J. Plasma Fusion Res. SERIES, Vol. 5 (2002) 22V224

End Loss Measurement of Neutral-Beam-lniected
Field-Reversed Configuration Plasma
ASAI Tomohiko, OKUBO Mamoru, INOMOTO Michiaki, OKADA Shigefumi and GOTO Seiichi
Plasma Physics ktboratory, Graduate school of engineering, Osaka University,
suita 565-0871, Japan(Received: ll December 2001 / Accepted: 1l July 2002)

Abstract
End loss flux measurements have been performed by using ion energy analyzer in order toinvestigate the confinement improvement in a field-reversed configuration (FRC) plasma sustained byneutral beam injection (NBI). The experimental results show that the energy confinement time of FRC
has been extended by about 100 Vo compared with the no NBI case. To understand these experimental
phenomena, some measurements have been performed on end loss flux. Previous theoretical studies
predict injected hot beam ion walls would play a role in improving the confinements on the edge layer bysome electrostatic effects. However, significant evidences suggesting these electrostatic effects on theedge layer due to NBI have never been observed in this work.

Keywords:
field-reversed configuration, neutral beam injection, improved confinement, end loss flux, particle energy
analysis

1. lntroduction

the injected beam power of several hundred kilo watts ismuch smaller than the reduced global energy losses of

A Field-reversed configuration (FRC) plasma
would have many advantages as a fusion reactor core
because of its high-beta nature and engineering
simplicity. Unfortunately, current FRCs are operated
only in a pulsed fashion. Therefore, in the present stageof FRC researches, application of a neutral beaminjection (NBI) is one of the most critical issues forseveral mega watts.

We calculated the trajectories of re-ionized beam

particles in the magnetic configurations with anexperimentally achieved equilibrium of FIX-FRC in theprevious work [1]. According to the calculation, it ispossible to infer that the dense region of beam ions areformed at the entrance of the mirror regions, andtherefore space charge would be formed around theseregions of around minimum lBl. Or the beam particles
ionized in the core plasma region have large orbits andstay mostly in the edge plasma region, and these ionsmight charge up the core plasma generating significant

steady state operation ofthe configuration.

The first results of using NB systems marked
improved confinement []. In the case of mirror ratio Ry- 8 and 320 kW (23 A, 14 kV) of NBI, configuration
lifetime is extended by more than 100 70 compared withthe case of no NBI. Furthermore, these lifetime

potential. These electrostatic developments would affecton ion particle loss from FRC plasma along open
magnetic field line, leading to some change in total

extensions are also observed to be dependent on beam
current 10. However, these NB effects on confinement
improvement are too large to consider that it is the only
act of current driving and/or electron heating, because@20O2 by The Japan Society of Plasma

Corresponding author's e-mail: tomohiko@ plasma.usask.ca
220

Science and Nuclear Fusion ResearchAsaiT. et al., End Loss Measurement of Neutral-Beam-Iniected Field-Reversed Confisuration Plasma

confinement property. In order to investigate theseelectrostatic effects, we measured the time evolution,
spatial distribution and velocity distribution of ion loss
flux at the end region of FIX device.
In this paper, we present the details of end loss flux

B. Neutral Beam Injection System
Two NB injectors with concave electrodes are
employed in our study. Extracted neutral beams focus ata distance of 800 mm from the electrodes on the beam
injection axis. Power supplies for the beam sources aremeasurements in FIX-FRC with NBI. We will also

the capacitor banks which are switched on-off with
Insulated-Gate Bipolar Transistor (IGBT) units. By

discuss about the electrostatic effect of large orbit beamions as one of the mechanisms of confinement

using this system, neutral hydrogen beams with 770 kW
of power (14 keV, 55 A) and 10ms pulse width areproduced. The injected neutral beam has a77 Vo degreeof neutralization and a 5.3 mm radius at the focal point.
In our experiment, The neutral beam injectors have been
mounted as shown in Fig. 1. The injectors are installedat an angle of l9o to the geometric axis so that the beam
particles ionized in the separatrix are outside the loss
cone of the mirror field in the case of the stronsest
mirror ratio of over 9.

improvement in these experimental results.

2. Experimental Setup
A. FIX Device
These experiments were carried out on the FIX
device [2]. The schematic view of the device is shownin Fig. 1. The FIX has unique plasma confinement
properties: translation technique, metal confinement
chamber, relatively low plasma density and strong
mirror field in which an FRC equilibrium is attained.
The FRC plasrna is produced by the field reversed theta
pinch method with a deuterium puffing system, and isinstantaneously moved along a guiding magnetic field
into the metal confinement chamber, which serves as aflux conserver, and then reforms to a quasi-steadyequilibrium state. Typically applied magnetic field in thestraight confinement region is 0.04 T. Each mirror coils
are spaced 3.4 m apafi; the mirror field strength is 0.08
- 0.40 T, and so the mirror ratio can be chosen tobe 2 10. Typical plasma parameters in the confinement
region are; electron density n" - 5.0 x 1Ore m-3, plasma
temperature Tun (= Ti + T") - 150 eV, separatrix radius r,- 0.2 m, and separatrix volume Ve - 0.4 m3. The
poloidal shape of the confined FRC is calculated fromexcluded flux measurement by using an array of 35
magnetic probes located just inside flux conserver wall.
A CO2 laser interferometer (10.6 pm) is installed
parallel to the y axis at e = 0.6 m from the midplane.

