J. Plasma Fusion Res. SERIES, Vol.5 (2002) 283-287

Formation and Sustainment of Low Aspect Ratio Torus
Plasma by EGH in the LATE Device
UCHIDA Masaki, HIGAKI Kenichi, YOSHINAGA Tomokazu, IGAMI Hiroe,
TANAKA Hitoshi and MAEKAWA Takashi
Kyoto University, Kyoto 606-8502, Japan(Received: ll December 2001 / Accepted:24May 2002)

Abstract
A plasma current of 1o - 3 kA is generated and maintained for I second by injecting a 2.45 GHz
microwave power of 5 kW without Ohmic heating power. Magnetic measurements suggest that closed

flux surfaces are formed. The electron density inferred from an interferometer is more than 1.0 x 10rr
cm-3 which is beyond the plasma cut off density, suggesting that electron cyclotron heating by modeconverted electron Bemstein waves may be responsible for plasma heating and current drive. The plasma
currents are observed to increase with the increase of RF power and equilibrium vertical field, and 1o = 5
kA have been obtained by 2 GHz klystron power of 53 kW.

Keywords:
spherical tokamak, current drive, plasma production, electron cyclotron heating, electron Bernstein wave1. lntroduction

plasmas since EB waves can propagate and be absorbed

Since Peng and Strickler [1] introduced theoretical

via EC damping in such overdense plasmas.
The LATE (Low Aspect ratio Torus Experiment)
device has been constructed to investigate the basic
physical mechanism of plasma production, heating andcurrent drive by the microwave power alone at electron
cyclotron frequency in the low aspect ratio torus plasma.
The main research objective of the LATE is to produce

advantages of t.he spherical torus (ST) concept in 1986,
there has been great interest in the ST. Their predictions
have been experimentally verified on small ST's [2] andthe next generation ST's recently began to operate [3,4].
Because of the narrow center column space in theST devices, the inductive flux of ohmic coils is severelyrestricted. Non-inductive startup and current sustainmentis therefore an important subject for ST. Heating andcurrent drive schemes by means of such as the coaxial
helicity injection t5l (CHI) and the electron cyclotron

the ST configuration by ECH without Ohmic heating. In

heating t6l (IICH) are now studied in several ST

power experiments with a 2 GHz - 350 kW - 0.1 sdevices. As for ECH, using a microwave at the electron
cyclotron frequency, plasma production, current start-up

klvstron.

and sustainment may become possible. We cannot,

2. Experimental Apparatus

however, take an advantage of the conventional ECH
because ST plasmas are essentially overdense. Theelectron Bernstein (EB) wave heating is considered adevice which is described in detail elsewhere [7]. Since
our study is concentrated upon the non-inductive plasma

this paper, we report an start-up and sustainment of Ip 3 kA for 1 second by a 5 kW ECH power from a 2.45
GHz magnetron and some preliminary results of highThe experiments are performed on the LATE

possible way to the effective heating of these ST
Corre sponding author's e -mail : m-uchida@ energy.lcyoto-u.ac.jp283@2002by The Japan Society of Plasma
Science and Nuclear Fusion ResearchUchida M. et al., Formation and Sustainment of Low Aspect Ratio Torus Plasma by ECH in the LATE Device

process [8]. Another system for a 2 GHz - 350 kW - 0. 1

production, the LATE has no ohmic solenoid. The
stainless vacuum vessel is a l-m-diameter and l-m-s klystron has the same structure except for the size of acircular waveguide and a quartz window.

height cylinder, which is evacuated with a turbo
molecular pump to base pressures of < 5 x l0-5 Pa.

