J. Plasma Fusisn Res.. SERIES, Vol. 3 (2000) 473476

Preliminary Pellet Injection Experimentin the GAMMA 10 Tandem Mirror
KAWAMORI Eiichirou*, TAMANO reruo, NAKASHIMA yousuke, yosHIKAwA Masayuki,
KOBAYASHI Shinji, cHo reruji, ISHII Kameo, MASE Atsushir)and yATSU Kiyoshi
Plasma Research Center, university of tsukuba, Tsukuba, Ibaraki 30s-g577, JAqAN])Advanced Science andTechnology Center for cooperative research, Kyushu university,Kasuga, Fukuoka, 816-8580 JAPAN(Received: 18 January 2000 / Accepted: 26 May 2000)

Abstract
In the GAMMA 10 tandem mirror, pellet injection experiments have been started as a solution forthe density limit problem. This is the first pellet injection experiment in open systems. We describe thepossibilities of confinement of pellet fueled particles. For that, we measure the number of end loss
particles and compare them with pellet fueled ones in various conditions of confining potentials. The
deterioration ofconfining potential with the pellet injection is a fundamental issue. The results show thatthe ion confining potential recover faster than central electron temperature due to the thermal barrier. We
also consider the operating space for fueling method. It is demonstrated that the operating space for pellet
injection exceeds gas fueled one on hot ion mode plasmas.

Keywords:
tandem mirror, pellet injection, density limit, confining potential

The major issue with pellet injection is a

1. Introduction
Density limit problems have been observed invarious tokamak devices, and pellet injection

deterioration ofconfining potential. One ofthe purposesof this article is to study the interactions between theplasma potential and the pellet injection.

experiments have been carried out as one of means tosolve this problem []. The density limit has also been
observed in hot ion mode plasmas [2] of the GAMMA
10 tandem mirror with gas-puff fueling. Pellet injection

In this paper, we show the possibilities ofconfinement of pellet fueled particles. For that, wemeasure the number of end loss particles and compare

experiments have been started as a solution for theproblem. This is the first pellet injection experiment andhas unique aspects in open systems. For example,
tandem mirror machine is suitable for measurement ofthem with pellet fueled particles. We also consider theoperating space in plasma parameters for the GAMMA
l0 hot ion mode under gas fueling and pellet fueling.

propagating pellet fueled particles along field lines.

2. Experimental Setup
2.1 The GAMMA 10 tandem mirror
The GAMMA l0 [3] tandem mirror consists of five

It is concerned that pellet fueled particles
immediately flow out along magnetic field lines in an
open system. However, confinement of particles fueledby pellet injection is possible with confining potentials
along the field lines in a tandem mirror.
*magnetic mirror cells. One is a central cell which is themain confining region of a plasma. Others areC o rre spondin g autho r' s e - mail : kaw amo ri @ p rc. t s ukub a. acjp

473

@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Kawamori E. et al., Preliminary Pellet Injection Experiment in the GAMMA l0 Tandem Mirror
12ICRF ; ICRF,.- r05B
E6xICRF

8"z5:

(a)

400.4
0.30.2lilT*[nhl,fi{AA4trW|i'|}t'] (o,rl/llI nu*r"r'o.i0l^.l''"\14{.14.2

Phg/BfuM-Z.s; 'a:ft;

Mt
Fig. 1 Schematic of the GAMMA10.

't.5.dl
xa-10.5(d)-itime [ms]Fig. 3 The time evolution of the diagnostic signals of atypical hot ion mode plasma with a pellet injected
at t = 150 ms. In due order, central cell line density(Nl-"), confining potentials {00: a barrier depth, @":Fig. 2 Schematic of the pellet injector.

ion confining potential), end loss ion currentminimum-B anchor cells and plug/barrier cells. The(1"n61o""),the soft x-ray signal (lsx).

plasma is sustained by ICRF heating with gas puffing in
the central cell. The confining potentials are formed byElectron Cyclotron Heating at plug/barrier cells.

installed in the GAMMAI0. The pellet injector is shownin Fig. 2. The injector is a pipe-gun type pneumaticpellet injector with eight barrels. The inner diameters of
the barrels are 0.39, 0.58, 0.79 and 0.99 mm. The pellet

The following instruments are mainly used forplasma diagnostics: Plasma densities are measured withmicrowave interferometers at several locations along theGAMMA 10 axial direction. End loss ion currents andion energy spectra at the ends are measured with end
loss ion energy analyzers (ELA). Plasma potentials in
the central cell and in the barrier region are measured
with heavy ion beam probes. The plasma potentials ininjector is mounted at a central cell midplane through a6m length of guiding tube.

