J. Plasma Fusion Res. SERIES, Vol.5 (2002) 159-162

Passive and Active Gorpuscular Diagnostic Techniques forStudying LHD Plasma High-energy Particle Physics
GONCHAROV Pavel R., LYON James F.r, OZAKI Tetsuo2, SUDO Shigeru2, SASAO Mamiko2,ISOBE MitSUtAKA2, KRASILNIKOV ANAtOIii V.3, KUTEEV BOriS V.A, SERGEEV VIAdiMir YU.A,BAKHAREVA Olga V.a, KHLOPENKOV Konstantin V.2, TAMURA Naokiand LHD Experimental Groups | / II2
Graduate University for Advanced Studies, Hayama 240-0193, Japan1

Oak Ridge National Inboratory, Oak Ridge, TN 37831-8072, USA2

3

National Institute for Fusion Science, Toki 509-5292, Japan

Troitsk Institute for Innovating and Fusion Research, Moscow region, 142092, Russia
a St.Petersburg State Technical lJniversity, St.Petersburg, 195251, Russia(Received: 11 December 2001 / Accepted: 19 July 2OO2)

Abstract
Corpuscular methods are extremely informative particularly in experiments with a magneticallyconfined plasma devoid of toroidal axial symmetry. Spatially resolved measurements of atomic fluxes
emitted by the plasma are required to study physical aspects of various plasma heating mechanisms, fast
particle confinement properties and the related subjects. An advanced passive diagnostic has beendeveloped that enables such measurements to be made on LHD. Another method has been proposed thatis aimed at achieving essential locality of measurements and simplifying the data interpretation. The
method consists in active measurements of hot plasma emitted neutral particles with an injected pellet
ablation cloud as an artificially created localized target for the charge-exchange process.

Keywords:
corpuscular diagnostic, neutral particle analysis, charge-exchange, high-energy particle, ion distribution,
plasma heating, drift orbit, stellarator/heliotron, Large Helical Device (LHD)

1. lntrodustionTwo diagnostics are presented, in which thedevices by means of these particle techniques is ratherwide. The following areas should be mentioned: the ion

measured fluxes and energy spectra of atoms escaping

distribution function evolution due to different heating
mechanisms; fast ion confinement and transport; itsfrom the high-temperature plasma provide theinformation about its ion component. The passive

influence on the radial electric field, which is asignificant factor for plasma microturbulence andgeneral confinement properties. As a matter of fact,

diagnostic proposed in ref. [] has been successfullyimplemented and started to operate recently. An active
diagnostic fundamentally similar to ref . [2] is being
developed. Semiconductor detectors are used for neutral
particle energy analysis in both cases.
The scope of experimental studies that can beperformed on quasistationary magnetic confinement

these areas are strongly interdependent.

For a fusion reactor modeling, in addition tounderstanding the heating mechanisms, an importantaspect is the ignition condition when the plasma is@2002 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author's e -mail: pavel@ nifs.acjp
159

Goncharov P.R. et at., Passive and Active Corpuscular Diagnostic Techniques for Studying LHD Plasma High-energy Particle Physics

heated by o-particles. Fast particle confinement is also

particle spectra. However, the X-ray spectra byimportant for reducing the heat load on the walls.

themselves interpreted as bremsstrahlung/recombination

Theoretical models exist of the energetic ion
distribution due to ICR and NBI heating on LHD, and ofcontain valuable information about 2", plasma density

the energetic ion transport (e.g. t3, l). In ref. [5] a self-

Technical description: The lowest particles' energy

consistent model is given of the radial electric field andthe fast ion orbits.

that can be measured is about 5 keV taking into accountthe so-called dead layer consisting of the 500 A front
electrode and the 40 pg/cm2 aluminium coating eliminating the visible light from the plasma. The maximumand Z"tt.

The diagnostics (Fig. 5) described below arecapable of direct measurements of LHD plasmaenergetic particle parameters with time and spaceresolution.

measurable energy about 4 MeV is determined mostly
by the range of the particles in silicon and the depletion
layer thickness which is 300 pm at the standard detector

2. Passive Silicon Detector-based Neutral

bias voltage of 50 V.

