J. Plasma Fusion Res. SERIES, Vol.1 (1998) 37-40

Gonfinement Transitions in the H-l Heliac
HARRIS Jeffrey* and the H-1 Team
Plasma Research Laboratory, Research school of Physical sciences and Engineering,
Institute of Advanced Study, Australian National University,Canberra, ACT 0200, Australia(Received: 30 September I997/Accepted: 12 January 1998)

Abstract
Experiments at low magnetic field (< 0.2 T) in the H-1 heliac show transitions in confinement atlow temperature and low heating power - 50 kW. These transitions exhibit many of the features seen inL-H transitions in large tokamaks and stellarators: steepening of the density gradient, increases in theradial electric field, suppression of turbulence, and increased ion energies. Future experiments in H-1
will attempt to extrapolate these phenomena to higher magnetic fields 0.5 < I < 1 T and heating
power P > 0.5 MW and study confiinement in finite-pressure helical-axis plasmas.

Keywords:
helical-axis stellarator, heliac, improved confinement, electric field, turbulence

1. Introduction
The H-1 device[l] in the Institute of Advanced

2. Confinement Transitions
This series of confinement experiments were conducted primarily in argon, although helium and neonshow similar behavior. The plasmas were initiated andmaintained with 50-100 kW of helicon wave heating at
f : 7 MHz. At magnetic fields that exceed a criticalvalue (that is experimentally found to depend on therotational transform and/or shear[4]) the density spontaneously increases by a factor - L.5 in - 1. ms, as
shown in the trace in Fig. 1. This increase in averagedensity is accompanied by a steepening of the density
profile and changes in the plasma potential that increase the radial electric field in the outer part of theplasma, as illustrated in Fig. 2.
Figure 2 shows that the maximum value of lE,l andits gradient occurs just outside the maximum in thedensity gradient, but it should be noted that in theseplasmas, the ion Larmor radius is - 0.2-0.3 of theplasma minor radius, which limits the effective resolution of the measurements. Study of a variety of discharges suggests that, globally, the radial electric field

Study at the Australian National University is a flexible

heliac[2] with major radius R : 1 rr, average plasma
radius ( a) = O.2 m, and M : 3 field, periods. Adjustment of the relative currents in the circular and helical
hard core windings permits variation of the low-shear
rotational transform profile from t * O.6 to 2. While
H-1 is designed ultimately to operate at magnetic fields
up to 8: 1 T with total plasma heating power P > 0.5
Nnry, it is limited during its initial period of operationby the present magnet power supply and heating systems to B < O.25 T and P: 100 kW. Nevertheless, experiments with low-power, low-temperature "model"
plasmas have demonstrated confinement transition phe-nomena similar to those seen in larger tokamaks andstellarators with high power plasma heating[3]. Thisregime has the advantage that material probes can beused to diagnose the plasma in detail.

*Corresponding author's e-mail: Jeffrey. Harri@ anu.edu. au5T@1998 by The:Iapan Society of Plasma
Science and Nuclear Fusion ResearchHarris J. et a/.. Confrnement Transitions in the H-l Heliac

increases in proportion to the ion pressure gradient,and that the bulk plasma rotation is negligible com-ilrt/(oE6F-P4pared to the poloidal E x B rotation in both the low
and high confinement modes[5]. Modelling studies suggest that the radial electric field is consistent with ambipolar ion and electron fluxes when both diffusive anddirect particle losses are included[6].
Substantial plasma turbulence and turbulent transport is observed in the plasma before the transition, butthese subside when the density jump occurs. Figure 3
shows the turbulent particle transport as measured bymultiple I-angmuir probes (which measure the plasma
density and potential fluctuations, and their mutual coherence and phase[4]) before and after transition; theo20 40 60 80

100f (ms)portion of the transport due to fluctuations drops by
two orders of magnitude. Note that this measurement
does not measure the underlying, non-turbulent transport due to diffusion and direct particle losses (bothlarge because of the low magnetic field and large ion
Larmor radii). The global nature of the fluctuations in
the low confinement mode (poloidal mode number rnFig. 1. Time trace of the line-average density in an H-1
discharge with 8= 0.615 T in a magnetic configura-tion with t = 1.41. The confinement transition occurs at t = 40 ms.

