H-1NF: The National Plasma Fusion Research Facility

Helical plasma

(Argon)

Helical conductor control winding

5 tonne support structure

Rotating 55 view Doppler tomography systemRotating 64 wire electron

beam tomography system

Diamagnetic energy monitor

Pentagonal central support column

14000Amp bus conductor and cooling

by Ding-fa Zhou

Photos: Tim Wetherell

11kV 3

switchgear

11kV :: 800Vtransformer

24 pulse rectifier

+ +

10 ea. 1MW DC-DC convertor

SVC switchedpower factor adjustment

1-4MVAr, 800V

+H-1 800VDC 14,000A

permanentharmonic filter

(11kV, 2.5MVAr)

2kHz PWM

1 second

critical accuracytime window

12MW Pulsed Magnet Power SupplyDC-DC Convertor/Regulator: ABB Aust. /Technocon AG

24 Pulse Rectifier: Cegelec AustraliaTransformers/Reactors: TMC Australia

Switchgear: Holec Australia and A-Force Switchboards, SydneyConsultant Engineers: Walshe & Associates, Sydney

“fish-eye” view of corrugated ECH waveguide to H-1 on left.(H.Punzmann)

puff

ech

rf pre-ion.

diamagnetism

ne

ECH plasmaThe figure shows an ECH produced plasma (200kW 28GHz gyrotron, 2CE at 0.5T). With a 10ms pulse, and rf preionization

of ~11017m-3, a diamagnetic temperature of 100-200eV was observed provided gas feed was carefully controlled (p < 210-6 Torr). A highly localised ionization rate was observed in the emission from argon doping, and at higher gas fills, a peak density in excess of 31018 was obtained, with a lower temperature. Impurity levels, estimated by comparison of spectral line intensities, were lower than in the rf discharges.

Microwave Source:

(Kyoto-NIFS-ANU collab.)

28 GHz gyrotron

230 kW ~ 40ms

The H-1 heliac is a current-free stellarator with a helical magnetic axis which twists around the machine axis (a 1m circular ring conductor,) three times in one toroidal rotation. It is a “flexible” heliac composed almost entirely of circular coils with the exception of the helical control winding, which also wraps around the ring conductor, in phase with the magnetic axis of the plasma. Control of this current produces a range of rotational transform from 1 to 1.5: (B0 =1T, r > 0.15-0.2 m), and 0.7 to 2.2

for B0 ~ 0.5 T, r> 0.1 m, allowing almost independent control of two of the three parameters: , magnetic well (–2% to +6%)

and shear in rotational transform., which can be positive (stellarator-type) negative (tokamak-like), or near zero (<0.1).

At 0.5 tesla, RF (20 ~150kW, ~ cH) produces plasma in H:He and H:D mixtures at densities up to <ne> ~ 21018m-3, with

temperatures initially limited to < 50eV by low-Z impurities. ECH ( = 2ce) produces considerably higher temperatures and

centrally-peaked density profiles.

Configuration Mapping: Electron Beam Wire Tomography A low energy (20eV, 100nA) electron beam traces out the

magnetic geometry, and is intercepted by a rotating grid of 64 molybdenum wires, 0.15mm diameter. The data, similar to Xray CAT scan data can be inverted to obtain images of the electron transits, shown below in blue. The advantage of this technique is that the exact position of the electron transit can be determined to within <1mm, allowing the magnetic geometry of the H-1 heliac to be precisely checked.

Point by point comparison with computation

Points are matched one by one, allowing matching of rotational transform to better than 1 part in 104. Small deviations from the computation can by quantified in terms of small errors in construction ~1-2mm. Super computer modelling allows these errors to be tracked down. In this example, a better fit was obtained by more accurate modelling of the vertical field coil pair.

RF configuration scansThe density and time-evolution of RF produced plasma

varies markedly with configuration as seen here, where kH is varied between 0 and 1.

Below is the density at 50 ms for a similar range in kH.

Configuration StudiesThe flexibility of the heliac configuration and the precision

programmable power supplies provide an ideal environment for studies of magnetic configuration. The main parameter varied in this work is the helical core current ratio, kH which primarily varies the rotational transform iota. Magnetic well and shear also vary .

Time (seconds)

De

ns

ity

(x

1018

m-3 )

573

456

Sudden changes in density associated with resonance at zero shear

Magnetic Fluctuationshigh temperature conditions:

H, He, D; B ~ 0.5T;ne ~ 1e18; Te<50eV

i,e << a,

mfp >> conn• spectrum in excess of 100kHz • mode numbers not yet accurately

resolved, but appear low: m ~ 1- 8, n > 0

b/B ~ 2e-5• both broad-band and

coherent/harmonic nature

• abrupt changes in spectrum for no apparent reason

• some Alfvénic scaling with ne, iota

Coil number

Poloidal mode number measurements

• Phase vs poloidal angle is not simple– Magnetic coords – External to plasma

• Propagation effects• Large amplitude variation

Expected for m =2

phase

magnitude

Major/minor radius 1m/0.1-0.2mVacuum chamber 33m2 good accessAspect ratio 5+ toroidalMagnetic Field 1 Tesla (0.2 DC)Heating Power 0.2MW 28 GHz ECH

0.3MW 6-25MHz ICHn 3e18T <200eV 0.2%

mode numbers related to rationals

B.D. Blackwell , D.G. Pretty, J.H. Harris, T.A.S.Kumar, D.R. Oliver, J. Howard, M.G. Shats, S.M. Collis,C.A. Michael and H. Punzmann

4-5, 7-8Mostly 0-3 3-8

2-4

2-44-5, 7-8

“bean-shaped” 20 coil Mirnov array

Data Mining, Alfvénic Scaling: The figure to the left shows Fourier fluctuation data a), and b) after data mining by SVD analysis, grouping of SVDs by spectral content, and clustering the groups according to phase similarity. The cluster marked with the red lines in the lower figure exhibits scaling in rotational transform with the shear Alfvén eigenmode frequency (although a scale factor of 3 is unaccounted for). This is clarified by normalization to ne in c) and the cluster is enlarged below.