Proceedings of ITC/ISHW2007

Particle kinetic-energy measurement in compact toroid injection
into a gas neutralizer
Dazhi Liu, Yoshiyuki Ito, Naoyuki Fukumoto, Tsuguhisa Sekioka, Yusuke Kikuchi and Masayoshi
NagataUniversity of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan

Particle kinetic-energy measurement in the experiments of compact toroid (CT) injection into a hydrogen gas
neutralizer is presented. The typical injected CT parameters are: (1 ~ 4) ×1021 m-3 in electron density and 30 ~ 70km·s-1 in velocity. By using microchannel plate detectors, kinetic-energy of ions and neutrals after CT passing theneutralizer has been measured with an electrostatic ion energy analyzer and evaluated by time-of-flight method,respectively. The pressure in the neutralizer was scanned. At the pressure ≈ 6 × 10-3 torr, charge exchange between
CT plasma and the neutral particles with high energy has been identified, which is characterized by significant
decay of CT plasma density and magnetic field profile. The velocity of the produced neutral particle flow is close tothe ion velocity of the CT plasma.
Keywords: compact toroid injection, charge exchange, neutralization, microchannel plate detector.

1. Introduction
A compact toroid (CT) is a dense, well confinedplasma configuration with embedded, comparable
toroidal and poloidal magnetic field. Normally, CTs areformed by using a coaxial plasma gun through gas
discharge between inner and outer electrodes. Once
formed, a CT can be accelerated to high velocity by J×B
force, where J is the radial current density and B theazimuthal magnetic field associated with J. Typicalparameters of a CT are: several hundreds of km·s-1 invelocity and 1021m-3 in electron density. CT injection wasproposed as a promising fueling technique forreactor-grade tokamak by Perkins and Parks in 1988[1, 2].
An accelerated CT is able to penetrate into the center of atokamak provided that the kinetic energy density of theCT exceeds magnetic field energy density of the tokamak.
This can be described as: ρCTV2/2>B2/2μ0, where ρCT is theCT mass density, V is the CT velocity and B is the fieldstrength at the tokamak center. In addition, recent
experiments on injection of supersonic gas jets as a new
fueling approach have been successfully conducted onsome tokamak facilities [3-5]. However, the ability topenetrate into tokamak plasmas needs to be enhanced forlarger fueling efficiency. Thus a new technique foracceleration of plasma jet has been proposed by
Rozhansky et al. [6]. The jet was accelerated to 100
km·s-1 by using a plasma gun, leading to deep penetration
into plasmas in Globus-M.
In the present paper, we report a new approach toproduce neutral gas jets with high velocity using a CT
author’s e-mail:d-liu@eng.u-hyogo.ac.jp

injector. The CT will be injected to a neutralization region
filled with high pressure hydrogen gas (up to 10-2 torr). It
is conjectured that charge-exchange collision will occur
between high velocity ions and the neutral H atoms andH2 molecules leading to the production of a high velocity
neutral flow. Therefore, kinetic energy measurements of
the neutrals and ions after the neutralization are crucial toclarify the charge exchange process.

2. Experimental setup

Fig. 1 Experimental setup.
The experimental setup is shown in Fig.1. Asingle-stage, coaxial plasma gun (HIT-CTI2) wasemployed to form CTs [7]. The power supply for theinjector was a bank of capacitor of 10 kV/0.6 mF.
Hydrogen was the working gas. The diameters of the innerand outer electrodes are 60 mm and 134 mm, respectively.
A segment of stainless steel tube (length = 109 cm) wasattached to the exit of the CT injector as the CT neutralizer.
A set of seven holes (dia. = 42 mm) were located in theProceedings of ITC/ISHW2007
neutralizer along CT trajectory for Langmuir and magnetic
probes. The neutralizer was located inside a stainless steelchamber. The pressure in the CT neutralization tube wascontrolled by a pulsed Piezo gas valve (PV-5020) withmaximum gas output of 100 Pa·m3s-1 for hydrogen. Theinline gas pressure was 1.8 ~ 2.5 atmosphere.
The vacuum of the system was attained by using a
500 l·s-1 turbo-molecular pump (TMP-1). The basepressure was 1×10-6 torr, typically. During the experiments
another turbo-molecular pump (TMP-2) with effective
pumping speed 50 l·s-1 was used to minimize the loss ofhydrogen gas.
A He-Ne laser interferometer (λ = 633 nm) and amagnetic probe array were routinely installed in theHIT-CTI2 for CT density (line-averaged) and magnetic
field measurements. A monochromator was used tomonitor Hβ emission (λ = 486.1 nm) at the exit of theneutralization tube (z = 165 cm). Three Langmuir probes(LP1~3) were located at z = 0, 88, 165 cm to measureplasma density. Magnetic probes (MPA1, MPA2, S2 andS3) were used to measure CT magnetic field and its
profile.
Microchannel plate detectors (MCP) were used todetect particles after CT passing the neutralizer. For
neutral detection, a deflection coil was used to remove
the charged particles in the flow; for ion detection, anelectrostatic ion energy analyzer (IEA), which consists ofan ion energy selection part and a MCP detector, wasemployed. High vacuum (< 2×10-6 torr) should bemaintained in the MCP chamber for proper operation. To
fulfill this requirement a pinhole (dia. = 0.8 mm and holelength = 3 mm) was used for differential pumping byusing a dedicated turbo molecular pump connected to theMCP chamber. At the maximum pressure in the CT
neutralizer, PN = 10-2 torr, the pressure in the MCP
chamber was 1.5×10-6 torr.