Particle Analyser (SDNPA)
Geometry of experiments: A compact quasilinear hori-To minimize the thermally dependent leakage
current the detectors and the input stages of thepreamplifiers are mounted on liquid nitrogen cooled
substrates. This results in a noticeably better energy
resolution [] compared to other NPAs using

zontal array of six ion-implanted silicon detectors and a
vertically movable collimating aperture provide a twodimensional scan of the non-axisymmetric LHD plasma
column. The angle between the sight lines in the horizontal direction is 4.6o. The angular sector observable
due to the vertical scan is 14". The geometry of experiments is shown in Fig. 1(a). The value of the effective
plasma radius has been calculated as a function of thedistance from the collimating aperture along each viewing chord. The results are shown in Fig. 1(b) for themiddle vertical scan position for one sample magnetic

semiconductor detectors. The preamplifiers are locatedas close as possible to the detectors for noise reduction
purposes.

The diagnostic includes a remotely computer
controlled precision electromechanical system forvertical plasma scan and aperture selection. A digital
data acquisition system with histogramming memory
modules builds the detector amplifier pulse height
distributions and stores the data. Thus the energy spectra
ofparticles incident on the detectors are obtained.
Physical basis: The general formulae for the quantity
measured by the passive diagnostic are as follows:

configuration.
Several collimating apertures with different areas
are available in order to be able to adjust the atomic flux
so that it does not exceed the maximum acceptable forthe detectors. Since the detectors are sensitive to X-rays,
one of the apertures is covered by a 13 pm beryllium
foil to measure the X-ray background. It should be
subtracted from the measured sisnals to obtain neutral

rqlan = -Q-s f g(E;dEe-',u.,'dr(l)4tt Jo "t,.\ln,f ,(E.r)dE (2)'s(EtdE=l\nn(ov)", + n.\ov),,,' '"/'"'

Here f (E)dE [s-'] is the measured atomic flux andg(E)dE [cm-:s-t1 is the local differential atom birth ratein the energy range (E, E+dE), C2 is the visible solid
angle, ,S is the analyser's collimating aperture area, theviewing chord is supposed to extend from r = 0 to r =L; nsis the density of targets for charge exchange, n. electron density, fi@,r) - ion distribution function,(ov)"* and (dV)."" - rates of charge exchange and radiative recombination processes. The Poisson factor e (4')
determines the attenuation of the atomic flux in theOistare dor! viswiE chqd, mplasma.

Fig. 1 (a) SDNPA six viewing chords; (b) effective

plasma radius vs. distance along the viewingr(E,r.1=

chords at the middle vertical scan position for F =3'6 m, P=O'22%'
r60[r^*, '(n,()d(,(3)

Goncharov P.R. et al., Passive and Active Corpuscular Diagnostic Techniques for Studying LHD Plasma High-energy Particle Physics&10@task is to find such free parameters that f@) fits the-o- tD str #281' i, detel or *B-4- lD sh f281i i, dete, or #3

Bmeasured atomic spectrum. The latter approach is more

powerful and is endowed with more useful information.
Sample initial results: To illustrate the capabilities ofthe diagnostic, two examples are given. Figure 2 showsa typical X-ray background spectrum and total energy
spectrum measured in similar LHD shots by one of theSDNPA detectors (#3). The X-rays do not strongly affect the measured energetic tails of the neutral particle
distribution. On Fig. 3 the neutral particle spectra areshown measured for NBI heated plasma along three different directions, for detectors 6, 3 and 5. The angularseparations between these directions are equal. Angulardependence can be clearly seen. The viewing chord ofdetector 6 is the most tangential one. The plots in Figs.
2 and 3 are in logarithmic scale.

h.

i-q

o1@c=%.

@ ""% to&"L(J^LM10AAIAAAAAAw rk.9o-A

1051015?o25303540
Energy, keVFig.2 Total spectrum (circles) and X-ray background(triangles) measured in similar LHD shots (ECRH+NBl heated plasma)

3. Active Neutral Particle Analyser ("pelletcharge-exchange", PGX)
Active methods employ measurements of neutral
particle flux caused by charge-exchange between plasmaions and atoms of an artificially created target. Acollimated neutral beam can be used for this purpose.
This method was the first one to make possible
measurements of local parameters of the ion component
in a hot plasma. Since its invention this technique has
been the most progressive and at the same time the mosttechnically complex type of particle diagnostics.

