Plasma and Fusion Research: Regular Articles

Volume 11, 2405037 (2016)

Depth of Influence on the Plasma by Beam Extraction in a Negative
Hydrogen Ion Source for NBI∗)
Shaofei GENG1) , Katsuyoshi TSUMORI1,2) , Haruhisa NAKANO1,2) , Masashi KISAKI2) ,Katsunori IKEDA2) , Masaki OSAKABE1,2) , Ken-ichi NAGAOKA1,2) , Yasuhiko TAKEIRI1,2) ,Masayuki SHIBUYA2) and Osamu KANEKO1,2)
1)

SOKENDAI (The Graduate University for Advanced Studies), 322-6 Oroshi, Toki, Gifu 509-5292 Japan2)
National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292 Japan(Received 29 November 2015 / Accepted 24 February 2016)

Experimental measurements with Langmuir probe and laser photodetachment in beam extraction region ofa cesium-seeded negative ion source for NBI has been conducted in order to investigate the response of charged
particles to applied external field. The profiles of probe saturation current and H− ion density in the direction
normal to the plasma grid (PG) surface were measured by scanning the tip position. By comparing the results
before and during beam extraction, the charged particle responses due to the beam extraction have been analyzed.
During beam extraction, probe saturation current increases since H− ions are extracted and electrons flow into theextracting region for charge neutrality. The maximum increment of the probe saturation current due to electron
flow appears in the range of 15 - 25 mm apart from the PG surface. Meanwhile, the maximum decrement of theH− ion density is at around 18 mm from the PG. In the region far from the PG, probe saturation current increment
is low as well as the decrement of the H− ion density. The extrapolations of the profiles suggest that the depth of
the influence on the plasma by beam extraction is 42 mm from the plasma grid surface.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: negative hydrogen ion source, beam extraction, plasma response, Langmuir probe, photodetachment
DOI: 10.1585/pfr.11.2405037

1. Introduction
Neutral beam injection (NBI) is one of the most desirable methods for plasma heating and current drive fora fusion reactor since the heating mechanism of NBI has
been well understood and NBI is essentially not aﬀectedby the edge plasma condition [1]. In order to realize a highpenetration depth of a neutral beam to the plasma and heat
the core of the plasma in a fusion reactor, high energy neutral beam is required [2]. However, the neutralization efficiency of a positive hydrogen ion beam decreases as thebeam energy increases. On the other hand, the neutralization eﬃciency of a negative hydrogen ion beam can keep
as high as 60% even the beam energy reaches 1 MeV [3].
Therefore, a negative ion based neutral beam injector plays
essential roles in a magnetically confinement fusion device.
In a negative ion based neutral beam injector, the beam
quality, power and stability are determined by the negativeion source. Research oriented to improvements of the negative ion source is necessary. A lot of eﬀorts have been
contributed to the improvements of negative hydrogen ion(H− ) sources, in order to increase the performance of negative ion based NBI systems [4–13]. It has been observedthat the H− ion rich plasma in which the H− ion density
author’s e-mail: geng.shaofei@nifs.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

is much higher than the electron density, is generated in
the beam extraction region [9]. However, the H− ion richplasma is aﬀected by the extraction field. During beam extraction, the ratio of the H− ion density to the electron density decreases. In order to investigate the distribution of theplasma response to the extraction field and understand theprocess of the response, the local plasma information before and during beam extraction is necessary. Experiments
to measure the local plasma response have been carried out
in the beam extraction region of a negative ion source.

2. Experimental Setup
The experiments were conducted on the Research &Development Negative Ion Source at the National Institutefor Fusion Science (NIFS-RNIS) which is schematicallyillustrated in Fig. 1. The ion source is divided into a driver
region and an extraction region by a filter field. Plasma
is generated in the driver region by filament-arc discharge
and diﬀuses to the extraction region. A bias voltage is applied between the plasma grid (PG) and the plasma chamber to suppress electrons [14]. Having the advantages ofsimplicity and measuring local plasma parameters, a Langmuir probe was applied in the experiments. As illustratedin Fig. 1, a four-pin probe was installed in the beam extraction region through the bias insulator. The arrangement ofeach tip of the probe is shown in Fig. 2. The probe was ro-2405037-1

c 2016 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 11, 2405037 (2016)

Fig. 1 Schematic illustration of the negative ion source andprobe position.

