J. Plasma Fusion Res. SERIES, Vol.4 (2001) 373-378

Spectroscopic Studies of a High MachNumber Rotating Plasma Flow
ANDO Akira*, ASHINO Masashi, sAGI Yukiko, INUTAKE Masaaki, HATTORI Kunihiko,YOSHINUMA Mikirou, IMASAKT Atsushi, TOBARI Hiroyuki and yAGAI Tsuyoshi
Dept. of Electrical Engineering, Tohoku University, Sendai 9g0-g579, Japan(Received: 5 December 2000 / Accepted:27 October 2001)

Abstract
Characteristics of an axially-magnetized rotating plasma are investigated by spectroscopy in theHITOP device of Tohoku University. A He plasma flows out axially and rotates azimuthally near themuzzle region of the MPD arcjet. Flow and rotational velocities and temperature of He ions and atoms
are measured by Doppler shift and broadening of the HeII (,?, = 468.58nm) and HeI (2 587.56nm) lines.
=
Rotational velocity increases with the increase of axially-applied magnetic field strength and dischargecurrent. As discharge current increases and mass flow rate decreases, the plasma flow velocity increases
and Z1 increases. Ion acoustic Mach number of the plasma flow also increases, but tends to saturate atnear 1. Radial profile of space potential is calculated from the obtained rotational velocity. The potential

profile in the core region is parabolic corresponding to the observed rigid-body rotation of the coreplasma.

Keywords:
plasma acceleration, plasma rotation, HITOP, MPD arcjet, spectroscopy, supersonic plasma flow, shockwave

1. lntroduction

self-induced azimuthal magnetic field.
It has been proposed and examined experimentally

A high beta plasma flow with supersonic velocity is
one of the interesting topics in MHD researches. Plasma

to operate an MPD arcjet with a magnetic field appliedexternally in the axial direction in order to enhanceplasma acceleration performance. There are several
acceleration mechanisms to an MPD plasma in an

acceleration in supersonic and super-Alfv6nic region has
attracted attention in relation to the formation of cosmic

jet in astrophysics. It has been proposed that a high beta
plasma would be confined stably in high Mach number

externally-applied magnetic field [2,3].
In an axial magnetic field 'B., interaction between
Brand, J, results in azimuthal electromagnetic force"/, x

plasma flow, where the static plasma pressure isbalanced with the dynamic pressure of the flow
according to Beltrami-Bernoulli' s conditions I I ].

8., which rotates the plasma azimuthally. In a divergent
nozzle type of the axial magnetic field, the rotational
flow energy is expected to convert to the axial flowenergy. In addition, the electron Hall current ,/e interacts
with 8", which arises from the divergent magnetic field.
This force also accelerates the plasma in the axial

Characteristics of a supersonic plasma flow has
been investigated in MPD (Magneto-Plasma-Dynamic)
arcjet plasma. A high power quasi-steady MPD arcjet is
operated in the HITOP device of Tohoku University. A
plasma in an MPD arcjet is accelerated axially by"/" xB6force, where I is radial discharge current and Be is*Corresponding author's e-mail:akira@ ecei.tohoku.ac.jp

J /J@2001 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchAndo A. et al., Spectroscopic Studies of a High Mach-Number Rotating Plasma Flowlens connected with a optical fiber. The viewing angle
can be changed from 90 deg. (perpendicular direction)
to 60 deg. (oblique direction) to the plasma flow. Ion
temperature Z; and axial flow velocity Uf of He ions aredirection.
A plasma flow velocity and temperature are very
important parameters of an MPD arcjet plasma in order
to investigate plasma acceleration mechanism and toobtain a high-B and supersonic plasma flow for several

calculated from the Doppler line broadening and theDoppler shift of the HeII line spectra, which aremeasured by viewing the plasma flow obliquely. Thoseof neutral He atoms, T^ and U f , are also obtained fromMHD studies.
Purpose of the present work is to clarify acceleration mechanism of the MPD arcjet with an externallyapplied magnetic field. Several parameters such as anaxial and rotational flow velocities, density and ion/
electron temperatures are measured in an axially-

those of HeI line spectra.