The other more recent idea of a local ion
255075100

1150

component diagnostic is to use an injected solid pellet
ablation cloud as an artificial source of charge-exchange
atomic flux that can be measured in a similar way. Such

175

Energy, keVFig. 3 Neutral particle spectra from NBI heated plasmameasured along three different directions.

a diagnostic was realized on TFTR for studies ofwhere the integration is along the direction of sight; ,tr-ry
is the mean free path of an atom with respect to ionizing
events, namely the ion impact ionization, the electron
impact ionization and the charge-exchange process. The
expressions above are basic for the passive NPA data

energetic alphas and tritons [2].
LHD is equipped with a diagnostic pellet injector
capable of injecting impurity pellets with velocities ofthe order of 102 m/s. A PCX diagnostic is being
developed based on the existing injector combined witha natural diamond detector. Such a combination can be a
relatively compact and powerful active neutral particle
analysis system.
The locality of measurements in this case isinterpretation.

Various practical approaches are possible. For
instance, using the known cross-sections of the relevant

determined by the overlap region of the toroidally

elementary atomic processes some simplifying

elongated pellet ablation cloud and the detector's coneof sight. The detector observes the pellet trajectory frombehind. The angle between the viewing chord and theinjection axis is 2.5o horizontally and 1o vertically. PCX
experiment geometry is shown in Fig. 4 (a) and (b). Fig.
5 shows the relative location of the diagnostics.
Suppose that the required spatial resolution / isassumptions can be made to obtain a working equation
for l(E)dE that reflects the ion distribution in a lesscomplicated lnanner. The other routine involves
modeling f(E) using the cross-section data and the
independently known plasma parameters. The unknown
values are treated as free parameters. In this case ther61

Goncharov P.R. et al., Passive and Active Corpuscular Diagnostic Techniques for Studying LHD Plasma High-energy Particle Physics

If energy losses and scattering of protons in theabout 10-1 m. A pellet at the velocity vpsr of 103 m./swould pass this distance in the time T about l0-a s. Let
us assume that the total number of counts N needed tobuild a spectrum is of the order of 103-104. This would
translate into the required count rate CR = NIT = 107108 s-1. Therefore the analyzer should be able to handle
each hit event in the time interval 10-100 ns. If Cft^u" isthe maximum acceptable count rate, the number ofcounts that can be collected N < CR^u*llvp"r. The
operating speed of the analyzer should be high to enablemeasurements to be made with the desired spatialresolution. Although there is a certain flexibility in vo"1,cloud are negligible and the typical linear size of thecloud L"1ou6 is small compared to the linear scale ofspatial variation of the proton distribution function/\_|L-,^..u, ldtnJtvt I . then the total emission of atoms H0\orlwith energy E from the visible area of the pellet ablation
cloud per second is given by the formula:f(E)dE = Fo(E)viS"r,"d j(E)dE,(4)where Fs(E) is the fraction of the incident protons neutralized by the cloud, fi@) is the proton distributionfunction, 4 is the velocity corresponding to the kinetic
energy E, and S"1ou6 is the area of the ablation cloud visible to the detecting element.

higher CR.* is preferable.

The count rate(b) ,.

dN"oE; for the atoms with energy E

measured by a detecting element of an energy analyser
can be calculated as a product of f(E), the atomic fluxo8

attenuation factor and the geometrical coefficient S"/

u 06
F4ttL2:6;.M(rlA/
5*= Ff (E)viS"rouo fr(E)e-"'E't'--:q -.
sE aI
4EL'Ru0.230 ua4 a6 3.0 40
DidtrdqEviilirEcM,

12(5)Here Su is the analyser's collimating aperture area, I is
the distance between the pellet cloud and the collimator.
A piezoelectrically driven variable width slit isused to adjust the atomic flux to the detector. A fastrecording oscilloscope and a set of discriminators areplanned to be used to obtain the detector pulse height

11m

Fig. 4 (a) PCX viewing chord and pellet injection axis; (b)effective plasma radius vs. distance along theviewing chord for F = 3.6 m, I = O.22To

spectra. Initial measurements are to be made in LHD
experiments with this new diagnostic.

Acknowledgements
The authors would like to thank members of NIFS
technical personnel for their kind cooperation.

References

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t2l S.S. Medley et al., Rev. Sci. Instrum. 67, 3122
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t3l S. Murakami et al., J. Plasma Fusion Res. SERIES
2,zss (1999).

[4] S. Murakarri et al., Nucl. Fusion 40,693 (2000).
[5] S. Murakami et al., Plasma Physics Reports 23,512(reel).

oi"cJ,.

t6l R.K. Fisher er al., Fusion Technol. 13, 536 (1988).

Fig. 5 Relative position of the diagnostics and neutralbeam injectors on LHD
t62