J. Plasma Fusion Res. SERIES, Vol.5 (2002) 175-178

Neutralization Effect on Energetic Proton
Gonfinement in LHD
SASAO MamiKol, KRASILNIKOV Anatolii V.', ISOBE MitsutaKa, SEKI Tetsuo, SAITo Kenji,SAIDA Tomoya, MURAKAMI Sadayoshi and the LHD-Experimental Group
National Institute for Fusion Science, Toki 509-5292, Japan
1

Troitsk Institute for Innovating and Fusion Research (TRINITI), Troitsk, Russia(Received: 14 December 2001 / Accepted:24 June 2002)

Abstract
The neutralization effect for energetic protons in Large Helical Device (LHD) was estimated fromthe high energy neutral flux measured by Natural Diamond Detectors (NDD). The neutral- and partiallyionized-particle-distributions are calculated with a code for the incoming H0, Heo transport (AnalyticalCalculation of Helium Neutral, ACHEN-Code). The electron density dependence of the effective He+
density at the ICRF resonance region derived from measured neutral flux of high energy particles is well
reproduced by the neutral-transport calculation with the incoming neutral temperature of about 10 eV andthe neutral density at the Last Closed Flux Surface (LCFS) of 10-3 x n". The estimated loss rates are
consistent in factors with the saturation level of decaying time of tail temperature after the termination ofthe ICRF heating.

Keywords:
Neutralization, High-energy particle, High energy tail, Large Helical Device, Diamond Detector, ICRFheating, Neutral particle

1. Introduction
In Large Helical Device (LHD), high-energy
neutral flux and spectra are measured by Natural
Diamond Detectors (NDD) [], Si-diods l2l, andperpendicular ions are well confined in the center regionof the LHD. However, deviation from the classicalprediction was observed in the outer region of theplasma, indicating ripple-induced transport and charge-exchange-losses [7]. In separate experiments, therelaxation time (td""ay) of T4 in decaying phase after
ICRF termination is studied. It is consistent with theclassical prediction when /6""u, < 0.lsec, but it saturates
at around 0.15-0.2 sec, indicating existence of loss
which has a time constant of this range [6].
Among various loss mechanisms of high-energy
ions in magnetically-confined fusion plasma,neutralization loss is one of the key processes. This isconventional Neutral Particle Analyzers (NPA) t3l,
during NBI sustained, ICRF sustained, or ICRF assisted
NBI discharges. With a help of a code to calculate H0,Heo, He* distribution (ACHEN-Code) [4], the effective
temperature, I"n, of an energetic tail formed by ICRFwaves is obtained for most of ICRF discharges since the3'd Champaign [5-7]. Comparison of measured effective
perpendicular lemperature of ICRF driven H* minority
ions with the classical Stix model indicates thatCorresponding author's e-mail: sasao@ nifs.ac.jp
t Present address: Departrnent ofQuantum Science & Energy Engineering,
Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan.
s asao @ qs e. to hoku. ac.j p175

@2002 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchSasao M. et al., Neutralization Effect on Energetic Proton Confinement in LHD

l^"u,(E) to the proton energy distribution functionfi (E)loss of energetic particles that are neutralized throughcharge exchange processes with residual neutrals orthat separates the z-dependent and E-dependent factors,

partially ionized ions in a plasma. The magnitude of thiscontribution is strongly related to their orbits, because
excursion from the magnetic flux surface to outer region
where the neutral density is higher, enhances theneutralization probability.
In the present work, the neutralization effect has
been studied by analyzing the high energy neutral fluxof a vertical viewing chord measured by NDD during
ICRF-heating experiments. Here the discharge gas ishelium with small amount of H-minority. The measured
neutral flux is compared with that estimated from thedensity of protons which are coupled with ICRF, andneutral and partially-ionized particle density in a corel^"u" (E) = r1g s g(E) frlD lnrk) nn"*(il dz '(1)has been used for the analysis. Here e is a vertical coor-dinate along the viewing line. In the present study, weconsider H0, HeO, and He*, as particles resposible for theneutralization, but the contribution of the He' is mostimportant, so we use the profile of He* but the energy
dependence of the sum of all contributors. We first calculate the z-dependent factor, no(z) ns"*(l,and integratealong the line of sight, by using the calculated He'profile (ns",(z)) and the ICRF power deposition profile nor-malized in to l4 7o proton density(no(z)), because thecomparison of measured effective perpendicular temperature with the classical Stix model indicates that 14
Eo of total protons resonate to the ICRF waves [8]. Thenu"*(z) profile depends on the neutral temperature 20,
across LCFS and its density r0. These parameters arediscussed in the next section. Examples ofz-dependence
are shown in Fig. l. The velocity (energy) dependent
factor g(E) is neutralization efficiency and is summationof contributions from all possible atoms/ions (denoted asplasma. The latter is calculated using a code to calculate