Plasma rotation velocity is measured by viewing
the plasma flow perpendicularly near the MPD muzzle.

Radial profiles of the velocities, IJ! and U!, aremagnetized MPD plasma. Dependence on discharge
current, mass flow rate and intensity of the appliedmagnetic field are reported and pressure balance of therotating plasma flow is investigated in this paper.

obtained by moving the lens vertically (from Y = -6cm
to Y = 6cm).

Electron temperature and density profiles aremeasured by movable double- and triple-Langmuirprobes. Blow-off plasma characteristics are alsomeasured by other diagnostics, such as a microwave
reflectometer, a multi-channel magnetic probe array and2. Experimental Apparatus
Experiments are performed in the HITOP (HIghdensity TOhoku Plasma) device of Tohoku University
I4,5,61. The HITOP device consists of a large cylindricalvacuum chamber (diameter D = 0.8m, length L = 3.3m)
with eleven large and six small magnetic coils, which
can generate an uniform magnetic field up to 0.2T.
Various types of magnetic field configurations can beformed by adjusting the coil currents. In the present

a time-of-flight neutral particle energy analyzer in theHITOP device.

3. Experimental Results
Radial profiles of line intensities, rotational
velocities and temperatures measured at 3cm
downstream from MPD outlet (Z - 3cm) are shown in
Fig. 1. Profile of the HeII line intensities is strongly
peaked in the discharge region, while that of the HeI
line intensities is rather broad.
Rotational velocity of He ions, U1e, increases

experiments most measurements have been done in an
uniform magnetic field.
The MPD arcjet, which is installed at one end of
the HITOP, has a coaxial structure with a center

tungsten rod cathode and an annular molybdenum
anode. Inner diameter of the anode is 3cm. A quasisteady discharge continues for lms with a pulse formingnetwork (PFN) system and a fast puffing of Helium gas.
Maximum discharge current is 10kA with a typicaldischarge voltage of 200V.
A high density (more than l0r6cm-3) plasma isformed in the muzzle region of the MPD arcjet. The
plasma is accelerated in the axial (Z-axis) direction by "/,linearly with the radius in the core region, whichindicates that the plasma rotates as a rigid body. The
rotational velocity attains its maximum value at a radius
which corresponds to the inner radius of the anode. Itsangular frequency is 2 x l06rad/sec when Ia = 8.7kA and
8. = 0.1T. The rotation is in the direction of theazimuthal electromagnetic force J,x Bz.

Ion temperature I obtained from the line

x Be force, and expands in the large vacuum tank along
the field line. Then, a high density, highly ionizedplasma flow is produced in the HITOP.
The blow-off plasma characteristics are measured

broadening of the HeII lines is almost constant at 30eV
near the mtzzle region. Temperature of the He neutral
atoms Zu is about 10eV in the flow core region.
The maximum rotational velocities U!, U! andat a muzzle region by a spectrometer with a focal lengthof lm. Two He lines, that is, HeI (i, = 587.56nm) andtemperatures Ty, Tu are also measured as functions ofdischarge current 16, mass flow rate h and axial
magnetic field strength B. as shown in Fig. 2. As shownin Fig. 2(a), Uf increases linearly with 16. Z1 also

HeII (1. = 468.58nm), are measured in the present
experiments. These line spectra are obtained in every
0.1ms time interval during a shot with the spectral
resolution of 0.02nm. The spectroscopy data in thisincreases linearly with 16 for the smaller values andincreases steeply for.fa of more than 7kA. On the otherhand, Uuo of the neutral atoms is almost independent ofIa. As rit decreases, U? and 7i increases as shown in Fig.

paper are taken at lms after the discharge is fired. Lineemission from the plasma is viewed through a focusing
3'14

Ando A. et al., spectroscopic studies of a High Mach-Number Rotating plasma Flow
162(b). The large increase in Z1 is also observed for lowerm. Dependence of U! and Z" on ru is quite different
from those of U;a and 21. This discrepancy is probably(a)T12i{FgoF

troE4aellrA-He ll.ccaused by the decrease in collisional interaction between

YHe ions and atoms for low rh case.It is noted that Ulot

increases with an increase of B. as shown in Fig. 2(c).

l.ooThis corresponds to the azimuthal acceleration by J,xa020FatE 10E5o
E:"cJ.9 - -r0Er -zoBr.