Fig. 3 Time evolution of probe negative saturation current.

Fig. 2 Location and scanning direction of each probe tip. Theprobe is rotated by 30◦ .

tated by 30◦ so that the probe tips had diﬀerent positions iny direction and four profiles can be obtained from one scan.
The probe can work in electrostatic and photodetachment
modes. In electrostatic mode the probe worked as a regular Langmuir probe. In the photodetachment mode, a laser
beam was irradiated into the plasma and H− ions in thelaser column were detached by photons. In this process,the excess electron detached from an H− ion by a photon
conserved the charge neutrality. A probe tip was located in
the laser beam co-axially and biased at 40 V. The detachedelectrons were collected by the DC biased probe tip. Thena current increase named photodetachment current was observed from the probe and it was proportional to the local
H− density. Then the local H− ion density can be measured by the photodetachment current. The detail calibration method and photodetachment experimental configuration can be found in Ref. 13.

3. Plasma Response to Extraction
Field
The negative ion source is a pulsed ion source. In a
duty period, the filaments are warmed up for 6.9 s at first,
and then the arc discharge starts and sustains for 9.9 s. At
the final stage of the arc discharge, extraction voltage is applied and sustains for 1 s. Figure 3 shows the time evolution of negative saturation current measured by each probe
tip and Fig. 4 shows the time evolution of H− ion density.
The term “negative saturation current” refers to the probe
current which contains electrons and H− ions. In Figs. 3Fig. 4 Time evolution of H− ion density.

and 4, time 0 indicates the moment when the extraction
voltage is applied. It can be observed that during beam extraction, the negative saturation current has an increment
and meanwhile, H− ion density decreases. The increaseof negative saturation current is attributed to the electron
density increase since H− ion density shows a decrement.
During beam extraction, a part of H− ions are extracted. As a consequence, the H− ion density decreases.
Meanwhile, electrons flow from the driver region to the extraction region for charge neutrality. An increase of electron density can be observed. By scanning the probe position in z direction normal to the PG, profiles of electron
density increase and H− ion density increase can be investigated.

4. Profile of Negative Saturation Current Increase
Figure 5 shows the profile of negative saturation current increase ΔIneg which represents the electron density
increase during beam extraction. The origin of the horizontal axis indicates the position of PG. Near the plasmagrid, ΔIneg is lower and has a peak as the increasing distance from the PG. The peak ΔIneg appears in the range of
15 - 20 mm from the PG at 1.3 V bias voltage of the PG.
The peak moves to the range of 20 - 25 mm by increasing
the PG bias voltage to 5.7 V as shown in Fig. 6. The linear extrapolations of the profiles show the influence of theextraction field on the negative saturation current reaches
42 mm from the plasma grid.

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Plasma and Fusion Research: Regular Articles

Volume 11, 2405037 (2016)

Fig. 5 Profile of increase of negative saturation current measured by each probe tip at PG bias voltage of 1.3 V.

Fig. 7 Profile of H− ion density before and during beam extraction.

Fig. 8 Profile of reduction of H− ion density.
Fig. 6 Profile of increase of negative saturation current measured by each probe tip at PG bias voltage of 5.7 V.

5. Profile of H− Ion Density Reduction
The profile of H− ion density before and during beam
extraction is shown in Fig. 7. In the whole measurementregion, H− ion density decreases during beam extraction.
The density reduction of H− ions ΔnH− shown in Fig. 8 indicates a peak exists and appears at ∼ 18 mm from the PG
at PG bias voltage of 2.4 V. The linear extrapolation of theprofile indicates the reduction of H− ion density diminishes
to zero at 42 mm from the PG. This characteristic is similaras the one of the profile of the negative saturation currentincrement.

