I. Plasma Fusion Res. SERIES, Vol.4 (2001) 391-394

Radial Electric Field Control with a Biased Hot Gathode inthe Tohoku University Heliac
KITAJIMA Sumio*, NAKANISHI Shigeyuki, HARADA Koumei, SUGAWARA Koujiand HASHIZUME Hidetoshi

Tohoku University, Aramaki aza-Aoba, Aoba-ku, Sendai 980-8579, Japan(Received: 5 December 2000 / Accepted: 5 October 2001)

Abstract
The influence of a negative biased electrode on the plasma parameters in the Tohoku University
Heliac (TU-Heliac) with an emissive hot cathode made of LaB6. is investigated. It is found that thisnegative biased hot cathode, which works as an electron source, induces the strong negative radial
electric field, the increase in line density and the increase in energy confinement time. The formation of
the negative radial electric field depends on the power input through the hot cathode.

Keywords:
plasma, helical axis stellarator, heliac, electrode, biasing, hot cathode, radial electric field, H mode

1. Introduction

fects by negative biasing with this small electrode inTU-Heliac. This is because this cold electrode can notcollect a sufficient radial current in negative biased
cases. Furthermore, we can not clarify the dependenceof a stored energy on the input power through theelectrode because of the difficulty in the power control

The high confinement mode can be triggered by abiased electrode inserted in the Tokamak plasma Il-3].

Radial electric field control in a helical axis stellarator isalso one of very important research subjects for understanding the mechanism of the H mode. Furthermore ina heliac configuration a center conductor coil is surrounded with a helical plasma, thus it is possible to substitute a center conductor coil can for a biased electrodein order to control the radial electric field if this coil canis set inside a vacuum vessel and insulated electricallv
from the vacuum vessel.

ofthe cold electrode. For above reasons we changed theelectrode into a hot cathode made of LaB6. It isconvenient to control the emission current and the bias

voltage, namely the input power through the hot
cathode.

with a small spherical electrode made of a stainless steel

In this paper the experimental results of radial
electric field control with a hot cathode and the influence on the plasma parameters in TU-Heliac arepresented. We also studied the dependence of a storedball [4]. We have observed the substantial plasma para-energy on the input power through the hot cathode.

We have investigated the influence of biasing
electrode on the plasma parameters in the TU-Heliac

meter changes after the electrode biasing: the increase in

2. Experimental Resultsline density, the steeping of electron density profile, thefluctuation suppression and the formation of a strong
positive radial electric field and large plasma rotation.
However, we have never observed any significant ef" Corresponding

TU-Heliac is a small heliac without an inner helicalwinding (number of field period is 4, major radius is 48