: 1 or 2, large coherence lengths) causes the fluctua-

1.5

tion-induced transport to peak at mid-radius.

n" (1Ql8 6-3)

Improvement in the heating and confinement of
the ions is also observed at the transition. Figure 4
shows that the profile of ion temperature (average ion

1.00.54o4E'. (kV/m)

IU)aAIZ2IIo(\lo

41 (m-zs-t1\Ilow modetIEor\*l4/I.. At^U,IEICDhigh modeFo-2-4

o

1

0.5

oo.5

1.5

1

1.5

r/arlaFig.3. Fluctuation-induced particle flux in H-1 discharges
below and above confinement transition threshold.
as measured by Langmuir probes. Note that thismeasurement only includes the contribution of theturbulence, not the underlying non-turbulent flux.
The background noise level is -10-3 times thepeak value, and the flux drops by a factor of 100 atFig.2. Radial profiles of plasma density (n ) and radial
electric field (F,) in H-1. Profiles taken 5 ms before(dotted) and 5 ms after (solid) transition to improved confinement. The plasma boundary r/a = 1
is determined by the last closed magnetic flux surface, and is also the location of the helicon waveantenna.

the transition.

38

Harris J. et a/., Confrnement Transitions in the H-l Heliac

field available in H-1 at present make scaling studies

energy measured by a retarding-field energy analyzer)
rises in the high confinement mode. The electron temperature profile (Fig. 5) is hollow both in the low andhigh modes - presumably because the proxmity of therf heating antenna and the non-resonant helicon heating scheme.
The energy and particle content of the plasma bothincrease by factors - 1.5 in the high confinement mode,
while the coupled rf power (as measured from thecoupling resistance) remains constant. Thus, the apparent confinement times ( - 1 ms) are increased with thetransition. However, the low plasma temperatures andthe presence of substantial neutral densities in theseplasmas make more precise energy and particle balan-impractical.

Additional experiments have shown a variety ofphenomena that occur in some transitions, includingdithering transitions and discharges in which the turbulence persists in the high confinement mode, but withfluctuation-induced particle flux that actually flows inward in the inner region of the plasma; this reversal has
been shown to be approximately uniform around theflux surfaces[7].

3. Discussion
These experiments have demonstrated that thelow-power, low temperature plasma in H-l can serve as
an accessible experimental paradigm for the study ofmagnetic confinement with strong radial electric fields.
The results show many of the phenomena seen duringconfinement transitions in larger devices with 10-100
times the heating power, suggesting that at least someof the underlying physics is similar.
An important challenge is to increase the plasma
parameters to bring these experiments closer to fusion

ces very difficult, and the limited power and magnetic

100

I { I t { In,nnmode60

plasma conditions. In December, 1997, H-1 will beshut down for 6-8 months to permit the installation of a
new magnetic field power supply which will provide
magnetic fields up to B : 1 T. Operation at B : 0.5
and 1 T will permit ECH with a 200 kW, 28 GHz gyrotron that has already been installed and tested in collaboration with teams from Kyoto University and the National Institute for Fusion Science in Japan. Additional
ICRF power (- 250 kW) will also become available.
The availability of two heating schemes will permit
much more flexible studies of confinement and electricfield effects. The increase in power will permit experiments at higher plasma temperature and pressure, andultimately, flux surface fragility at finite pressure, andrlitril+{++s)
l-20

0.5

1.5
r/aFig.4. lon temperature (measured with retarding-field energy analyzer) in low and high confinement modes.

stability phenomena such as ballooning modes.

Acknowledgments
20

These experiments represent the work of the entire

H-l scientific team: M. G. Shats, D. K. Rudakov, B. D.

oBlackwell, J. Howard, G. G. Borg, D. Miljak, and R. L.
Dewar, as well as the H-l technical support group: G.

10

C. Davies, J. Wach, R. Kimlin, R. Davies, and C.
Costa.

0.5

This work was supported by the Australian National University and by the Australian Department ofIndustry, Science, and Tourism as part of the Major

1.5r/aNational Research Facilities program.

Fig.5. Electron temperature (measured with Langmuirprobes) in low and high confinement modes.
39

Hmis L et al., Confrnement Transitions in the H-1 Heliac

References[5] M.G. Shats eral., Phys. Plasmas 4,3629 (1997\.
[6] S.A. Dettrick, H.J. Gardner and R.L. Dewar, in[1] S.M. Hamberger e/ a/., Fusion Technol. 17, I23

these Proceedings, p.222 (1998).

(1eeo).
Harris etal., Nucl. Fusion 25,623 (1985).
J.H.
[2]F.
Wagneret al., Phys. Rev. Lett.49,1408 (1982).
[3]M.G.
Shatse/ a/., Phys. Rev. Lett. 77, 4190
[4][7] M.G. Shats er a/., Phys. Rev. ktt. 79, 2690
(Lee7).

(ree6).

40