(b)

Axial flow velocities Uf, U: and temperatures fi,.9

Zu are measured atZ = 40cm as functions of Ia, m and B,ol.ln

and are shown in Fig. 3. Dependence on Ia and rit arevery similar to those in Fig. 2. On the other hand, theuniform axial magnetic field strength does not affect the9-r rllo@=a_I I lm

E-30

axial flow velocity. Uf is almost constant as Bz'deincreases, though Z1 increases with the increase in ,B,, asshown in Fig. 3(c).

40Assuming that the axial velocity does not change inso9^J>

the two different measurement position, we cancalculate the total velocity Uf of He ions by using theobtained data of Ul and, U? as

Eg-zob.
oF.E,.=

tor,t[l.,rzt utzu;=Vlui) +luil0(l)Then total Mach number Mf is expressed as
Fig. 1 Radial profiles of (a) spectral line intensity, (b)rotational velocity and (c) temperature. Open andclosed circles are those obtained from Hell andrlt

Ml=ai' L,(2)

Figure 4 shows dependence of the calculated Ufand M! on 1o and h. lt is difficult to measure Z" in theHef lines, respectively. la=8.7kA, rh=O.OGg|sec, B,= 0.1L

m=0.0679/sec, B=0. 1 T, Z=3cmfrI,l (7,+7,\ / m,lo=7.5kA, rir=o.o67g/sec, Z=3cm7.5kA, B=0.1T,2=3cm'9il 20--- E(c)

1110

olF

(l)E>-v(uo.E3. ro6-.-oEJoat0ooac)

oclFrs IIJ3]F tlo-.tta*l1l(U.llL0 6.9E-30ots- 20EsEro10aaEPo
Discharge Curent lo [kAJ5-0Mass Flow Rate ri [g/sec]ao

00.04 0.08 0j2[l0.16

Magnetic Field B,

Fig. 2 Rotational velocities Ul, U! and temperatures 7,, T" as functions ot (al lo, (b) m and (c) g,. Open and solid symbols
are those obtained from Hell and Hel lines, respectively.
375

Ando A. et dl., spectroscopic Studies of a High Mach-Number Rotating Plasma Flow
td=7.5[kA]

O,O : rh=0.0679/sec Bz=0'1m

rir=O.067[g/sec], Z=40[cm]ld=7.s[kA], B=0. 1 m, z=4olcml

a,1 : rir=O.139glsec l=40[cm](c)6>9n6o do';i ->XE>N.25 s-20 ll,1sF-10 P:

61"'i1010080080o2020ooo'lollJ

5ElLloo

3x.l
0Fd,:_o)(goEaoo ooFioilso 0
Mass Flow rate m [g/sec]

Discharge Current lo [kAJ

0.04 0.08 0.12 0.16

Magnetic Field B, FlFig.3 Axial velocities Ui, Ui and temperatures 7,, T"as functions of (a) /a, (b) m and (c) 4. Symbols are the same as
those in Fig. 2.

Total Mach Number: Hel (O), Hell(O)Total Flow Velocity : Hel (I ), Hell( ! )j30

8 o.4o.z
Eo24681(b)oooo

Bto saturate at I in the region ofhigher current and lowerotratr

H 0.4ot

mass flow rate. When Mf is saturated, Ty strongly
increases as shown in Fig. 2 and Fig. 3. These saturation
phenomena of Ml seems to be related to the occurrenceof a shock wave.
A radial profile of space potential @.(r) can beestimated from the obtained profile of UP in the rotatingoaElt,Ltr30 :)2010I

0.05EFLaIt40oo

0.62-co6The total ion velocity increases with the increase inthe current and with the decrease in the mass flow rates.
As is shown in the figure the total Mach number tendsC)

lo=7.skA, B.=0.17

Fthe figure.