3. Experimental Results and Discussion
A. Experiments with a 2.45 GHz - 5 kW

Both toroidal magnetic field coils and verticalmagnetic field coils can be operated steady. A 11.4-cmdiameter center stack can flow 60 kAT which generates
a fundamental ECH resonance layer (B = 875 G) for
2.45 GHz microwave at a radius of R = 13.7 cm. The
vertical field coils consist of two sets of mirror-type

Magnetron
Experiments are performed in quasi steady-state.
The microwave power is injected after both the toroidal
and the vertical coil currents become steady, which havecoils.
The microwave power of < 5 kW from a2.45 GHzCW magnetron is converted from a TE61 rectangular
waveguide mode into a TE11 circular mode which isinjected into the plasma through a circular quartz

been ramped up to specified values in advance.
Figure 1 shows time histories of a discharge for thewindow. The power is launched from the outboard sideon mid-plane at an oblique angle to the toroidal field.
The injected wave is linearly polarized with the electricfield parallel to the equatorial plane. It is expected inthis setup that some portion of the wave is modeconverted into the EB wave via the so-called OXB

the vertical coil current is raised up to a specified value,

toroidal coil current of ft = 59.4 kAT, the 2.45 GHz
microwave power Ppp = 5 kW, and H2 gas pressure ps =6 x 10-3 Pa. When the microwave power is injected after
the flux loop signal @6, wound around the vessel (seeFig. 2), falls in - 20 ms and then is kept constant. The
negative variations of the flux signal correspond to thevariations such that the applied vertical field is
weakened, which means a net toroidal currents flow inthe opposite direction of the vertical field coil currents.
In this shot, the vertical field strength By is ramped up
in the midst of the discharge. The flux loop signal <D6 isplotted with subtracting the contribution from this By,d--\tiii.<\\.|Current center
Ip:3.lkA(R-18.6cm, r-2cm)

1/,\ti- 'z/te)

r_7/l

R:34cm

1.5

0.5-0.5

time (s)Flux loop coils

Fig. 1 Time traces of a discharge for /t = 59.4 kAT, Ppe =5 kW, and H, gas pressure p0 = 6 x 10-3 Pa. (a) in-I

0jected microwave power; (b) vertical coil current;(c) flux loop signal Ou; (d) soft X-ray emission sig-nal from a SSB diode looking for the whole

0.2 0.4 0.6
R (-)

0.8

Fig.2 A contour map of the poloidal flux with /p = 3.1 kA

plasma; (e) line-integrated electron density along
the vertical chord at B= 34 cm (see Fig. 2).

for the discharge shown in Fig. 1 at t= 1.65 s.

284

Uchida M. et al., Formation and Sustainment of Low Aspect Ratio Torus Plasma bv ECH in the LATE Device

ramp, and thus; shows the net variation of poloidal flux
produced by ttLe plasma current. When the vertical fieldcoil current is ramped up at a rate of 160 A/s from I =1.0 s to I,l s, the plasma current is increased. Thecurrent is sustained and monotonically increased until
the microwave power is turned off. In the case that theBy is ramped t:arlier in a discharge, a plasma current ismaintained for I second after the Bu ramp (dotted line).
The soft X-rav emission signal and the line-integrated
electron density are also increased with the By ramp,
indicating that the plasma parameters are improved. The(c) 30
20

a1oN-10_20-30

line-averaged electron density at the end of thedischarge is n,, - 1.0 x l0rrcm-3 even if the plasma isfilled uniformly in the entire vessel, and is beyond theplasma cut off density (7 x 10r0 cm-3).
The magr.itude of the plasma current is calculated

01020304050
Rprobe tip position (cm)from the measured flux variation in six loop coils (seeFig. 2). Assunring that the plasma current is a toroidal
determined by using the least square method such thatthe measured flux data is fitted the most. In theFig. 3 (a): Radial profile of the ion saturation current at z= +6.8 cm; (b): Flux variation in the No. 5 flux loop
coil as a function of the location of the probe tip;(c): Flux contour inferred from the magnetic mea-

discharge shorvn in Fig. I, a toroidal current of Ip - 2.6
kA flows at (R , Z) = ( 18. 1 cm, -l .5 cm) just before theBy ramp (/ = t.0 s), and Ip = 3.1 kA at (R, Z) = (18.1

taken place. The process of the mode conversion is notcm, -2.O cm) at the end of the microwave pulse (/ =clarified.

