I. Plasma Fusion Res. SERIES, Vol.3 (2000) 202-205

Dynamic Behavior of Detached Recombining Plasmas
for Plasma Heat Pulse in the Divertor Simulator NAGDIS-I|OHNO Noriyasu*, NISHIJIMA Daisuke, MOTOYAMA Masataka, HATTORI Norifumi,
UESUGI YoshihiKoI, TAKAMURA Shuichi and WENZEL UWC2lCenter

Department of Energy Engineering and Science, Graduate School of Engineering,for Intergrated Research in Science and Engineering, Nagoya University, Nagoya 464-8603, JAPAN2Max-Planck-Institut
fiir Plasmaphysik, EURATOM Association D-10117 Berlin, Germany(Received:18 January 2000 / Accepted: 15 May 2000)Abstract
We will report on a comprehensive study of the dynamic response of the detached recombining
plasma in the linear divertor plasma simulator. The plasma heat pulse was generated by rf heating, which
can give a modification of the electron distribution functions. The negative spikes were appeared in thetime evolution of the Balmer series spectra, which indicate the transition between ionizing plasmas andrecombining plasmas. Detailed analysis on the time evolution was done with collisional-radiative code toreproduce the time evolution of the electron temperature.

Keywords:
detached recombining plasma, volumetric plasma recombination, edge localized mode, divertor

necessary to do well both in the reduction of plasma

1. Introduction

heat flux with the plasma detachment and good
confinements such as H-mode. In the design activity ofIn recent years, there has been a growing interest in

plasma detachment in edge plasma physics ofmagnetically confinement fusion research, which is oneof the most effective methods to reduce the plasma heat
flux to plasma-facing components [-3]. Recent studies
on detached plasmas in fusion-related divertor plasma
experiments make it clear that a volumetric plasma
recotrbination in detached plasmas plays an essential

ITER, H-mode with Edge Localized Mode (ELMy Hmode), which brings an intermittent plasma heat pulse
into the divertor plasma region, is one of the mostimportant operation candidates as a good stationary
confinement mode. Moreover, very complicatedrole in strong reduction of ion particle flux along themagnetic field, resulting in a decrease in the heat flux tothe divertor plasma region [6,7], which have not beenclearly understood yet. Therefore, we should understanddynamics of the detached plasma to the intermittent

phenomena associated with the ELM were observed inplasma-facing components. Continuum and a series ofvisible line emissions from highly excited levels due tothe radiative and three body recombination (EIR) wereplasma heat pulse induced by ELM.

We will report on a comprehensive study of thedynamic behavior of the detached recombining plasma
associated with EIR in the linear divertor plasma

clearly observed in tokamaks with a divertor configuration and linear divertor plasma simulators [4,5].

In next generation fusion devices, which are

simulator, NAGDIS-II (Nagoya University Divertorintended to have a long or steady state plasma, it is*Corresponding author's e-mqil: ohno@ ees.nagoya-u.ac.jp
202@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchOhno N. et al., Dynamic Behavior of Detached Recombining Plasmas for Plasma Heat Pulse in the Divertor Simulator NAGDIS-II

Simulator). The plasma heat pulse was generated by the3. Experimental Results and Discussion

modulation 6f discharge currents in the dc discharge

Plasma heat pulse generated by the rf heating wasintroduced into detached recombining plasmas in thedivertor plasma test region. The duration of the pulse is
0.5 msec. The rf power is 2 kW. Figure 2 shows thetypical time evolution of the line emission intensities
from excited states (2p- n'd:T) of helium atom at theneutral helium pressure P of 5.85 mtorr, when theplasma was detached from the target plate. The time
evolution is found to show very complicated dynamicbehaviors. The T" of the bulk plasma is estimated to be

and/or rf heating.

