J. Plasma Fusion Res. SERIES, Vol.7 (2006) 18-21

Diagnostics of Charge Exchange Neutrals from Multi-Ion-Species
NBI-Heated Plasmas on LHD
GONCHAROV Pavel, OZAKI Tetsuo, LYON James1 , SUDO Shigeru, SERGEEV Vladimir2and LHD Experimental Group
National Institute for Fusion Science, Toki 509-5292, Japan
1 Oak Ridge National Laboratory, Oak Ridge, TN 37831-8072, USA
2 St.Petersburg Polytechnical University, St.Petersburg, 195251, Russia（Received: 5 October 2004 / Accepted: 10 January 2005）

Abstract
Fast ion kinetics and thermalization in neutral hydrogen beam heated hydrogen and noble gas target plasmas are studied on LHD heliotron by passive multi-chord and active radially scanning diagnostics of charge
exchange (CX) neutral particles. In the passive method the natural creation of fast atoms within the plasma
and their coming out to the periphery depend on the overall neutral balance. In active measurements the ionization / neutralization balance in an impurity pellet cloud is essential. The application of the relevant atomic
elementary process data to the analysis of NPA measurements is described. The eﬀect of the target plasma
ion composition on pitch angle scattering intensity observed on energetic ion spectra is discussed.
Keywords:charge exchange, pellet charge exchange, multi-component plasma, neutral particle analysis,
fast ion distribution, particle beam, plasma heating, heliotron, Large Helical Device (LHD)
1．Introduction
The formation and evolution of suprathermal ion distribution function tail under the influence of the tangential hydrogen neutral beam injection (NBI) is studied on
LHD by the neutral particle analysis (NPA) technique.
The multidirectional passive analyzer [1] and active pellet charge exchange (PCX) diagnostics [2,3] are used.
In this paper the emphasis is put on the application of
the relevant atomic elementary process data to the analysis of these NPA measurements.
Calculating the fast ion distribution function fromthe energy resolved naturally occurring chargeexchange neutral particle flux requires the accurate
modeling of the measured quantity [4], which assumes
the knowledge of the type, density, and radial profilesof the charge-exchange centers.
In local PCX measurements a solid impurity pellet
is injected in order to create an artificial localized target for the enhanced charge exchange process in theplasma. To obtain the local ion distribution function
from the measured charge exchange atomic flux originating from the pellet cloud, one needs to know theenergy dependent neutralization ratio for the fast ionsentering the cloud. This is governed by the neutralization / ionization balance equations in the cloud [5].
The next section describes the elementary processes

indispensable for the passive NPA data analysis in caseof multi-component plasmas. Then, polystyrene pellet
charge exchange measurements are considered and finally, the application of the CX cross-section data to thestudies of Ze f f influence on fast ion spectra is discussed.
2．Interpretation of passive neutral fluxes
The formation of the flux of neutral atoms from aquasistationary plasma column and the attenuation ofthis flux on the way out to the periphery are determinedby the three groups of elementary atomic processes, viz.
charge exchange, recombination and ionization. The
charge exchange process is of primal importance forcalculating both the local source function of atoms in
the plasma and the loss of atoms along the observation
direction.
H+ CX cross-sections [6] on He0 and He+ shown in
Fig. 1 need to be taken into account in calculations offast proton distributions resulting from neutral hydrogen beam injection into helium plasma. In studies ofhigh-energy proton distributions in multi-species plasmas including Ne and Ar components the charge capture cross-sections by protons from the atoms and ionsof these noble gases are required: H+ + Nek+ → H0 +C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchCorresponding author’s e-mail: pavel@nifs.ac.jp

18

Diagnostics of Charge Exchange Neutrals from Multi-Ion-Species NBI-Heated Plasmas on LHD

As an example of the analytical and empirical behaviour at typical suprathermal energies, Fig. 1 illustrates the experimental H+ /Ne0 and H+ /Ar0 CX crosssections [13,14] shown by circles in comparison with
Gryzinski’s theoretical model estimations [7,8] shownby solid curves.
3．Pellet charge exchange
In order to calculate the local ion distribution function from the time and energy resolved atomic fluxΓ (PCX) (E, r (t)), besides the knowledge of the geometry of experiments, one needs to have the informationabout the energy dependent dimensionless factor F 0 (E)[2]. This factor determines the fraction of particles thatenter the pellet ablation cloud as ions and exit as neutralatoms.
As it was shown in [15] and [5],F0 (E) = 1/(1 + σ0→1 /σ1→0 ),where σ1→0 and σ0→1 are hydrogen neutralization andionization cross-sections in the cloud respectively.
Figure 2 shows F0 (E) calculated in [5] for apolystyrene (−C8 H8 −)n pellet ablation cloud. The
cloud was assumed to consist of 25 % C1+ , 25 % C2+
and 50 % H0 . The relevant elementary atomic processes
involving carbon in this case are as follows:

Fig. 1 Lower: experimental cross-sections of electron
capture by H+ from He0 (solid squares) and from
He+ (open squares) [6]; middle: experimental(open and solid circles) [13,14] and theoretical(solid line) [7,8] H+ / Ne0 CX cross-sections; upper:
experimental (open and solid triangles) [13,14]
and theoretical (dashed line) [7,8] H+ / Ar0 CX
cross-sections.