3. Experimental Results
The experiments are carried out in the GAMMA 10hot ion mode plasmas. The barrel which is used in theseexperiments is the one of inner diameter 0.79 mm. We

the plug regions are determined by the energy analysisof end loss ions with ELA. The ion temperature isdetermined from the diamagnetic signal. Schematic ofcompare the results with a confining potential andwithout it.
The temporal evolution of the diagnostic signals ofa typical hot ion mode plasma with a pellet injected at t
= 150 ms is shown in Fig. 3. The ion confining potential
Q" and the thermal barrier depth @6 are formed withECH at plug regions from 130 ms to 200 ms. At thetime ofpellet injection, the central cell line density rises

the GAMMA 10 and the diagnostic instruments ofplasma are shown in Fig. L

2.2 Pellet injection system
The electron stored energy of the GAMMA l0
plasma is lower than that of other devices. Therefore, asubmillimeter pellet injector is required for fueling. For
this purpose, the ATF pellet injector made by the Oak

up to 2 times of that before injection. Subsequent

Ridge National Laboratory [4] was modified andincrease of end loss ion current is observed at both ends.
474

Kawamori E. et al., Preliminary Pellet Injection Experiment in the GAMMA 10 Tandem Mirror
2AThe thermal barrier depth @6 reduces to almost 0 V. The

soft x-ray signal, which correlates the central-cell(t)electron temperature, sharply decreases immediately
after the pellet injection. This implies that the electron
stored energy is insufficient to the pellet ablation.
The spatial profiles of line integrated Ha emission
in cases of gas fueling and pellet fueling are shown inFig. 4. In the case of gas puffing, the tendency thatparticle source are slightly localized at the plasma
periphery is observed. In the case of pellet injection, afueling into plasma core is observed.

No Gas pufl. | . Pellet

15Eoa ------>
1080lt65@.*185S95(D6044u=Limiterooa <-o.=at,co32A (r,'5o10y [cm]15l020Fig. 4 The spatial profiles of line integrated Ho emission
in the cases of gas fueling and pellet fueling.

4. Discussions
In Fig. 5, the number of end loss ions is plotted as afunction of the number of pellet fueled particles invarious conditions of potential confinement. In the casewith confining potentials in both plug/barrier regions,the number of end-loss particles is estimated to be lessthan several percent of pellet fueled ones. In the case ofno confining potential, on the other hand, the number ofescaping particles increased to several tens percent.

r] w/o plug potential

O with plug potentials
X with one side plug
ooo.9

too.

t-.

Under the condition that only one side potential isoformed, the number of the opposite end-loss particles isincreased to the same as the sum of the both ends loss
with no confining potentials. Furthermore, it is observedthat the number of flowing out particles decreases with.""ry5ood'8";
z?oo

the magnitude of the potential. Therefore, potential

f.--j erJ!

confinement of pellet fueled particles along field lines isclearly demonstrated.
Fig. 6 shows the operating space of the GAMMA

l0 hot ion mode. The vertical axis is average ion

6Noerret [x 1 01 sparticles]

temperature and the horizontal axis is average density atthe central region. We define average density asi""" = NL",/d. Here, d is the diameter of plasma (usually36 cm). In the case of pellet injection, we plot the time
traces of the parameters from the start of ablation to lms
after the time of maximum density. This time length

Fig.5 Comparison between number of pellet fueled
particles and number of end loss particles.

seems to be comparable to the particle confinement time

under these low electron temperature, since theseexperiments are carried out without the heating of1000

5sl

central cell electron. The solid line in Fig. 6 represents
the limit considering ion finite radius effects [5]. Whenthe characteristic time of ion heating becomes smaller
than electron drag time under the strong ion heating. ion
larmor radius become large. Consequently such ions arelost by charge exchange reaction near the plasma edge,
since the ion orbit get across the plasma boundary. Fromthe particle balance between inflow of neutral gas andloss by charge exchange reaction for peripheral fueling,t.:llF

100dr;* [" tottmo ]Fig. 6 Operating space for the GAMMA 10 hot ion modeplasmas.

it is shown as475

Kawamori E. et al., Preliminary Pellet Injection Experiment in the GAMMA 10 Tandem Mirrorll.-=i-ynfollowings:

i. Potential confinement of pellet fueled particles along\ov)*rofield lines is demonstrated.
where y0 is velocity of neutral flas, (ov)sx and rp arc
charge exchange rate coefficient and the plasma radius,
respectively. 1fl)cx increases with ion temperature inthis experimental condition. Thus, a operation with different fueling scheme may exceed the limit. From theresult in Fig. 4, pellet injection is more effective fuelinginto a plasma core than gas puff fueling. The above discussion provides the reason why the plasma parametersin the condition of pellet fueling exceed the limit withgas puffing.
Sustaining the confining potential is an essential
requirement for steady-state operation in tandem
mirrors. At the time when a pellet is penetrating into aplasma , the plasma potential along field lines seemed tobe flat. After the pellet injection, however, the ion
confining potentials recover faster than the central cellelectron temperature as shown in Fig. 3. The result
implies that the thermal barrier, which insulate central
cell and plug cell electron, is effective.

. It is clarified that the density obtained with the pellet

5. Conclusions[5] T. Tamano, in bulletin of l5th meeting of the Japan

fueling exceeds the limit with gas puffing due tofueling into the plasma core.

. After pellet fueling, the confining potentials recoverfaster than central electron temperature due to thermalbarrier.

. In future work, we are going.to study radial diffusion
processes of the pellet fueled particles.

Acknowledgments
The authors wish to thank the GAMMA l0experimental group for contributing data for this study
and carrying out the experiments.

Reference:tllc. Vlases e/ al., Nucl. Fusion 27, 351 (1987).

[2] T. Tamano, Phys. Plasmas 2,2321 (1995).

t3l K. Yatsu e/ al., Nucl. Fusion 39,1707 (1999).
[4] A.L. Qualls er al., in Proceedings of the l3th Symposium on Fusion Engineering, 2, p.1244.

For steady state operation in the GAMMA l0

Society of Plasma Science and Nuclear Fusion Re-search (in Japanese), (1998).

tandem mirror with pellet injection, we conclude the476