In the case of the modulation of discharge currents,
the time evolution of plasma parameters and helium
Balmer series spectra is found to depend strongly on theneutral pressure in the divertor plasma test region. It isfound that the EIR does not work enough to suppress anincrease of ion flux to the target plate during the heatpulse for the detached recombining plasma at a
relatively low neutral pressure. It is found that small
concentration of the energetic electrons plays an

about 0.3 eV before the rf pulse is on. At the beginning

important role in the dynamics response of the detachedplasma [8].

of the rf pulse, all line emissions are found to drop
rapidly in time, which means that the EIR becomes

In the case of rf heating, we can control theduration and power of the heat pulse by changing theweak probably due to an increase in the kinetic energyof electrons. After the first negative spike, the emission
intensities start to increase except for n'-10. Especially,
the intensities from low excited states n' - 3 and 4 areincreased to be larger than those before the rf pulse isduration and amplitude of the rf pulse. We haveexperiments on three different types of the heat pulse
injected to the detached plasma: l) long pulse - 0.5msec, 2) short pulse - 0.05 msec and 3) train of shortpulses, which are related to the frequency control of theELM.
In this paper, we would like to mainly focus on theexperimental results on l) long pulse rf heating.
Experimental results on 2) and 3) will be reported

on. They peak during the rf pulse, and gradually
decrease in time after the rf pulse is off. It is found thatthe second negative spikes clearly appear at t- 1.6 msec

for n' - 3 and at t - 1.3 msec for n' - 4.
In order to quantitatively specify the characteristics
of dynamic behaviors of helium Balmer series emission,

somewhere else.

especially the negative spikes mentioned above, we have

2. Experimental Setup

of a helium atom by using collisional-radiative (CR)The experiments were performed in the linear
divertor plasma simulator, NAGDIS-U shown in Fig. l.
The maximum magnetic field strength is 0.25 T. Bulk
helium plasmas were produced by the dc discharge
between LaB6 hot cathode and hollow anode. Themodel (Goto-Fujimoto code) including recombining as

analyzed the emission intensities from the excited states

well as ionizing components [9,10]. The population
densities of the excited states of a helium atom arethought to be governed by i) EIR and ii) electron impact
excitation from the ground state.
Figure 3 shows the dependence of emission
intensities from n'-3 and 8 on the electron temperature
I" as a parameter of plasma density calculated with theplasma density can be controlled by changing thedischarge current. The plasma column is terminated by
the target plate with water cooling installed at the end of
the vacuum chamber. The neutral pressure P can bewith a frequency of 13.56 MHz and a maximum power

CR model. It is proven that the electron temperature T"eein which the emission intensity is minimized exists. The
reason is that in the low 7i region, lower Z" gives larger
emission intensities because the population densities ofthe excited states are governed by EIR. On the otherof 10 kW.

hand, at the relatively high 7", the emissions strengthen

Spectra of light emissions from helium atoms aredetected at the axial positions of X = 1.06 m (Up), 1.39m (Middle), and 1.72 m (Down) from the discharge
anode electrode. Fast scanning probes are also installedat the same positions to measure plasma parametersj
Time-averaged heat load to the target plate is estimatedby measuring the temperature rise of the cooling water.

with the rise of Z" since the electron impact excitation isa dominating process. In short, the transition from therecombining phase to the ionizing phase and/ortransition from the ionizing phase to the recombination
phase give the negative spikes of emission intensities atI""q. By comparing Fig. 3(a) and (b), the value of theZ""q depends on the excitation levels because thepopulation densities in lower excitation levels are easy

controlled from 1.0 mtorr to 20.0 mtorr by feeding asecondary gas near the target plate.
Plasma heat pulse is produced by using rf generator

203

Ohno N. et al., Dynamic Behavior of Detached Recombining Plasmas for Plasma Heat Pulse in the Divertor Simulator NAGDIS-II

toioo.i"$***'n*3i4Tr

r6t Rosiq

10ei(a)z108-o.=lr r'lr r'lr rc=.Gi

Fig. 1 Schematic

Hotrofof the experimental apparatus,

NAGDIS.II.

