ORIGINAL RESEARCH

Effect of Incident Energy and Foil Thickness on Foil Strippingand Scattering Efficiency in Charge Exchange Analyzer Detector

M. Kazemi • M. Habibi • M. Tafreshi

Published online: 21 July 2011

� Springer Science+Business Media, LLC 2011

Abstract Compact neutral particle analyzer (CNPA) is

used to measure ion temperature in tokamaks. Calculating

of stripping efficiency and scattering efficiency are most

important parameters affecting on CNPA performance. We

studied the dependent of these parameters with the thick-

ness of carbon foil and incident energy of neutral hydrogen

atoms. In low carbon foil thickness variation of the carbon

foil stripping efficiency (gi) and scattering efficiency of the

ions (gsc) with incident energy is very salient. For foil

thickness between 200 and 600 angestrom, scattering effi-

ciency of the chamber will be smaller than 0.11.

Keywords Incident energy � Carbon foil thickness �Stripping efficiency � Scattering efficiency

Introduction

Measuring of ion temperature in tokamaks and other

plasma confinement devices requires to use advanced

diagnostic techniques. One of these methodes is neutral

particle analyzing by Compact Charge Exchange Analyzer

(CNPA) diagnostic. As it shown in Fig. 1, as the incoming

flux of neutral atoms passes through the stripping cell,

many of them become ionized. The ions are deflected by a

magnetic field and measured by channeltron. Each channel

shows the specific energy of primary neutral atoms. The

first use of this techniques (secondary charge exchange

atomic flux) for measuring ion temperature has been

reported by Berezovskii in T-10 tokamak et al. [1]. By

developing the neutral particle analyzing devices, other

parameters like D–T density ratio in the plasma core, were

measured [2]. Neutral beam Plasma heating in fusion

machines need NPAs for controlling and optimization.

Recent studies demonstrate this fact. Specified NPA,

Compact Neutral Particle Analyser (CNPA), developed by

the A. F. Ioffe Physical-Technical Institute, has been

installed on TJ-II [3]. CNPA has many advantages consist

of: (1) It can be installed and easily moved anywhere near a

confined plasma device. (2) Sensitivity of this device for

neutrons and c rays can be very low by good shielding. (3)

Enlarging of analyzer scale increases the influence of outer

magnegtic field on secondary ion trajectories from strip-

ping foil or gas chamber [4]. Specific CNPA shown in

Fig. 1 consist of 6 parts [5]: stripping and acceleration

system, stipping foil, analyzing magnet for producing

magnetic field in perpendecular direction of Ion speed to

deflect them, Hall probe to measure the magnetic field

among the magnet plates, analyzing electrode condenser

for mass resolution (this part causes D? particles impact

on second row of channeltron), shielding mask at the

entrance of the system to reduce sensitivity of light and crays of hot plasma which causes undesirable noise on

external signal, detector to measure the ion energy.

Two kinds of CNPA are used to obtain ion temperatures.

Difference of these two typs is only relevance to stripping

method: stripping with gas chamber (He gas) or utilizing of

Stripping foil (carbon foil). The main advantage of strip-

ping with carbon foil in contrast with gas chamber method

is that the vacuum system of the experimental setup is

sufficient for stripping. In addition, for particles with

energies higher than 10 keV, stripping in a foil is far more

effective than stripping in gas. This may be important to

M. Kazemi (&) � M. Habibi

Amirkabir University of Technology, Tehran, Iran

e-mail: moslemkazemi@aut.ac.ir

M. Tafreshi

AEOI, Plasma Physics and Fusion Research Center, Tehran, Iran

123

J Fusion Energ (2012) 31:179–183

DOI 10.1007/s10894-011-9447-7

study the energetic ions on the tail of the distribution

function [4]. As reported in [6], fast ion diagnostic exper-

iment methode (FIDE) can’t detect hydrogen ions with

energies higher than 90 keV. While CNPA that shown in

Fig. 1 can detect hydrogen ions with energies more than

150 keV [6]. In this paper the dependent of stripping effi-

ciency and scattering performance with the thickness of

carbon foil and incident energy of neutral hydrogen atoms

has been investigated by MATLAB software.

Stripping Efficiency

A key parameter of the CNPA is total detection efficiency

(g) of incident atoms. It is determined by stripping effi-

ciency of the carbon foil (gi) and some factors that affect on

secondary ion trajectories, namely (1) scattering efficiency

of the ions by the stripping foil (gsc) (2) acceleration effi-

ciency after stripping (gE), (3) capability of system dis-

persion by magnetic and electric fields (gc), and (4)

efficiency of the detectors (gR). Thus, detection efficiency

of CNPA is calculated by multiplying five above parame-

ters:g ¼ gigscgEgcgR [2]. Stripping efficiency of the thin

carbon foil (300 A) as a function of energetic hydrogen

atoms incident on the foil is presented in Fig. 2. The

equilibrium fraction of hydrogen atoms that emerge from

the foil, integrated over all scattering angles. As it shown in

Fig. 2, by increasing the incident hydrogen energy, the

equilibrium fraction of H? increases. As illustrated in

Fig. 3 in high energy regime of neutral atoms, the main

parameter to calculate the detection efficiency is the

stripping efficiency [2].