H0, He0, He+ distribution (ACHEN-Code). Once thecalculated H0, He0, He* density distributions and source
neutral temperature are verified, it is possible to evaluate
the total neutralization loss.

2. Experimental Setup and Data Analysis
Two sets of NDD of vertical line of sight are usedin the present work [4]. Each detector assemble has adouble slit system, but the geometrical efficiency, 4,i), that can transfer one electron to a proton.
s@) = Ziloivlp.nl(zp)lns., (zp) -and the viewing area, s are determined by the detector
size (2 mm) and the changeable aperture of 525 mmapart. The aperture of the center chord, which provides(2)

We consider only H0. Heo. and He*. because the impurity concentration is low in LHD. Here 4p is a representative point through the viewing line, such as theresonance point of the ICRF wave. Finally the derived
proton spectrum,fo @) is fit into an exponential line,

most of measurement of present study was fixed at 2mm(n, = 1.53x10-7, s = 2xl0-a m2;. The details of themeasurement setup are discussed in ref. 1.

A simplified relation [6] of measured neutral flux,
fp (E)dE = vLdvLt@/2-l/'(Ttlt2

exp{-((mv12l2| T)ll,(3)6 lotoassuming that the parallel velocity distribution is that ofthermal ions.

5 loto EoE

troo-l toto 3&tu

3 't0'" F
410z toto Eozr ro'o $93. The Incoming Neutral Temperature andthe Neutral Density
In the procedure to derive the effective

o5000.20.40.60.8

perpendicular temperature 21, the calculated He+profile(nr.,(z)) is assumed. However this calculation contains

two free parameters, the neutral temperature 7"0,01

incoming across LCFS and its density n0. The electron
desity dependence of nHe+ at certain radius inside theplasma core is determined by To, while the overall
absolute value is affected by n0. Some examples of time
traces of ICRF sustained discharges of LHD at Br= ).$/,
T, R* - 3.15 m, are shown in Fig. 2, together with thoseFig. 1 ICRF power deposition profiles of 38.47 MHz(type-S) and of 40.47 MHz (type-6) at B. = 2.75 f ,F, = 3.6 m are shown by solid lines. Dotted lines
indicate those multiplied by the nr". (z) profile calculated with 7a= 10 eV, and n0 = 10-3x n", whereD"=1x1013/cm3.
176

Sasao M. et al., Neutralization Effect on Energetic Proton Confinement in LHD

1.522.533.5

22.5

20.5

210 t8oEg

6096i;?lrro'+;40Ei020Eg

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3 !8,EEEE

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2Fig. 2 Time traces of ICRF sustained discharges l#24045, 24051,240691 of LHD at 4= 2.64 T, R, = 3.75 m, together withthose of Ir, and measured neutral flux @. The minority concentration is kept nearly constant (7%) while the electron density varies from 0.3 x 1013/cm3 (#24069],,1 x 10r3/cm3 (#24051\ to 2 x 1013/cm3 (#240451, and L are changing
accordingly. Notethatthe neutral beams are injected after t=2 sec, and short but strong gas puffing at 1.5 sec on#24045.

1or0(#24069), both Zt and (D are increasing. The dependence
1eVof He* density estimated from O upon ne (Fig. 3) is1oev

It
1-.-------30eV----- a 24051
r 24o4sV 24069

3oev(no=0.02%n 1'e'I

compared with the calculated ns"*(z) at p - 0.5, where
the ICRF resonance surface exists, for three cases of ?0,(30 eV, l0 eV, I eV). As the 70is diminished, the attenuation ofneutrals become severe, and the He* density
decreases. Good agreement is obtained for Z0 = l0 eV.

