Programmable BT Supply: Installed, Working Well Coming Soon : Programmable BP Supply

Flexible BT and BP field control will benefit MST

• 32 H-bridge modules in series/parallel• IGBT switching supply• 10 kHz Pulse Width Modulation• Arbitrary demand BT waveform• Output 40 MW (25 kA at 1600 V)• General Atomics PCS will control, allow sophisticated feedback control

• Each module has 4 IGBTs, 0.25 F capacitor• Module output 1600 A at 900 V• 2 Modules in series for 1800 V• 16 in parallel for 25 kA• Long cables to load makes it easy to parallel modules

BT Transformer connection scheme upgraded

H-bridge

H-bridge

H-bridge

H-bridge

Vdiff, 2 bridges in series

13.5 14.0 14.5 15.0 15.5msec

-300-200

-100

0

100

200300

Vo

lts

V to GND of one bridge

13.5 14.0 14.5 15.0 15.5msec

-400-200

0200400600800

1000

Vo

lts

1101004025

Vdiff, 2 bridges in series

13.5 14.0 14.5 15.0 15.5msec

-400

-200

0

200

400

Vo

lts

V to GND of one bridge

13.5 14.0 14.5 15.0 15.5msec

-500

0

500

1000

Vo

lts

1090603039

(note -- low voltage tests shown --normal operating voltages about 10 X these test voltages)

After (40:1)

• Sawtooth spike voltages clamped to supply voltage

• Common mode voltage half of output voltage

• Better transformer current distribution gives better spatial BT uniformity

Before (20:1)

IGBT TEMPS 0,1,2,3

-4 -2 0 2 4 6 8sec

21.0

21.2

21.4

21.6

21.8

22.0

De

g C

1100303044

Temp Sensors monitor IGBT stress

• IGBT lifetime limited by ∆T per pulse• Air-cooled (no heatsink)• Sensors monitor baseplate temperature• Baseplate ∆Tb reveals die ∆T• ∆Tb < 1 C gives adequate IGBT life• Gives confidence despite complicated waveforms, high power output

H-bridgemodule800 V at1600 A

H-bridgemodule800 V at1600 A

LEM current monitor

H-bridgemodule800 V at1600 A

H-bridgemodule800 V at1600 A

LEM current monitor

H-bridgemodule800 V at1600 A

H-bridgemodule800 V at1600 A

LEM current monitor

0

1

15

Rogowskicurrent monitor

BT transformer40:1

twisted-quad transmissionline

BP system may have 1.5 x capacitors,3 x switching modules of BT system

Existing BT system: 32 modules

Possible BP system: 96 modules

Maximizing MST's Inductive Capability with Programmable BT and BP Power SuppliesD. J. Holly, J. R. Adney, B. E. Chapman, K. J. McCollam, and J. C. Morin, University of Wisconsin - Madison

• It's easy to parallel IGBT modules with 50 feet of transmission line to load• Stacking modules two high (1800 V) requires care• Stacking modules three high (2700 V) will require better isolation• Voltage transients from plasma have not been a problem• Tektronix makes a better x100 probe than I can make• Do not use a 10 kHz digitizer to record a 10 kHz signal!• Design with failure modes in mind, e.g. add baffles between modules so arc doesn't propagate

250 V

V

550 kA

I

100msect

-250 V

BP demands more power than BT

• Single-turn current > 550 kA• Single-turn voltage up to 250 V• OFCD operation is most demanding• Limit voltage to 250 V to limit risk of arc at poloidal gap• BP transformer ratio 40:1• Use IGBTs to achieve high bandwidth• Capacitor energy per module may be less than in BT supply

Possible configuration:Four amplifiers, eachdriving ten turns

• Each amplifier produces 2500 V at 14 kA• Can use three 900 V modules in series• Each amplifier has 24 modules (3 x 8)

• OFCD needs 250 V for full 100 msec• Must limit starting bank voltage to 250 V to avoid gap breakdown• Boost regulator could keep output cap bank at full voltage• Requires more IGBTs but fewer capacitors, less energy storage• Output cap bank must be big enough to absorb energy from plasma

• Plasma current: 600 kA• Discharge duration: 80 msec• Best confinement time: 12 msec• Typical ne = 1019 m-3

• Highest Te = 2 keV

• R = 1.5 m, a= 0.52 m• 50 mm thick aluminum wall • Transformer drives current in shell to create BT

MST Reversed Field Pinch

Programmable BT supply provides great flexibility

Supply is modular, expandable

Acknowledgments

The project has benefited greatly from Bob Ganch's skillful and careful help.This work was supported by U. S. D. O E.