Fig.2

The CT magnetic filed profile in the same shot isshown in Fig.3. It clearly demonstrates that the CT still
kept its integrity and field profile at this location.
However, the CT-shaped profile vanished at the S3location (z = 128 cm).

Fig. 3

3. Experimental results and discussion
3.1 Observation of charge exchange process
The injection experiments were performed with thefollowing CT parameters at the exit of the injector (z = 0):
2.5 kG in CT magnetic field, (1 ~ 4) ×1021 m-3 in electron
density and 30 ~70 km·s-1 in velocity. The CT velocity wascontrolled by gun discharge voltage (Vgun). For instance,
when Vgun= 6 kV, the CT velocity was about 40 km·s-1.
Fig.2 shows typical discharge waveforms whencharge exchange occurred, from top: Hβ emission, MCP
intensity and the plasma density measured by Langmuirprobes at z = 0, 88, 165 cm. When PN < 6×10-3 torr, nocharge exchange phenomena was observed. The plasma
density at z =165 cm (LP3) is about 6% of that at the z = 88cm (LP2). The peak of Hβ coincides with the onset of LP3
signal.

Typical discharge waveforms when charge
exchange occurred. Vgun= 6 kV, PN =6×10-3 torr.

Magnetic field profile of the CT at z = 80cm.

At Vgun= 6 kV, a comparison of plasma density was
made at different neutralizer pressure PN, from 10-6 to
10-2 torr, see Fig. 4. At PN ≥ 6×10-3 torr the charge
exchange (CX) occurred and the electron density
decreased by one order of magnitude compared withthose shots without occurrence of charge exchange.
Meanwhile, the CT magnetic field also demonstrated thesame trend. In Fig. 5, the field strength decays
significantly, about 50%, compared with those shots inwhich no CX occurred. The square marker shows thevalue of natural decay of CT magnetic field according toFukumoto et al. with magnetic field decay time τd = 24 μs[7].

Proceedings of ITC/ISHW2007
collision between the ions and neural atoms and molecules
is the dominant ion momentum loss channel at high ionenergy. It can be justified by the atomic cross-section data:
for Ep= 200 eV, σm = 5×10-19 m-2 and σcx = 4×10-19 m-2,
where σm is the momentum transfer cross section and σcx isthe charge exchange cross section for P+H collision [8].

Fig. 4 Comparison of plasma density at different PN
and w/ and w/o CX occurred. Vgun = 6 kV.

Fig.6 MCP signals for neutral kinetic energy
measurement w/ and w/o CX occurred.

Fig.5

Comparison of CT magnetic field decay atdifferent PN and w/ and w/o CX occurred.
Red lines are with CX. Vgun = 6 kV.

3.2 Particle kinetic energy measurement
Fig. 6 shows typical raw MCP signals in the neutral
kinetic energy measurement. The difference in these twosignals (red curve) is attributed to more production ofneutrals when CX occurred. Correspondingly, the velocityof neutrals can be determined by the time of flight (TOF)
method, VH ≈ 262/68.2 = 38 km·s-1, which is close to theinjection velocity of H+ in the CT plasma at Vgun= 6 kV.
This result also verified our observation of CX occurred inthe high PN shots.
The kinetic energy of ions was measured with IEA.
Fig.7 shows the energy spectrum of ions at different PN.
Vgun was fixed at 6 kV in the measurement. The amplitudeof the signals of high energy ions (> 200 eV) decreases
with increasing PN . This is because the charge exchange

Fig.7 Ion kinetic energy spectrum at different
PN.

4. Conclusions
We have performed the injection of compact toroid
into a hydrogen gas neutralizer filled with variable gaspressure. At pressure ≈ 6 × 10-3 torr, charge exchangeprocess between ions in the CT plasma and the neutral
particles has been identified, which is characterized bysignificant decay of CT plasma density and magnetic
field profile.
By using microchannel plate detector, the velocity ofneutral particle flow produced in the charge exchange
process, has been measured. It is close to the ion
directional velocity in the CT plasma prior to CX. The

Proceedings of ITC/ISHW2007
fraction of ions with high kinetic energy (>200 eV)
decreases with increasing neutralizer pressure because ofion momentum loss mainly caused by the charge
exchange process.

Acknowledgements
The author, Dazhi Liu, thanks the Japan Society for
the Promotion of Science (JSPS) postdoctoral fellowshipfor financial support.

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