H+ + C1+ → H0 + C2+ ;H+ + C2+ → H0 + C3+ ;H0 + C1+ → H+ + C0 ;H0 + C2+ → H+ + C1+ .
The possible recombination processes and the electron impact ionization processes are neglected. Thecross-sections of hydrogen ionization by charge exchange with C1+ and C2+ ions are given in [16]. The
H+ /H0 CX cross-section is very well known. The crosssections of the electron capture by protons from C1+Ne(k+1)+ and H+ +Arl+ → H0 +Ar(l+1)+ . The experimental data on these cross-sections is not abundant. However, various theoretical estimations exist, among which
historically the first one is the approximation suggested
by Oppenheimer, Brinkman and Kramers based on thequantum perturbation theory. This so-called OBK approximation, e.g. monograph [7], allows one to obtainan analytic expression for the charge transfer probability amplitude. More profound theoretical models andscaling laws exist, e.g. [8,9].
The authors of [10] propose a treatment of the charge
state distribution in neon plasma involving their upgraded code [11]. When fast ion distributions are of interest, it is enough to know the high energy behaviour ofthe charge exchange cross-sections for σE-correctionof the neutral particle energy spectra, while the absolute values of the cross-sections are not required. The
asymptotics of the charge capture cross-sections at highenergies is thoroughly discussed in [12].

Fig. 2 Estimation of the equilibrium neutral fraction energy dependence for a polystyrene (–C8 H8 –)n pellet ablation cloud [5].
19Diagnostics of Charge Exchange Neutrals from Multi-Ion-Species NBI-Heated Plasmas on LHD(a)Fig. 3 Orthogonally measured PCX spectrum obtainedby digital processing of natural diamond detector signals [3]. Target plasma Rax = 3.53 m,
β = 0.20 %, B = 2.856 T, ne = 1.5 × 1013 cm-3 , tangential NBI heating (175 keV, 4 MW NBI #1,150 keV, 1 MW NBI #2).

(b)and C2+ ions were calculated in [5] using OppenheimerBrinkman-Kramers scaling not applicable at lower energies E < 10 keV.
Figure 3 shows both raw and F 0 (E) corrected PCX
pitch angle scattered particle spectra measured orthogonally from tangential NBI heated hydrogen plasma [3].
4．CX spectra from multi-species plasmas
The influence of the target plasma Ze f f on the intensity of the pitch angle scattering of NBI-produced
fast ions has been studied. A substantial enhancementof the pitch angle scattering of fast ions from tangential NBI has been observed for plasmas with higher Z
ion species in comparison with hydrogen plasma. The
asymptotic behaviour of H+ /Nek+ and H+ /Arl+ charge
exchange cross-sections at high energies [7] was usedfor σE-correction of the measured neutral particle spectra.
Figure 4 shows the energetic proton distributions
along three viewing directions from ne = 0.4 ×
1013 cm−3 , central T e = 2 keV target plasmas. Fig. 4 (a)refers to 131 keV hydrogen NBI into hydrogen (Z = 1)
plasma; Fig. 4 (b) 132 keV hydrogen NBI into neon
(Z = 10) plasma and in Fig. 4 (c) 135 keV hydrogen
NBI into Ar (Z = 18) / He (Z = 2) mixture plasma.
Figure 5 shows the corresponding contour plots of+H energy and pitch angle distributions in the axesE⊥ , E|| for these three target plasma ion compositions
respectively. The sectorial gaps are due to the fact thatthe measurements are made along several discrete directions [1]. It can be seen that the angular diﬀusionof energetic protons is very sensitive to the backgroundplasma ion composition in similar discharges. The in-(c)Fig. 4 H+ energy distributions f (E, θ) along three directions from H, Ne and Ar / He target plasmas.

terpretation is that the Legendre operator describing theangular spread on the right hand side of the FokkerPlanck equation contains the term proportional to Ze f f ,which is responsible for the ion contribution to the pitch20

Diagnostics of Charge Exchange Neutrals from Multi-Ion-Species NBI-Heated Plasmas on LHD

Fig. 5 Contour plots of the H+ energy distributions f (E⊥ , E|| ) from H, Ne and Ar / He target plasmas.

1 2
Z ni strongly
ne i idepends on the target plasma ion species.

[3]

angle scattering. The value Ze f f =[4]

5．Summary
The analysis of neutral particle measurements frommulti-ion-species plasmas requires the the overall neutral balance modeling and reliable experimental data on
the charge exchange cross-sections. However, in studiesof suprathermal distribution tails it is enough to know
the asymptotic behaviour of the cross-sections at highenergies.
Active charge exchange measurements with impurity
pellets require the knowledge of the cloud charge statecomposition and the relevant cross-sections.
The enhancement of the pitch angle scattering of
beam ions in plasmas with higher eﬀective charge
has been observed and interpreted in terms of FokkerPlanck equation for energetic ions.

[1]
[2]

[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]

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