J. Plasma
J.
Plasma Fusion
Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 6(2004)(2004)492–495
000–000

Compact Torus Plasma Injector in High Repetition Rate Operation
SHIMAMURA Shin, NITOU Junsuke, HAYAKAWA Masahide and OOTA Fuminori
Department of Physics, College of Science and Technology, Nihon University, Tokyo, 101-8308, Japan(Received: 9 December 2003 / Accepted: 24 February 2004)

Abstract
Compact torus plasma injection with high repetition rate has been experimented. Using the same polarity
oscillating current as a power source, a magnetized coaxial gun could generate a series of compact torus plasma withthe same helicity sign in a high repetition rate. A series of generated compact torus plasmas was injected into a drifttube. The second generated compact torus plasma interacted with the decayed first one. Accumulation effect wasobserved though the experiments were conducted in the drift tube. The repetition frequency of the compact torus
plasma was increased from 50 KHz to 70 KHz. The ratio of accumulation effect increased remarkably by increasing
the repetition frequency.
Keywords:
compact torus, spherical torus, high repetition rate CT injector, coaxial helicity injector, fueling, current drive,
sustainment

1. Introductionof the first gun current, the injected first plasma decayed. The
second gun current pulse generated the next CT plasma. The
second CT plasma moved into the tube and interacted withthe former decayed CT plasma.
Therefore we observed the accumulation effect by theinteraction between the second plasma and the first one,though the experiments were conducted in the drift tube.

It is promising that injection of compact torus (CT)plasma (CT plasma also includes spherical tokamak) is a
useful way for starting up an initial plasma, for putting onfuel particle and for sustaining of main tokamak plasma andspherical tokamak plasma. A CT plasma can be injected
deeply and fueled particle the interior of main tokamak.
Comparing with the other sustainment methods the CT
plasma injection supplies many torus plasma components ofmain tokamak plasma. The CT plasma itself has all plasma
parameters and components of main tokamak plasma.
For effective fueling and sustainment, single pulsed
injection of small CT plasma is not sufficient in many cases[1]. Therefore high repetition rate CT injector is required.
We have experimented on the CT plasma injection withhigh repetition rate and studied effect of an increasing
repetition rate. The CT plasma was generated by a small
magnetized coaxial gun (be also called as Coaxial Helicity
Injector: CHI). Using same polarity oscillating current as apower source, the magnetized coaxial gun can generate aseries of the CT plasmas with high repetition rate.
A series of the generated CT plasmas was injected intoa drift tube. A metal (SUS) made drift tube was used formeasurement of magnetic field structure of the injected CTplasmas. A transparent drift tube that was made of pyrex glass
was used for an observation of cross sectional view of theplasma in the drift tube by a fast-gated image intensifier(FGII) camera.
The first plasma that was generated by the first current
pulse of gun moved into the tube. According to the decreasing

2. Estimate of helicity injection
It is possible that repetitive CT injection can sustain
helicity, particle and many other components of torus plasma.
We tested a simple model of repetitive helicity injection. In
the repetitive CT injection case, we estimated the helicity
from the following next helicity balance equation [2]dH = – H + f 2V φg gτHdt
where H is the helicity of the CT plasma in the drift tube.
The second term of the RHS consists of voltage pulse Vg thatis repetitively supplied to the magnetized gun and magnetized
flux φg threaded electrodes. A function f includes frequency
or repetition rate. The helicity in the drift tube decays withtime constant τH . When the CT plasma is resistive, the helicity
decay time can be estimated by the magnetic energy decaytime. For simplicity, it is assumed that the injected CT plasma
relaxes faster than the helicity decay time.
At the beginning the injection term is given. The flux φg
is constant and the gun voltage Vg is applied as repetitivepulse. The gun voltage pulses with two different repetition

Corresponding author’s e-mail: shimamra@shot-gun.phys.nihon-u.cst.ac.jp1492©2004 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Shimamura S. et al., Compact Torus Plasma Injector in High Repetition Rate Operation

Fig. 2 Setup of pyrex drift tube experiment.

Fig. 1

A model of accumulation effect by increasing repetition
frequency.

rates are shown as broken line (square pulses) in Fig. 1 (a),(b). The decay time constants τH are estimated by resistive
diffusion time of the magnetic field and are assumed as thesame in both calculations. The numerical solutions ofresulting normalized helicity are plotted as solid line
(sawtooth like pulse) in Fig. 1(a), (b). If we select the proper
repetition rate the normalized helicity is accumulated andfinally sustained continuously.