1.65 s). In this discharge, the current position is hardly
changed durinl; the By ramp. The loop voltage producedby the By ram'r is about 3 mV and the resulting Ohmic
heating is negligible. These results suggest that theincrease of the, plasma current and the improvement ofplasma param()ters are not attributed directly to the By
ramp but to riome change in the coupling between
current-carrying electrons and the microwave. Further
experiments are left for the study on this mechanism.
Figure 2 illustrates the contour map of the poloidal
flux inferred fi'om the fit for 1p = 3.1 kA. This indicatesthat the closed flux surfaces are created. The outermost
surface is limited on the casing of the center stack, andhas an aspect ratio ofA - 1.4. The current center isTo check this flux surface closure, radial profiles of
the ion saturation current are taken with floating double
probes scanning shot by shot. In Fig. 3, the radial profile
at z = +6.8 cm is shown together with the magnitudes of
05 and poloidal flux contours for which measurements
are performed. The abscissae in the Fig. 3 (a) and (b)filament current, its magnitude and position issurement.

represent the location of the probe tip. The ion
saturation current increases abruptly near the outermost

magnetic surface which is inferred from the magnetic
measurement, and, in accordance with it, @5 decreases
due to disturbing the flow of the plasma current. These

results show consistency with the magneticmeasurement. The corresponding measurement in thevertical direction ttt R = 24.5 cm is also performed,confirming the consistency with the magnetic

slightly outside the ECR layer. The microwave
interferometer chord is shown in Fig. 2, and the lineaveraged density exceeds the plasma cut off density as
mentioned above. These results suggest that electron

measurement.

The magnitudes of the flux variation in experimentswith 3-5 kW microwave power and the various verticalfield strengths, are summarized in Fig. 4. Since thelocations of the current center are almost the same
through these experiment, @6 indicates a quantity

cyclotron heating by mode-converted electron Bernstein
wave may be responsible for plasma heating and current
drive. The inje:ted waves may be mode-converted to theEB waves via the OXB process, or possibly XB process

proportional to the plasma current. The flux variation ofwith multiple wall reflections. For the density profile@6 increases almost linearly with By under a fixedroughly infelred from Langmuir probes and themicrowave power, but become saturated and decreased
when By is too strong (crossed symbols: Pnr = 3 kW).

microwave in,.erferometer. both the orocesses can be285

Uchida M. et al., Formation and Sustainment of Low Aspect Ratio Torus Plasma by ECH in the LATE Device

0.50.8o Ppp=S kW, Ramp-up Bvo Ppp=S kW, Steady Bv
X PRr=3 kW, Steady Bv

0.4.2GHz,Pnr=53kW

r' 2GHz, Pss - 19kW
I 2GHz, PRF = gkw
0.6

oO<-lP =3 kAIgo 2.45GH2, e*r=+skw

o,rT18*4(ox":' xf

E' 0.2E 0.4ra)aIT\lllp=5kAo'g,tlt

0.1

0.0

ooo.g

at!

0.2

lr\ooQtd- tp=3kA

X

010200,030

10203040

Bu (R=20cm) (Gauss)

By(R=2Ocm) (Gauss)

Fig. 5 The flux variation in the No. 5 loop coil versus thevertical field strength for P^, = 9, 19, 53 kW at 2Fig. 4 The flux variation in the No. 6 loop coil versus thevertical field strength.

GHz. Results with 2.45 GHz microwave (P". - 4.5kW) are also shown.

With higher microwave power (filled circles: Pnn = 5kW), this saturation is shifted to the stronger By regime
and accordingly reached a larger (D6. In addition, when
By is ramped up as shown in Fig. 2 (open circles: P*" =5 kW), @6 increases without saturation.