LL

Discharge Current lo [kAJ0.2
Eof10 o=FsE 0.8

E=o20 o6-c;1o40;1g 0.8
E
2 0.6oo'outlet region of MPD arcjet. In the downstream regionof the plasma flow 7" is measured by electrostatic probeand obtained 5-10eV. Though I" is probably lower than11, it is assumed that Z" equals to 4 in the calculation ofMl. The same dependence of He atoms is also plotted in6oplasma. The radial force-balance equation for a plasma
rotating with the azimuthally-rotational velocity Ue is'6

given by

6- - (Jo2 ap
M"?
-fi + JtB,-J"Bs =Qo=lJ-J0

0.15
0.1
Mass Flow Rate rh [g/sec]

EoF

(3)

Here, M is an ion mass and P = Pi + p" is a totalpressure. A generalized Ohm's law with the assumptionof negligible resistivity can be expressed as,Fig. 4 Total velocities U,'(open squares) and total Mach
number M/ (open circles) of He ions as functionsof (a) /, and (b) m. Those of neutral atoms areplotted as solid ones.

en(E,+UsB,)-JeB, +J-Bo+%,=o
ctr

376(4)

Ando A. et 41., spectroscopic studies of a High Mach-Number Rotating plasma Flow

Then the rotational velocity Ue is obtained as

rr - E, , T, Olnn MUr've--U-*---reT

cyclotron angular frequency and pr= r/TMTrleB,is ion
cyclotron radius. As the E x B drift term dominates therotational velocity in eq.(5), the plasma rotates in thesame direction as that of the E x B drift. Using theexperimental result shown in Fig. l, radial profiles ofE.(r) and @.(r) in the rotating plasma are calculated andare shown in Fig. 5. The derived potential profile in thecore region is almost parabolic. The ratio of the threeterms in eq.(5) is 2:l:0.35 in this case.
The increase of rotational velocity with the increaseof .B., as appeared in Fig. 2(c), is mainely due to theincrease of 8,, which has been confirmed by other(s)The rotational velocity is composed of three terms, E x

B dift, diamagnetic drift and centrifugal force drift.
When the density profile n(r) is assumed to begaussian, n(r) = 710 exp(-flrs2), the equation can benormalized with non-dimensional variables, X = a/{);, y= a*tldli and | = (pi/rd',

X2+X=I-|(6)experiments.

Here, ar = Uelr is an angular frequency of the rotation,@zxa = -E,/rB. is that of E x B drift, Qi = eBzlM is ion

4. Gonclusion
Flow velocity and temperature of an axiallymagnetized MPD arcjet plasma are measured by15()

ooxEI

10

spectroscopy in the HITOP device. With the extemallyapplied magnetic field 8,, the plasma is accelerated not,(a)only axially by J,x 86 force, but azimuthally by J,x B,force, where./, is radial discharge current and Bsis selfinduced azimuthal magnetic field.

50-5

The axial and rotational velocities and ion and atom
temperatures are measured by the Doppler shift andbroadening of the HeII and HeI lines.
The axial and rotational velocities increase with theincrease in discharge cuffent and with the decrease inmass flow rates. It is noted that the rotational velocity

D -10-15ooot,oao

16
128

increases with the applied magnetic field strength, while
the axial velocity is independent of it. Total ion velocity

4and Mach number are calculated and the Mach numberof the flow tends to saturate at near I in the operation00Eoregions of higher discharge current and lower mass flow-10rate. The strong increase of I; is observed in these-20

operation regions.
Radial profile of space potential is evaluated from-30
lrJ

the experimental data of rotational velocity. The
potential profile in the core region is parabolic-40

corresponding to the rigid-body rotation.

-50
20(d)

5. Acknowledgments

0This work was supported in part by a Grant-in-Aid

S -zoo -4O/

-60

'80-g'

for Scientific Research from the Ministry of Education,Science and Culture of Japan. Part of this work wascarried out under the Cooperative Research ProjectProgram of the Research Institute of Electrical-2-101

Communication, Tohoku University.

y (cm)

Fig. 5 Radial profiles ol lalUf , (b)ar. (c)F,and (d)d".
377

Ando A. e/ al., Spectroscopic Studies of a High Mach-Number Rotating Plasma Flow

Plasma Physics (ICPP96, Nagoya) 1,2014 (1996).

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