3. Experimental apparatus
The schematic drawing of experimental device is shownin Fig. 2. The magnetized coaxial gun q generates and injects
CT plasmas into the drift tube in high repetition rate. The
diameters of inner, outer electrode and these lengths are 2cm, 4 cm and 20 cm respectively. Fast acting valve w
supplies He gas from this port. Drift tube e is set on in frontof the magnetized plasma gun. In this experiment drift tubeof pyrex glass is used mainly for the observation of wholeplasma by FGII camera. The diameter and the length of thedrift tube are 10 cm and 100 cm respectively. An externalsolenoid coil r generates axial magnetic guide field on thedrift tube. The center conductor t is set on the axis of pyrex
drift tube all the way long.
One of the discharge circuits to generate repetitive
current pulses with the same polarity is shown in Fig. 3 (a).
The capacitance and the magnetized coaxial gun areconnected with a transformer. Both primary circuit andsecondary circuit have start switch .The primary circuit of thetransformer is switched on and when the current of the circuit
reaches the peak value the secondary circuit is switched on.
In order to study the accumulation effect the capacitor of theFig. 3 Discharge circuits (a) and gun currents (b).

gun was decreased to 4.4 µF. Figure 3 (b) shows two current
waveforms that flow into the magnetized coaxial gun. The
solid line is the waveform in 8.8 µF capacitor and thefrequency is f = 50 KHz. The broken line is the waveform in
4.4 µF and the frequency is f = 70 KHz. The doted line is thewaveform that the switches of primary and secondary circuit
are turned on simultaneously. This case allows alternative
polarity current and then the helicity of each generated CT
plasma changes the sigh alternatively.
A CCD camera with fast-gated image intensifier wasused to take a side view of CT plasma in the pyrex drift tube.
Two band-pass filters were prepared for the spectroscopic
observation. Central wavelength of one filter was 468.6 nm
for the first ionized He ion line. Another one was 587.6 nm forthe excited neutral He atom. We took two pictures for eachfilter in same conditions from different shots. Intensity ratio
for each pixel of the two pictures transform into a distributionof plasma temperature by means of line pair method.
2493

Shimamura S. et al., Compact Torus Plasma Injector in High Repetition Rate Operation

4. Experimental results

The ratio of flux change for the second CT plasma to
the first CT plasma is typically 0.7. This figure also shows
the ratio of the peak gun current normalized by the firstcurrent. The gun current ratio of the second pulse to the firstone is 0.6. A slight accumulation effect was observed in thisrepetition rate.
We observed a series of injections of luminous CTplasmas. The side view of CT plasmas looked like a convex
parabolic. The shapes for each CT plasma were similar to thesolutions analyzed by P.M. Bellan [3] even though theexperimental conditions are more complicated and theanalysis is not strictly applicable in this experiment.
Figure 6 shows a temperature distribution in the drift
tube. The contrast of the picture is changed from the originalfor printing. The highest temperature of the luminous parts isaround 10 eV.
Plasma density was not measured in this experiment,
however, averaged density from the former measurement by
He-Ne laser interferometer showed 1.5 × 1020 m-3 at peakdensity.
Figure 7 shows a contour map of the flux change on
axial distance-time (z-t) plane. Four or more blobs of the CT
plasma were ejected from the magnetized coaxial gun. The
period of repetition becomes 18 µs ( f = 70 KHz) whichcorrelate with the period of the gun current pulse. The fluxchange ratio is shown in Figure 8. The ratio of flux change
for the second CT plasma to the first CT plasma is 1.0~1.1.

The first experiments were carried out by using 8.8 µF
capacitor. Magnetic flux loops lapped around on the pyrex
drift tube measured the flux change in the guide field. The
flux change was measured at four positions on the drift tube.
Figure 4 shows a contour map of the flux change on axial
distance-time (z-t) plane. Axis of abscissas is the time fromthe start of current in the primary of the gun circuit. Axis ofordinates is the axial distance from the gun muzzle. Thevalues of flux change are shown on the contour in unite of×10-5 Wb. Three or more blobs of the CT plasma are ejectedfrom the magnetized coaxial gun. The period and thefrequency of repetition are 24 µs and 50 KHz respectively.
These values are correlated with the period and the frequencyof the gun current pulse. The typical drift speed is 40 Km/s.
Accumulation effect is estimated by the ration of peak
flux change for the numerical order of injected CT plasma tothe peak flux change for the first one. Figure 5 shows theflux change ratio for the different external guide fields.

Fig. 4

Contour map of flux change on Z-t plane. Repetition
frequency 50 KHz.

Fig. 7

Contour map of flux change on Z-t plane. Repetition
frequency 70 KHz.

Fig. 5 Flux change ratio in 50 KHz repletion frequency.

Fig. 6 Temperature distribution in the drift tube.

Fig. 8 Flux change ratio in 70 KHz repetition frequency.

3494

Shimamura S. et al., Compact Torus Plasma Injector in High Repetition Rate Operation

References

The gun current ratio of the second pulse to the first one isalmost the same as 0.6. Increasing the repetition frequency
from 50 KHz to 70 KHz, accumulation effect increased from0.7 to 1.0~1.1.

[1] T. Ogawa et al., Nucl. Fusion 39, 1911 (1999).
[2] C.W. Barns et al., Phys. Fluids 29, 3415 (1986).
[3] S. Shimamura et al., J Plasma Fusion Res. l3, 494(2000).
[4] P.M. Bellan, Spheromaks (Imperial College Press, 2000)
p.162.

5. Summary
Increasing 1.4 times the repetition rate, the ratio of fluxchange raised from 0.7 to 1.0~1.8. The ratio of the second
gun current to the first current was both same. The
accumulation effect was increased remarkably.

4495