6. Discussion

In the Cs-seeded negative hydrogen ion source, the H−
ions are mainly produced on the PG surface. One can expect the maximum response of plasma appears near the PG
during beam extraction. However, the measurement results
indicate that the maximum response of the plasma is far
away from the PG. During beam extraction, the stable stateof the plasma is changed since the extraction voltage is applied and a part of the H− ions is extracted. If we compare
the diﬀerence of the plasma stable state before and during beam extraction, a change is the supply of H− ions tothe extraction region is reduced. The reduction exhibits itseﬀect far from the PG. A possibility is that the transportof H− ions from the PG surface to the extraction region is
not simple because of the magnetic field near the PG. If weconsider the movement of a H− ion in only y-z plane shownin Fig. 2, a H− ion experiences gyration in filter magnetic
field, since the filter magnetic field exists in large area inx direction with a strength of around 0.006 T. The H− iontemperature was measured to be around 0.1 eV [15, 16].
The gyroradius is calculated to be around 9 mm or the diameter of the gyration is around 18 mm. One of the possibilities is the H− ions experience long distance travel toreach the extraction region. During beam extraction, thetravel of H− ions may be disturbed by the extraction fieldand some H− ions cannot finish a full gyration and reachthe extraction region. As a consequence the maximum reduction of H− ion density appears at around 18 mm fromthe PG. Further investigations on H− ion dynamics are required to understand this phenomenon.
In the negative hydrogen ion source, plasma is produced in the driver region and diﬀuses to the extraction
region across the filter field. Therefore, low density andlow temperature plasma can be found in the extraction region. During beam extraction the plasma in the extraction
region is indirectly influenced by the extraction field. On
the other hand, the plasma in the driver region has highdensity and high temperature. This plasma is not influenced by the beam extraction field. In the experiments, it
has been found that the depth of influence of the beam ex-2405037-3

Plasma and Fusion Research: Regular Articles

Volume 11, 2405037 (2016)

traction field on the plasma can reach 42 mm apart fromplasma grid surface. We may regard the position which is
42 mm apart from plasma grid surface, as the connection
point of driver region and extraction region.

7. Summary
A four-pin Langmuir probe and photodetachment
technique have been applied to the extraction region of theNIFS-RNIS to investigate the influence of the extraction
field on the plasma. The maximum response of the plasma
to the extraction region appears in the range of 15 - 20 mmfrom the PG at 1.3 V bias voltage of the PG and moves tothe range of 20 - 25 mm at 5.7 V bias voltage of the PG. The
profile of H− ion density reduction has a peak at around
18 mm from PG at PG bias voltage of 2.4 V. Both of theLangmuir probe and photodetachment measurements suggest the depth of the influence of the extraction field on theplasma reaches 42 mm from the PG.

Acknowledgments
The authors would like to express their gratitude to thetechnical staﬀ of LHD-NBI group for the excellent operations of NBI test stand. This work is supported by SOKENDAI Course-by-Course Education Program GGXX100,
SOKENDAI Grant No. 70207003GGRR003, JSPS KAKENHI Grant No. 25249134 and NIFS Research Program
No. ULRR702.
[1] T. Oikawa, Y. Kamada, A. Isayama, T. Fujita, T. Suzuki,
N. Umeda, M. Kawai, M. Kuriyama, L.R. Grisham, Y.
Ikeda, K. Kajiwara, K. Ushigusa, K. Tobita, A. Morioka,
M. Takechi, T. Itoh and JT-60 Team, Nucl. Fusion 41, 1575
(2001).
[2] B.B. Kadomtsev, F.S. Troyon, M.L. Watkins, P.H.
Rutherford, M. Yoshikawa and V.S. Mukhovatov, Nucl. Fusion 30, 1675 (1990).
[3] K.H. Berkner, R.V. Pyle and J.W. Stearns, Nucl. Fusion 15,249 (1975).
[4] J. Paméla, M. Fumelli, F. Jequier, M. Hanada, Y. Okumura
and K. Watanabe, Nucl. Instrum. Methods Phys. Res. Sect.
B Beam Interact. Mater. At. 73, 289 (1993).
[5] Y. Okumura, Y. Fujiwara, A. Honda, T. Inoue, M.
Kuriyama, K. Miyamoto, N. Miyamoto, K. Mogaki, A.