author's e-mail: sumio.kitajima@ qse.tohoku.acjp
391@2001 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchKitajima S. et al., Radial Electric Field Control with a Biased Hot Cathode in the Tohoku University Heliac-350 V at t = 3.75 ms and a hot cathode current 1" flowsup to 5 A (Fig. 2 (a)).The top position of the hotcathode is at r = 87 mm. Figure 2(b) shows the line
density and the density at r = 130 mm, where r is theminor radius which is the distance from the axis of thecenter conductor coil. The line density increases byfactor of 3 without gas feed. In contrast, the density at rcm, average radius of the last closed flux surface is -7cm) [5]. In this experiment the plasma was producedwith low frequency (a < a"i, rrr";: ion cyclotron
frequency) ohmic heating. The low frequency ohmic
heating power (/= 18.7 kHz, P.u* = 35 kW) is supplied
to RF coils which are wound outside toroidal field coils
like vertical field coils [6].
The experimental set-up is shown in Fig. l. The
electrode is a cylindrical hot cathode made of LaB6, l0mm in diameter and 17 mm in length.The leg of the hot
cathode for electrical connection is 10 mm in length andthe total length of the hot cathode is 27 mm. The
maximum emission current /" is about 8 A. The hot
cathode is inserted horizontally from the low field side
and located at a toroidal angle @ = 270o and negative
bias voltage V" is applied against to the vacuum vessel,
which works as an electron source. The target plasma issustained only by low frequency ohmic heating. The
following results are obtained in the target plasma at n"= 0.6 x 1012 cm-3 (Helium), T" - 25 eV and the magnetic
field on axis is 0.31 T. The line density is measured
along a vertical chord passing through a magnetic axis
with a 6 mm microwave interferometer which is placed
at a toroidal angle 0 = 90o.
Typical temporal behavior of the heliac plasma
parameters is shown in Fig. 2, which clearly shows thatthe plasma parameters change substantially after theelectrode biasing. The hot cathode is biased up to V. == 130 mm, where is about 10 mm outside the last closed
flux surface (LCFS), remains constant during thebiasing. This indicates that the electron density profile
becomes steep. On the other hand, the electron temperature decreases to about half (Fig. 2 (c)). In Fig. 2(d), the difference among the floating potential signals
measured at three different radial points shows that thestrong negative radial electric field is formed by the hotcathode biasing and the polarity of this radial electricfield changes from positive to negative at t = 3.75 ms,
although the absolute value of the electric field is small.
Figure 2 also shows that the radial electric field isformed more rapidly than the growth of the electrondensity, which indicates that the radial electric field isthe cause of the increase in the electron density. Thisdelay time in the density and the potential may be
associate with the particle confinement time.
The dimension of the hot cathode is somewhat toolarge for the small size plasma in TU-Heliac. Therefore,
we carefully covered feeders of the cathode withinsulator in order to isolate the cathode feeders from

0-200-400-600
32I<Eo030F^

0\'-5^
2i'-o0e"20
1000-5U-1 00

Fig. 1 Experimental set-up of the hot cathode, which ismade of LaB., 10 mm in diameter and 17 mm inlength. The /eg of the hot cathode for electricalconnection is 10 mm in length and the total

Fig. 2 Time evolutions of (a) hot cathode bias voltage %and emission current /", (b) line density andlength of the hot cathode is 27 mm. The feedersof the cathode are covered with insulator. The hot
cathode is inserted horizontally from the low fielddensity at r= 130 mm, (c) electron temperature at
r= 100 mm and (d) floating potential yr at r= 85.
100 and 110 mm. The top position of the hot
cathode is at r= 87 mm.

side and negative bias voltage V" is applied
against to the vacuum vessel.
392

Kitajima S. et al., Radial Electric Field Control with a Biased Hot Cathode in the Tohoku University Heliac

plasma electrically. The behavior of biased plasmathat the strong negative E, (> - 40 Y/cm) is produced.
The electron density increases by a factor of 5 near themagnetic axis, though the electron temperature decreasedepends on the hot cathode position. Figure 3 shows (a)the dependence of the plasma space potential on top
position of the hot cathode, (b) the line density and (c)the emission current of the hot cathode. In this case thehot cathode bias voltage V" is -350 V and the filament

to -12 eY and has an almost flat radial profile, whichsuggests that the stored energy increases by a factor of
2. Furthermore, the normalized fluctuation level iscurrent I for heating is 23 A. The plasma spacepotential is measured at r = 100 mm. The total hot
cathode length is 27 mm and the LCFS is at r = ll7
mm. In.Fig. 3 the plasma potential drops and the line
density increases at r = 90 mm and this point corres-suppressed after biasing. From these results it seems
some improvement of plasma confinement occurs, but
we need to survey the dependence of the stored energy
on the power input through the hot cathode in order todiscuss an improvement of plasma confinement.
Figure 5 shows that the dependence of the productof the electron temperature and the line density on thepower input through the hot cathode. We can regard thisproduct of the electron temperature which is measured

ponds to the position where the end part of the cathodeis exactly inside of the last closed flux surface. Namely,
the plasma space potential begins to drop at the position
where the whole part of the cathode is immersed insideof the plasma.
Figure 4 shows typical radial profiles of (a) theelectron temperature, (b) the electron density, (c) theplasma space potential and (d) the normalized fluctuation level of the ion saturation current. These aremeasured with a triple probe placed at a toroidal angle Q= 0o. Before biasing, the plasma potential is slightly
positive which indicate that the weak positive E, (< 20V/cm) exists. On the other hand, after biasing plasma
potential clearly changes to negative which indicates'. O ''. Beforc Bising---*-

During Biring V

35302520F15105543O

During Biasing

1002BeforeBiasingtso (a) oooooooooJ^ -50'aeot2v1****^rsr= 100 mm

0-50
0.5
0.4
0.3
0.2(c)

0.1-2-4'v-'o.-o..o..