C. End Loss Analyzer

A cross-sectional view of the particle energy
analyzer is shown inFig.2. This analyzer has a set ofretarding potential grids. By sweeping the retarding bias
Gmund glid

Entrance slil
Signal out

Ion collector

Electron repeller

Entrance gridIon repeller

l0A mmElectron Repeller GridIIII

MachineGround III
II
I

tlillallONmm

Fig.1 Sectional drawing of the confinement part of thei/ !rIII
I-200v

TTI

TI
I
I
I
I

Densiry controlling

FIX device. The NB injectors are mounted on thetapered part of the confinement chamber. The impact parameter is 100 mm and the beam injection
axis is inclined 19'with respect to the geometric

IAl\ItI-24VaII!

-270V

crid Ground c,f;doll*tot

Fig. 2 Schematic view of the energy analyzer and grid
potential. The ion repeller bias voltage can be

swept to measure axial velocity distribution.

axis.

22r

AsaiT. et al., End Loss Measurement of Neutral-Beam-Iniected Field-Reversed Configuration Plasma(solid line).

voltage, parallel component of energy E1 of loss ions ismeasured. A typical collector current under theThe detailed propriety of this system have beentested in our previous study [3,4]. The cutoff angle ofthe entrance aperture is 85 degree i.e. the ratio E1lE11=132, and the measurable range is enough to obtain thecondition of swept bias voltage shown in Fig. 3. A
typical axial velocity distribution measured by using thissystem is shown in Fig. 4. The horizontal axis shows theaxial velocity and the vertical axis shows the collector
current signal differentiated by the ion repeller voltage
which corresponds to v"/(v11) where /(v11) is axial

energy distribution in our case.

3. Experimental Results

velocity distribution function. The distribution isapproximated by a shifted Maxwellian velocity

We have conducted NBI experiments focusing ourattention on ensuing improved confinement anddependency of end loss flux on the improvement. In
following experiments, the beam pulse of NBI has been
started about 2 ms before FRC formation because theoutput of the beam injector reaches a stable anddistribution function.f (v,,)*".p[-(" - un.*)'lr*] ,(1)

where v11o* is the axial flow velocity and v,6 is the axial

reproducible value l-2 ms after it is turned on. Even inthermal velocity. The measured velocity distribution
shows a good agreement with a shifted Maxwellian

this situation, the formation and the first-pass of theFRC translation are not influenced by the beam. The
injected beam power and mirror ratio are fixed to 320
kW (14 keY,23 A) and Rr - 8 respectively.
Typical time evolutions of total particle inventoryN(r) (= n,G)'vp(t)) of FRC plasmas with/without NBI
are shown in Fig. 5. Here, time evolution of theseparatrix volume %(t) is calculated from axial
distribution of the excluded flux and averaged electron
density D"(/) is estimated from interferometric

100

3 r'4
< t.2

rl.llrrl

1.0E!lg 0.8
E o.u

rl
tl
l1tl

lt

ii o.+ii'iii'r

Itl
rl
ll
ll
rl
rl
ll

IIH

po'od o.2

I o.o2U)

measurement as J n.d(,12r". The translated FRC plasmas

l\,lijU,itl,,\JljUl 0300

have almost same initial value of N before 80 ps
regardless of NBI. After 80 ps, the beam effect on theinventory change appears and the value ofNdecays at a400

Time []rsl

drastically different rate. Here, we define the particle
confinement time 21,, as the e-folding time of the particle

Fig. 3 Typical time evolution of the collector current signal (solid line) and bias voltage on the ion repeller
grid (dashed line).

inventory decrease, just after 80 ps. The particle
confinement time ?N has increased from 160 1ts to 270
trrs with the application of NBI. This result shows thatl.
0.8?i0.68Ra
h6fit

0.4
0.2
0010

Parallel Velocity [x104 m/s]

100200300

Time [ps]Fig. 5 Time evolution of total particle inventory N in FRC

plasmas with/without NBl. The injected beampower Po estimated from the extracting voltage

Fig. 4 Typical axial velocity distribution obtained fromthe energy analyzer. The solid line is fitted one using a shifted Maxwellian distribution function.

and the current is about 320 kW (14 keV, 23 A).

222

AsaiT. et al., End Loss Measurement of Neutral-Beam-Iniected Field-Revened Confrguration Plasma
0.20

I o.rstJ'

5 o.toLl\- 0.05
0.00

0100

200

0300

Time [ps]shown in Fig. 8. However, significant difference. with NBI

pbetween cases, both with and without NBI, has neversw/oNBI;300'a€ zooIxt

been observed.

