.P. Hughes and S.R.P. Smith, Rev. Sci. Instrum.
72(6), rO75 (1988).

Salpeter approximation has been used.

t3l H. Bindslev J. Hoekzema, T. Huges and J.

The calculation was done for vertically elongatedplasma. As a receiver 20 dB gain conical horn antenna

Machuzak, Phys. Rev. Lett. 83, 3206 (1999).
[4] E.E. Salpeter, Phys. Rev. 72,1528 (1960).
t5l T. Simosuma, S. Kubo and M. Sato et a/., FusionEng. and Des. 53, 525 (2001).

was used. The position of antenna was changed
according to the values of scattered angle. The resultsshows (see Fig. 5) that for the wide frequency range,

196