Relevance of stripping efficiency with ion energy after

collide with carbon foil are obtained experimentally as

below [7]:

gi ¼ 1� exp �0:058 E0:72out

� �: ð1Þ

In this equation Eout is dependent to thickness of carbon

foil and incident energy of neutral hydrogen beam. This

dependency is presented in Eq. 2:

Eout ¼ffiffiffiEp� 1:6� 10�3 � d

h i: ð2Þ

These two equations are plotted in Figs. 4 and 5 by

Fig. 1 A scheme of CNPA analyzer: (1) stripping and acceleration

system; (2) stripping foil; (3) bending magnet; (4) Hall probe; (5)

analyzing electrostatic condenser; (6) shielding mask; (7) detectors;

(A0) atomic flux emitted by plasma, (A?) secondary ions)

Fig. 2 Equilibrium fraction of hydrogen atom fragments that emerge

from a 300 A thick carbon foil

Fig. 3 Detection efficiency K(E) as a function of atom energy.

Equilibrium fraction of H?ions after stripping is simulated [6]

180 J Fusion Energ (2012) 31:179–183

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MATLAB for special range of carbon foil thickness

(1–600 A) and incident energy of hydrogen beam (1 to

6 keV).

As it shown in Fig. 6, gi for 6 keV incident energy

equal to 0.09 in comparison with experimental result for

300 A0 in Fig. 2 is approximately 0.1.

Scattering Efficiency

The second parameter of total detection efficiency was

scattering of ion emerged from carbon foil (gsc). On the

other hand this factor describes the number of ions arriving

to the aperture of CNPA bending magnet. Because of

specific area of the aperture some particles scattered in

specific direction can affect by magnetic field of the device.

Value of gsc depend on E (hydrogen energy) and D (carbon

foil thickness) like gi. This dependency has been shown in

Eq. 3 [7].

Fig. 4 Variation of exit

hydrogen energy versus incident

energy and foil thickness after

passing through carbon foil

Fig. 5 Ratio of exit H? particle

after hydrogen beam passes

through carbon foil.

Approaching to high ratio of

exit H? particle needs to afford

thinner carbon foil as it

possible. Producing thinner

carbon foil needs more

technology and accurate

processing

Fig. 6 gi for 300 A0 carbon foil thickness versus incident energy

J Fusion Energ (2012) 31:179–183 181

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gsc ¼ 0:5 1þ exp �pð Þð Þ � A: ð3Þ

In this equation P and A are described as below:

A ¼ 1� sin2 #0

2

� �4 exp �pð Þ

1� exp �pð Þð Þ2

" #�12

8<

:

9=

;ð4Þ

p ¼ 4:33� 10�4 d

Eð5Þ

in Eq. 4 value of #0 is equal to 0.1 radian. E in Eq. 5 refers

to average of H? and H0 energies as illustrated in Eq. 6.

E ¼ Eþ Eout

2ð6Þ

By combination of these equations we can describe the

dependency of gsc to incident energy and carbon foil

thickness directly. In Fig. 7 ratio of arrived particle versus

incident energy and foil thickness has been plotted. In this

figure the evolution of gsc with different incident energy

and carbon foil thickness obviously illustrated.

The important region of Fig. 7 is specified by a black

circle. In the range of 0–0.3 keV (approximately), gsc

extremely changes by incident energy and after that the

slope of evolution is decreased. Whatever thickness of foil

increases, tonality of color become nearer to red color more

slowly. YZ view of Fig. 7 (from left) for four special

carbon foil thickness are plotted in Fig. 8.

These two parameter (gi, gsc) indicate the performance

of stripping and accelerating systems. Because these two

mechanisms has been attached in one chamber (see Fig. 9)

it is suitable that we merge gi and gsc to one parameter

named gchamber. Behavior of gchamber versus incident energy

and foil thickness are plotted in Fig. 10.

It is clear from Fig. 10 that the diagram of Fig. 7 has

been flattened with the diagram of Fig. 5 specially in

thinner carbon foil.

Results and Conclusion

The main goal of this study was investigation of total

detection efficiency for CNPA. Discussed equation in this

Fig. 7 gsc as function of carbon

foil thickness (angstrom) and

incident energy (KeV)

Fig. 8 YZ view of Fig. 6 (from left) for four special carbon foil

thickness

Fig. 9 Assembly of the accelerator-stripping unit

182 J Fusion Energ (2012) 31:179–183

123

paper refers to hydrogen incident beam. CNPA make

detection of hydrogen and deuterium beams simultaneity.

In low carbon foil thickness variation of gi and gsc with

incident energy is very salient. Since neutral atom beam

emitted from tokamak plasma have Maxwellian distribu-

tion [8], estimated detection efficiency for hydrogen beam

with energies up to 1 keV should be done with thicker

carbon foil. With this consideration for foil thickness

between 200 and 600 angestrom, gchamber will be smaller

than 0.11. Variation of other isotope of hydrogen with

some negligible effects are the same.

References

1. A. Medvedev, V.S. Strelkov, Plasma Physics Reports. 32(5),

411–417 (2006)

2. V.I. Afanasyev et al., Reviwe of scientific instrument. 74, 4 (2003)

3. R. Balbın, 32nd EPS Conference on Plasma Phys. Tarragona, vol.

29C, (2005)

4. F.V. Chernyshev, Instruments and experimental tech. 47(2),

214–220 (2004)

5. NPD Loffe Institute Catalog for CNPA. pp3.(2008)

6. A.L. Roquemore et al., Rev. Sci. Instrum. 56, 5 (1985)

7. A.V. Bortnikov et al., Ion and electron energy spectra evolution

during plasma current disruption in TVD. Appendix 1 pp3

Moscow (1992)

8. J. Wesson, Tokamaks, chapter 10, 3rd edn. (Oxford University

Press, Oxford 2004), p. 422a

Fig. 10 gchamber versus incident

energy and foil thickness

J Fusion Energ (2012) 31:179–183 183

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