Eo{.''=c.^tu I

The absolute value of nu"* (z) and hence n0 can beobtained from O by normalizing it with q, s J no(z)nn *(z)dz calculated for each T0, thus obtained. In thepresent case, the n0 of 0.4 x lO-3 n", obtained with 70 =10 eV is consistent with the measured density depen-t,o+oE\'.

1eV1no=3olon")\ b.

t'\\1o€v(no=o'04%n")

dence .

108

00.511.5

24. Neutralization Loss and Conclusion

2.5

High-energy neutral particles detected are actuallyparticles lost from the plasma, and directly related to theneutralization loss. The local information on theneutralization loss and its energy dependence can be
estimated by using the 70 and no derived from thecomparison of the density dependence and absolute
neutral flux with the calculation. Using these parametersn"-av. 1cm3;

Fig. 3 The electron density dependence of nHe* k) estimated from <D upon n" (symbols). The lines arethose calculated at p = 0.5, where the ICRF resonance surface exists. Good agreement is obtainedby the neutral-transport calculation with f0 = 10
eV and n(neutral)/n =4 x 104.

obtained in the analysis described in the previoussection (with Z0 = l0 eV, n0 = 0.4 x l0-3 n"), and theneutralization loss profiles are estimated as in Fig. 4.

of Zs, and measured neutral flux @, which is themeasured proton spectra integrated over E, defined by@ = It^,^(E)/s@) dE= r1rs .[nrk) ns",(z) dz.

Here, we used the effective neutralization probability ofg(E) as defined in eq. (2), including three charge
exchange rates. A short neutralization time causes large
neutralization loss. Figure 4 indicates that the neutralization loss is not negligible when the density is less than
10r3/cm3 in the peripheral region (p > 0.7), especially

(4a)
(4b)

As the electron density varies from 2 x l0t3lcm3(#24045), I x l0r3/cm3 (#24051) ro 0.3 x 1013/cm3
17'7

Sasao M. er aL, Neutralization Effect on Energetic Proton Confinement in LHD
T.=2kev,n(noutral)/n.(o) = 10processes. However, the present calculation shows that{, T o=10ev

particles of energy higher than 100 keV have longer
confinement time, while the saturation in the Stix
diagram shows the faster loss for higher-energy.!.r..^.

(q-. \_.n - lxl0Oo13particles.

In this study, the orbit effect that the excursion ofhigh-energy particle orbit from the magnetic flux

cm,.q-r.*.r,.

surface is not considered. Actually this effect is strongly
coupled to the neutralization loss, and affects more forhigher energy particles. The study on the combinedeffect of orbit excursion and neutralization is now indNprogress.
OReferences

[]
00.2

0.4

0.6

0.8

Isobe M., Sasao M., Iiduka S., Krasilnikov A.V.
et al., Rev. Sci. Instrum. 72, 6lI (2001).

[2] Osakabe M. et al., Rev. Sci. Instrum., 72,788

1(2001).

pFig. 4 The neutralization loss time estimated at the electron density of 0.5 x 1013/cm3, 1 x 1013/cm3and 3 x1013/cm3 of helium plasma for energetic protons of[3] Ozaki T. et al., Rev. Sci. Instrum. 71,2698 (2000).
[4] M. Sasao et al.,Proc. "Advanced Diagnostics for
Magnetic and Inertial Fusion", (Varenna, 2001),
edited by P.E. Stott et al. (Plen]um Press, NewYork), pl29 (2002).
[5] Mutoh, T. et al., Phys. Rev. Let.85,4530 (2000).

50 keV, 100 keV, and 200 keV. when n(neutral)/n =4 x 104, and 7'o = 10 eV.

for ions in the energy about 50 keV. When the electron
density is I x 1013/cm3, the characteristic neutralization
time at the ICRF resenance region (0.4 < p < 0.7) is40 - 2OO ms, which is consistent with the saturation[6] Sasao M. et aI., in Fusion Energy 2000 (Proc. 18h

temperature after the ICRF termination [6], indicatingthat the neutralization is one of the dominant loss

t8l Saito, K. et al., Nucl. Fusion, 41, l02l (2001).

Int. Conf., Sorrento, 2000), IAEA, Vienna (2001)
CD-ROM file EX9/l .
[7] Krasilnikov A.V. et al., Nucl. Fusion 42, 759

level of decaying time (150 - 200 ms) of tail(2002\.

178