Nagase, Y. Ohara, K. Usui and K. Watanabe, Rev. Sci. Instrum. 67, 1018 (1996).
[6] M. Kuriyama, N. Akino, N. Ebisawa, L. Grisham, H.
Liquen, A. Honda, T. Itoh, M. Kawai, M. Kazawa, K.
Mogaki, Y. Ohara, T. Ohga, K. Ohmori, Y. Okumura, H.
Oohara, K. Usui and K. Watanabe, J. Nucl. Sci. Technol.
35, 739 (1998).
[7] P. Franzen, H.D. Falter, U. Fantz, W. Kraus, M. Berger,S. Christ-Koch, M. Fröschle, R. Gutser, B. Heinemann, S.
Hilbert, S. Leyer, C. Martens, P. McNeely, R. Riedl, E.
Speth and D. Wünderlich, Nucl. Fusion 47, 264 (2007).
[8] U. Fantz, P. Franzen, W. Kraus, M. Berger, S. ChristKoch, H. Falter, M. Fröschle, R. Gutser, B. Heinemann,C. Martens, P. McNeely, R. Riedl, E. Speth, A. Stäbler andD. Wünderlich, Nucl. Fusion 49, 125007 (2009).
[9] K. Tsumori, H. Nakano, M. Kisaki, K. Ikeda, K. Nagaoka,
M. Osakabe, Y. Takeiri, O. Kaneko, M. Shibuya, E.
Asano, T. Kondo, M. Sato, S. Komada, H. Sekiguchi, N.
Kameyama, T. Fukuyama, S. Wada and A. Hatayama, Rev.
Sci. Instrum. 83, 02B116 (2012).
[10] K. Ikeda, H. Nakano, K. Tsumori, M. Kisaki, K. Nagaoka,M. Osakabe, Y. Takeiri and O. Kaneko, New J. Phys. 15,103026 (2013).
[11] H. Nakano, K. Tsumori, K. Nagaoka, M. Shibuya, U.
Fantz, M. Kisaki, K. Ikeda, M. Osakabe, O. Kaneko, E.
Asano, T. Kondo, M. Sato, S. Komada, H. Sekiguchi andY. Takeiri, AIP Conf. Proc. 1390, 359 (2011).
[12] M. Kisaki, K. Tsumori, H. Nakano, K. Ikeda, M. Osakabe,K. Nagaoka, M. Shibuya, M. Sato, H. Sekiguchi, S. Komada, T. Kondo, H. Hayashi, E. Asano, Y. Takeiri and O.
Kaneko, Rev. Sci. Instrum. 83, 02B113 (2012).
[13] S. Geng, K. Tsumori, H. Nakano, M. Kisaki, K. Ikeda,
Y. Takeiri, M. Osakabe, K. Nagaoka and O. Kaneko, AIP
Conf. Proc. 1655, 040014 (2015).
[14] K.W. Ehlers and K.N. Leung, Appl. Phys. Lett. 38, 287(1981).
[15] S. Geng, K. Tsumori, H. Nakano, M. Kisaki, K. Ikeda,
M. Osakabe, K. Nagaoka, Y. Takeiri, M. Shibuya and O.
Kaneko, Rev. Sci. Instrum. 87, 02B103 (2016).
[16] H. Nakano, K. Tsumori, M. Shibuya, S. Geng, M. Kisaki,
K. Ikeda, K. Nagaoka, M. Osakabe, Y. Takeiri and O.
Kaneko, “Cavity ringdown method for negative-hydrogenion measurement in ion source for neutral beam injector”, 17th International Symposium on Laser-Aided
Plasma Diagnostics (Chateraise Gateaux Kingdom Sapporo, Hokkaido, JAPAN, 2015) accepted by Journal of Instrumentation.

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