O A-1.4,r-1.2
o-'4

B. High Power Experiments with a 2 GHz -350 kW KlystronA new antenna for 2 GHz - 350 kW klystron haso/"abeen installed to investigate higher microwave power
regime, and some preliminary results are obtained.
During the microwave pulse of 100 ms, the vertical field
is ramped up in first 80 ms and steady in the rest 20 ms.
The plasma currents are observed to increase withan increase of the microwave power and the equilibriumoo'100ox;,
200

300

400a'Brr(Ro, Zry) I lto(srb'm)

vertical field. In Fig. 5, the magnitudes of the flux
variation of (D5 at the end of microwave pulse arekA have been so far obtained with 53 kW microwave
power. It is noted that the current does not increase

Fig. 6 Observed plasma currents are plotted as a function of the vertical field at the current center. Solidline (a) shows a possible equilibrium vertical field
tor A= 1.4, r= 1.74,6= 0.41 1, and Bi (a) = -9.679
in ref. [7]. Dashed line (b) shows the vertical field
which satisfy the force balance with a given toroi-linearly with the microwave power. In the high power

dal plasma with A = 1.4 and rc= 1.2, using aplotted as a function of By. The plasma currents up to 5

olasma self-inductance and a mutual inductancein ref. [8].

regime, it is found that there is an overdense region
outside the magnetic surface, thus this may prevent themicrowave power from being absorbed in the coreplasma. The control of density profiles is needed forresulrs of both the 2.45 GHz RF and the 2 GHz RF
experiments are plotted. The solid line (a) represents a
possible equilibrium solution that is calculated for agiven aspect ratio of A = 1.4 with some constraints on
the geometric parameters describing the plasma crosshigher power experiments.
In Fig. 6, the obtained plasma current is comparedwith an equilibrium model developed by Ludwig andAndrade [9] for low aspect ratio tokamak plasma. The286

Uchida M. et al., Formation and Sustainment of Low Aspect Ratio Torus Plasma by ECH in the LATE Device

section. This solution has an elongation r= 1.74 and a

The plasma current is observed to increase with themicrowave power and the equilibrium vertical field, andthe plasma current up to 5 kA has been obtained with 53
kW ECH power. The magnitudes of the obtained plasma
current are roughly consistent with the theoreticalequilibrium vertical field for the low aspect ratio

triangularity 6 = 0.411. Another comparison is madewith the vertical field which satisfy the force balancewith a given toroidal plasma with no internal pressure,using a self-inductance and mutual inductancedeveloped for an arbitrary aspect ratio by Hirshman andNeilson [0]. The dashed line (b) shows the equilibrium
vertical field calculated for A = 1.4 and r = 1.2. The

tokamak plasma.

magnitudes of the observed current are roughly

References
Y-K.M.
PengandD.J. Strickler, Nucl. Fusion 26,[1]

consistent with the equilibrium vertical fields in both themodels. This suggests that the low aspect tokamak

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t2l A. Sykes er al., Nucl. Fusion 39, l27l (1999).
t3l S.M. Kaye et al.,Phys. Plasmas E,1977 (2001).
t4l A. Sykes et al., Phys. Plasmas 8, 2101 (2001).
t5l B.A. Nelson et al., Phys. Rev. Lett. 72, 3666

configuration is formed by ECH power alone.

4. SummaryA plasmrL current of 3 kA is produced andmaintained for I second by 5 kW ECH power at 2.45(ree4).

GHz without Ohmic heating. The magnetic[6] C.B. Foresr er al., Phys. Rev. Lett. 6E, 3359 (1992).
[7] H. Tanaka et al., Proc. 2000 Int. Conf. Plasma
Pftys., Quebec, Canada,2000, BPl. 158.
t8l T. Maekawa et al., J. Phys. Soc. Jpn 48, 247

measurements indicate that the closed flux surfaces areformed and the outernost surface has an aspect ratio ofA - 1.4. The plasma current is decreased when the probe
is inserted insicle the closed flux surface, confirming theconsistency with the magnetic measurement. The central
density exceeds the plasma cut off density, suggesting
the heating by the electron Bernstein wave.
The new antenna for a2 GHz - 350 kW microwave
has been installed and preliminary results are obtained.

0980).

[9] G.O. Ludwig and M.C.R. Andrade, Phys. Plasmas
5,2274 (1998).
[10] S.P. Hirshman and G.H. Neilson, Phys. Fluids 29,790 (1986).

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