-100

OOOOOOOOO
00
100

50*l*<eo:.

Ra^**-***f(b)1\ = - 350V, I r= 23A0r ^*l70

r*l-f.r,*r-t*n*.

8590r (mm)80 90 100 1't0 120 130 140r (mm)95Fig. 4 Radial profiles of (a) the electron temperature 7i,(b) the electron density n", (c) the plasma spaceFig. 3 (a) The dependence of the plasma space potential

potential % and (d) the normalized fluctuation

on top position of the hot cathode, (b) the line
density and (c) the emission current of the hotlevel of the ion saturation current both before (2.5ms; open circles) and during (9.5 ms; solidtriangles) the biasing of the hot cathode. Thecathode. The plasma space potential is measured
at r= 100 mm. The total hot cathode length L*,noo"
is 27 mm and the last closed flux surface (LCFS) is
dt r1-spr = 117 mm.

magnetic axis is at r= 79 mm, the top position of
the hot cathode is at r= 87 mm and the LCFS is at
r= 117 mm.
393

Kitaiima S. et aI., Radial Electric Field Control with a Biased Hot Cathode in the Tohoku University Heliac

the negative radial electric field appear in this highstored energy group. It is also clear that the gradient forthe power dependence in the high stored energy group is
about 2 times as large as that for the low stored energy
group. This indicates that the increase in an energy
confinement time occurs by radial electric field control
with the hot cathode. The energy confinement time isabout 70 psec which is roughly estimated from thegradient in the high stored energy group.

at r = 100 mm and the line density as the stored energy
by reason that the electron temperature has an almost

flat radial profile. We neglected the low frequency
ohmic heating power because no significant changes
exist in a plasma current, a loop voltage and phase shift
between the plasma current and the loop voltage after
biasing. In Fig. 5 it is clear that there are two groups indata, low and high stored energy groups, ald the storedenergy jumps to the high energy group at a critical input
power (-700 W). The negative potential transition and3. Summary
The negative biased hot cathode induces the strong

5NFo Before TransitionTransition^ AfterT. (r= 100mm)Ao'c) 4*

negative radial electric field and the increase in line
Adensity. The plasma space potential begins to drop at theposition where the whole part of the cathode isrr

immersed inside of the plasma. The energy confinement^itime after the potential transition is about 2 times aslarge as the time before the transition.

Acknowledgment

H"3We are grateful to Shigeru Inagaki for his useful
discussions. Thanks are also due to Satoru Itoh andKazutoshi Kitago for assistance with the experiments.

<R'Referencestll

500 1000 1500v" I. (w)R.J. Taylor et al., Phys. Rev. Lett. 63,2365 (1989).

[2] R.R. Waynants et al., Nucl. Fusion 30, 945 (1990)'
t3l R.R. Waynants et al.,NucL Fusion 32,837 (1992).
[4] S. Inagaki et aI., Jpn. J. Appl. Phys. 36, 3697
(r9e7).

Fig.5 The dependence of the product of the electrontemperature T"at r= 100 mm and the line density
on the input power V"/" through the hot cathode.
The open circles and solid triangles indicate the[5] S. Kitajima et al., Jpn. J. Appl. Phys. 30, 2606
(l991).

[6] S. Kitajima et aI., I0th Intemational Conference ondata before and after potential transition. The topposition of the hot cathode is at r = 87 mm.

Stellarator, EUR-CIEMAT 26, 236 (1995).

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