E100

C.9aatr
5ttoo

10a-E
IJJ

106

\.//'

J/-re=lx101em-3

Eoc- ,,'.--

104(b)2p-8d;T

106
i

105

t\ --.-_

104\,\,\".\L103

to0.1

1e=5x1018m-3

103

102

@\J105

1.E.--Z)| .-2"
Ir/7'107

LoP'3d;T oe=5x1019m-3

5o'Te (eV)0.01
Fig. 3 Dependence of line emission intensities fromprincipal quantum number n' - 3 and 8 on theelectron temperature calculated by CR model asa parameter of plasma density.

0.001

01254
Time (ms)function could be close to the Maxwellian distribution
after the rf pulse. On the other hand, the first negative
spikes observed at the beginning of the rf pulse can not

Fig. 2 Time evolution of the line emission intensities
from principal quantum number n' from 3

through 10 in a fairly detached plasma at P- 5.85

be reproduced self-consistently, because the value ofthe

mtorr.

first negative spike differs from that of second one andall the line emissions decrease simultaneously. In thecase of such a rapid transition from recombining toionizing phase observed at the beginning of the rf pulse,
we should take non-equilibrium condition of atomic
processes into account, in particular, non-equilibrium
ionization becomes important. Usually, typicalequilibrium times of ionization and recombination

to receive the effect of the excitation process rather thanthose in highly excited levels. On the recombining
process, it tends to be reversed.
Based on the numerical results, we can reproduce

the time evolution of the ?" by tracing back the time
evolution of emission intensities from the negative
spikes in Fig. 2. Figure 4 shows the reconstituted time
evolution of T, in making the second negative spikes forn'- 3 and 4 to be an origin, where the electron
distribution function is assumed to be Maxwellian at any
time and the plasma density is l0rem-3. The
reconstructed time evolutions of T" from z'- 3 and 4
match well, which means that the electron distribution

processes are larger than several milliseconds in ourexperimental conditions. The characteristic time of thefirst rapid transition is found to be much smaller thanthe equilibrium time. On the other hand, someexperimental results indicate that the energetic electron
could be produced at the beginning of the rf pulse

204

Ohno N. et al., Dynamic Behavior of Detached Recombining Plasmas for Plasma Heat Pulse in the Divertor Simulator NAGDIS-II

probably due to the near-field acceleration. For instance,

detached recombining helium plasma to demonstrate
clearly the dynamic behavior of the volumetric plasmarecombination in a linear divertor plasma simulator.
The rapid transition from the recombining toionizing plasmas at the beginning of the rf heating andif there are arssumed to be fast electrons with an energyof 25 eV and a density of I x 10r7m-3 1O.5Vo of the bulk
plasma density), the ionization time for an excitedhelium atom (r?' - 6) can be estimated to be 2.0 x 10-6 s,slow transition from the ionizing to recombining
plasmas after the rf heating were observed, bothaccompanied with negative spikes in Balmer series line

where the cross section for the excited helium atom (n'- 6) is about 1.7 x l0-10m2[8]. For helium atoms inhigher excited levels, the ionization time is getting
smaller, which becomes comparable to the radiative
decay time for these highly excited levels. Thisestimation suggests that reionization due to the fastelectrons is playing an important role in the net
recombination, which could be related to the firstnegative spike. Neutral transport is also importantemissions.

Time evolutions of Balmer series line emission
were analyzed with collisional-radiative model, which

reproduces the time evolution of the electron
temperature.

region. We should need to use the time-dependent CR

Acknowledgements
The authors would like to express our deep
gratitude to Mr. M. Takagi for his great technicalmodel rather than the conventional CR model.

supports for this experiment.

because this reionizaiton effect depends on how fast thehighly excited atoms escape from the recombining

4. Gonclusiontll

We have performed experiments on injection of aplasma heat pulse produced by rf heating to theReferences
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