+4. Discussion
We will discuss the possible basis of the improved
confinement, which could be caused by the injectedbeam ions. The issues discussed here will be thegeneration of plasma potential or potential barriers in
the edge plasma region. The particle confinement time
zly is increased from 160 ps to 270 ps by the NBI as
shown in Fig. 6. In this experiment, the global energy
loss rate of 5.5 MW in normal FRC is reduced to 2.3?,*s0400

Time [ps]Fig. 8 Time evolution of parallel energy of end loss ionsfi, from FRC plasmas with r9u - 8 for the case withNBI (Pb - 320 kW) (open square), and with no NBI(closed circle).

Fig. 6 Time evolution of the amount of end loss flux
from FRC plasmas with R" - 8 for the case withNBI (Pb -- 320 kW) (solid line), and with no NBI(broken line).

.-:400100 200 300

6ioioio40ior[mm]Fig. 7 Spatial distribution of end loss flux from FRC plasmas with NBI (Pb - 320 kW) (closed circle), andwith no NBI (open square).

MW due to NBI, indicating that the energy confinement
time tB is increased by a factor of approximately two.
The injected NB power is 0.3 MW and is not sufficient
to directly make up for the diminution of 3.2 MW in

NBI leads to good confinement in the FRC plasma,
because the total beam particle deposit to the coreplasma is too small to explain this increase of particle
confinement tirne.
The end loss flux has been measured for same
plasma conditions by using the end loss analyzer toobserve the electrostatic effects which may be enhanced
by NBI. Time evolution of end loss flux amount isshown in Fig. 6. The loss flux has been reduced by NBI
especially in the early equilibrium phase (100 ps-150

global energy loss.
In previous theoretical work [5,6], it was predictedthat the electrostatic potential arising in the edge plasma
would reduce the particle loss. In our case, because of
the high mirror ratio, open field lines around the FRC
resembles a magnetic mirror configuration, andespecially the configuration near the X points may beps). Measured total amount of loss flux is consistentregarded as magnetic cusps. During NBI, thewith the time evolution of particle inventory in Fig. 5.
Spatial distribution of end loss flux has also beenmeasured at the equilibrium phase. Figure 7 shows theelectrostatic potential could be enhanced by the beam

spatial distribution at 150 ps. Here the vertical error bars

ends of the confinement chamber as estimated bynumerical calculation [1]. The hot beam ions near the X
points can possibly create an enhanced potential. This
beam-assisted potential could reflect the plasma ions

ions.

The density ofthe fast beam ions increases at bothindicate the standard deviations of loss flux over several

plasma shots. The spatial distribution shows nosignificant change duo to NBI. Time evolution ofparallel energy of loss ions En ( l/2 mi(v{o* + vfu;) tras
also been estirnated by using this energy analyzer as

which leak onto the open field lines. This would
especially admit the possibility of improving the particle
223

Asar T . et al., End Loss Measurement of Neutral-Beam-Iniected Field-Reversed Configuration Plasma

confinement in the mirror region and around the X

gradient on the separatrix, and thereby improves theloss ion flux. Therefore it is concluded that the NBI in
this experiment does not cause significant change ofplasma potential or potential barriers. As anothercandidate to explain the improved confinement, the hotcore FRC plasma confinement.

beam ions could heat the electrons [8] and suppress

In a FRC plasma, it is known that the parallel
energy component of end loss ions are accelerated by
the pressure drop between core plasma region and theend region [7]. If the potential change decelerates loss
ions, its parallel velocity distribution could change and/or the cross sectional area of loss flux could narrow
down. However, end loss flux measurements in thismicro/macro instabilities. For further investigation, weare preparing a YAG-laser Thomson scattering system
to measure the electron temperature in detail.

work have indicated that the beam ions have nosignificant effect on plasma potential or potential
barriers at the edge plasma region. Therefore, it isconsiderable that some other mechanisms such as

experiments. They are grateful to Mr. Yano for hiselectron heating and/or global stabilization due to fast
beam ions may be responsible for the improved
confinement property in NB-injected FRCs.

[1] T. Asai et al.,Phys. Plasmas 7,2294 (2000).
I2l A.Shiokawa and S. Goto, Phys. Fluids 5, 534

points. In our case, it is possible that the loss reductionin the edge plasma region decreases the pressure

Acknowledgements
The authors express their gratitude to the membersof the FIX group for their help in performing these NBI
technical assistance.

References( l 993).

t3l M. Okubo et al., Fusion Eng. Des. 34-35, 547
5. Summary

fl997\.

End loss flux measurements have been performed

t4l M. Okubo et al., J. Plasma Fusion Res. 70, 1102

in order to investigate the mechanism of improved

0994\.
L.C.
Steinhauer, Phys. Fluids 29,3379 (1986).
t5l[6] P-R. Chiang and M-Y. Hsiao, Phys. Fluids B 4,

confinement in NB-injected FRC. The total amount ofloss flux measured by the end loss analyzer is reducedin the early equilibrium stage due to NBI, which isconsistent with the time evolution of total particle
inventory. However, obvious changes have never been
observed on the spatial and velocity distribution of the3226 (1992).

[7] T. Otsuka et al., Phys. Plasmas 5, 3649 (1998).
[8] T. Asai et a/., J. Plasma Fusion Res.77, 1230
(2001).

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