BT at edge (black), scaled demand (blue)

20 22 24 26 28 30msec

-400

-300

-200

-100

0

100

gaus

s

BT supply output voltage

20 22 24 26 28 30msec

-1500

-1000

-500

0

500

1000

1500

Volts

Module output current (eight Modules overlaid)

20 22 24 26 28 30msec

-1.5

-1.0

-0.5

0.0

kA

1100903042

Can generate complex BT waveformsSupply shuts off for 350 usec, ramps toward 0

Modules share current load well

A

B

C

D

0.25F@

900 V

Each H-bridge module: Four IGBTs, floatingcapacitor bank

Existing 40-turn BP primary

Plasma is single-turnsecondary

Supply hits current limit, shuts off briefly(shown is test of partial system with 12 kA limit)

+-

demand fromWAVEFORM

GENERATOR /G.A. PCS

FILTER

PULSE WIDTHMODULATOR

andINTERLOCKS

FIBER OPTICSPLITTER/COMBINER

FIBER OPTICSPLITTER/

COMBINER 0

H-bridgeMODULEs

0L, 0R

LEM currentsensor 0OVER-

CURRENT0

FOUR #0TWISTED 15 m

OUTPUTFILTER

BT PRI 40:1

ROGOWSKI

INTEGRATOR

OVER-CURRENT

15

FIBER OPTICSPLITTER/

COMBINER 3 H-bridgeMODULEs15UL, 15R

LEM currentsensor 15

FOUR #0TWISTED 15 m

CAPACITORCHARGING

SUPPLY480 VAC

CAPACITORROOM

MST VACUUM VESSEL

• Lower voltage stress• Better turns ratio available• BT field more uniform

Sawtooth spike drives largecommon-mode voltage

H-bridge output is multipliedby transformer -- gives largecommon-mode voltage

Possible BP Supply scheme

2500 V14 kA

10 turns

2500 V14 kA

10 turns

2500 V14 kA

10 turns

2500 V14 kA

10 turns

There are now strong motivations on MST for increased plasmacurrent, increased pulse length, and flexible waveform control. For example, improved confinement plasmas at near MST's maximum plasma current, 0.5 MA, exhibit RFP-record Te and Ti that are several-fold larger than temperatures achieved at 0.2 MA. The maximum dc current added by ac oscillating field current drive is not yet known since the added current ramps up on a time scale longer than the duration of present MST discharges. Inductive currentprofile control has generated substantially improved confinement, but the optimal control waveform and, hence, the maximum confinement are not yet known. These and other considerations motivate the installation of flexible, programmable power supplies. A power supply for BT is already in operation. Based on IGBT switches, this supply provides waveform control with a bandwidth of several kHz at about 25 MW. The design of a similar but more powerful supply for BP is underway. Together, these supplies will provide MST with the most advanced inductive control capability of any RFP in the world.

Lessons from BT supply

Our experience building BT Programmable supply will help in building BP Programmable supply.

H-bridge

BP pri10 turns

BP pri10 turnsH-bridge

L

C1 C2

IoutId

Isw

IL

10-turn H-bridge without voltage regulator

10-turn H-bridge with boost voltage regulator

2-Stage Capacitor bank?

Longer discharges, better control

• More efficient use of BP transformer allows longer discharges• Better control gives longer IP flattop• Control of surface electric field allows reduced fluctuation, improved confinement• Better waveform control, longer experiment time gives better Oscillating Field Current Drive

OFCD current still growing when flattop ends

Flexible waveform control allows exploring OFCD parameters

Reduced risk at high currents

• Present BP supply can't be turned off once discharge starts• If plasma terminates early, BP core saturates and current skyrockets• High current operation is risky with present BP supply• Programmable BP supply limits current, removes risk to transformer

Discharge terminates early

BP core saturates

Large BP Ipri endangers windings

(note digitizer is saturated-- actual peak Ipri larger!)

BT programmable supply is running, giving new capabilities

• Flexible BT waveform control allows control of q(a)

Old BT supply: inflexiblecontrol of q(a)

New BT supply: can generatemultiple q(a) values in single shot

• Μany experiments require flexible control of both BT and BP

• Precise control of BT waveform may benefit OFCD

Non-sinusoidal oscillating BTmay help study and control of MHD activity