The National Plasma Fusion Research FacilityH-1NF Heliac and Engineering Subsystems

Tolerance Band DC-DC Convertor• PWM inherently voltage source - V out =Vin

• Tolerance band– fast responding current source - high performance

even in open loop, also removes 1 pole from FB loop– inherently variable frequency conflicts with desire for

phase interleaving in multiphase systems– inherent GTO protection (I<3kA)

Upper limit

Lower limit

GTO Current

12MW Pulsed Power Supply for the H-1NF Magnet

DC-DC Convertor/Regulator: ABB Aust. /Technocon AG24 Pulse Rectifier: Cegelec Australia Transformers/Reactors: TMC Australia

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

Specifications

H-1 was the first large scale “heliac”, a particular type of helical axis stellarator which was conceived in 1969 in mathematical form as a plasma configuration with strong inherent magnetic well for stability at high plasma pressure. A realizable form was invented at Princeton in 1982, and the first experimental device, “SHEILA” was built and operated at the ANU in 1985. The helical axis, strong plasma twist, “bean shaped” plasma and simple circular coils are features of this configuration. H-1 was constructed entirely (apart from the vacuum shell) by the Plasma Research Laboratory and RSPhysSE workshop staff, to extremely high accuracy (~1mm), in spite of the complicated interlinking shape and the large forces on the conductors (~ 5 Tonnes per coil). H-1 employs one small helical winding, which greatly increases the range of plasma shapes (“flexible heliac”).

Other heliacs have been built in Japan and Spain, and a next generation machine, a large superconducting “helias” is under construction in Germany, which although quite different, has the familiar bean shape in some cross sections.

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

H-1 is controlled by a PLC system (Programmable Logic Controller). Each of 4 PLCs is a small, real-time computer executing 20-40 complete monitoring cycles every second.

H-1 Control System

Main Control Screen (Overview)Subsystems are summarised in each box, red and green indicating faults

and normal operation. Each sub system has a detailed screen such as below.

Cooling Sub systemThe magnetic field coils dissipate up to 12MW of power, which

must be removed as heat. The high purity H-1 cooling water transfers this heat through a heat exchanger to an evaporative cooling tower.

Each coil’s temperature is monitored closely (bars lower left)

RF Heating Systems

Microwave Source:

(Kyoto-NIFS-ANU collab.)

28 GHz gyrotron

230 kW ~ 40ms

H-1 may be heated at both the electron cyclotron resonance (ECH) (microwave ~28GHz)and lower radio frequencies near the ion cyclotron resonance(6-26MHz, helicon and ICH). Several hundred kW are available.

Ion Range RF source (ex Radio Australia)

• 6-26 MHz, 250 kW• Refurbished by British Aerospace• New coax from NIFS• Use for helicon, ICRF heating• 6x30kW AWA transmitters for plasma production, and phasedexcitation

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

by Ding-fa Zhou

Photos: Tim Wetherill

Current Ramp Scans Plasma

Achieved:

• three confinement modes inone pulse, by varying magneticfield

• Can also vary configurationpulse to pulse or during a pulse

20 Amp ramp on a 4000 Amp pulse

Ripple

Causes “shimmer” inconfiguration

DB /dt EMF unwanted plasma current

Possible reconnections andheating at surface

Achieved:

• Ripple current << 1Amp

• DI/ dt (0-30Hz) < 10A/s(highly suppressed zero)

• 1-14kA, 800V, <1Amp precision: main magnet

• 1-14kA, 100V, <1Amp precision: control magnet

• Ripple and DI/ dt small to avoid “shimmer” andinduced currents (plasma conductivity ~copper)

• Minimize line disturbance (14MVA load)

• Protect heliac windings (100kA/50us crowbar)

Minimize Power Line Disturbance

H-1NF Power System

Compensation

permanent harmonic filter (11kV, 2.5MVAr)

1 second

critical accuracytime window

ANUACTEW

supply drop

H-1current

ANUACTEW

supply drop

H-1

compensation

total

-4% droop

DC current

AC line droop correctedV I

• Harmonics– 24 phase diode bridge, full

conduction angle

• Disturbances– ramp load smoothly by

dumping power

• Droop– Switched capacitor

“overcompensation”

Feedback ControlConstant-power power converter negative resistance

I in=P/V in I/ V ~ -1/V 2 input LC instability (45Hz)

Output filter capacitor and magnet second resonance (20Hz)400ms time constant

Feedback control is like PID but voltage derivative , (proportional/integral/derivative)Implementation with real time 68000 P, 600 s per cycle

programmed in “Hawk ” C

Mean value filters for quick response (top hat)

Current step: Disable Gyrotron and insert limiter into plasma

H-1NF Pulse SequencerThe PLCs implement a 32 step, software configured sequencer which is displayed below through a real-time Excel interface.

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