23 rd SOFT 20-24 September 20041/31 Control of non-axisymmetric magnetic fields for plasma enhanced performances: the RFX contribution P. Sonato, R.Piovan,

23rd SOFT 20-24 September 2004 1/31

Control of non-axisymmetric magnetic fields for plasma enhanced performances:

the RFX contribution

P. Sonato, R.Piovan, A.Luchetta and the RFX team

http://europa.eu.int/comm/research/energy/fu/fu_rd_asn_en.html

2/3123rd SOFT 20-24 September 2004

Outline

• Introduction to MHD instabilities in tokamaks & RFPs– Error field control in tokamaks– RWM stabilization in tokamaks– Error field reduction in RFX– Control of m=1 modes in RFX

• The new experiments on the modified RFX– The machine modification and the saddle coil system– Power supply– Magnetic measurements– Control system– Control strategies

• Conclusions


Introduction: MHD plasma instabilities

• The MHD instabilities limit the operational space of the plasma in any magnetically confined plasma operating at the highest performances

• MHD instability sources: – current gradients – pressure gradients

• MHD instability types:– Ideal instabilities– Resistive instabilities


Introduction: MHD description

• 2-D Fourier decomposition of the magnetic field:– Poloidal spectrum: m– Toroidal spectrum: n

m=1, n=3

Resonant surfaces in toroidal geometry


Introduction: MHD resonance

• Tokamak:– Internally resonant modes– Externally resonant modes

Resonance for rational q


MHD instabilities in the Tokamak – error fields

• The non-axisymmetric magnetic error fields are sources of instabilities: – coils misalignments– coil feed connections– inhomogeneity of conductive passive

structures– ferromagnetic structures– ripple– ……..

The error fields exert a braking torque against the

plasma rotation

• Problems of present error field studies:– Many sources of error fields are not

completely evidenced– Sidebands of correction coils


Error field control in DIII-D

• Static error field compensation to attain low density regimes

• Recent multi error field compensation with both N=1 coil and C-coils

• limit vs. Br(2,1)

Referencedischarge

Multi-modeError field

compensation


MHD instabilities in the Tokamak – RWMs

• Advanced scenarios require:– sufficiently high :

• high boostrap current fraction• flat or reversed shear

• Consequence: Resistive Wall Modes (RWMs) appearance– external kink modes:

n=1,2 and various m– stabilized only by an infinitely

conductive wall close to the plasma surface

– severe limit in


RWMs stabilization and control in DIII-D

• C-coil feedback control of RWMs and pre-programmed similar correction obtain similar improvement

• RWMs avoidance strategies:– Stabilisation by rotation through tangential NBI– Careful error field control – Feedback stabilisation with additional coils

No feedback

Pre-programmed

Feedback


Error fields & RWM extrapolation to ITER

• The RWMs can be stabilized by feedback control acting on the outer correction coils


MHD instabilities in the RFP experiments:mode classification


RWMs in the RFP experiments

EXTRAP-T2R

HBTX-1C

internally nonresonant on-axisRWM, m=1, n=-10

Internally resonanttearing mode m=1, n=-12


Error fields in RFX - ’92-’99

• The broad spectrum of internally resonant MHD m=1 tearing modes on rational surfaces can easily couple with harmonics of an error field

• Two main sources of error fields in the passive Aluminium stabilizing shell:– 2 poloidal insulating gaps – 2 toroidal insulating gaps

Axisymmetricequilibrium coils

poloidal gap

localcontrol

coils

with m=0Pre-programmed

Equilibrium

m=1,n=0Equilibrium

feedback

localfield error

minimization

short circuitedgap


Tearing modes in RFX - ’92-’99

• RFX always exhibited high amplitude m=1 tearing modes:– phase locked with respect to each other – locked with respect to the wall


m=1 mode control through m=0 mode coupling in RFX

1,9

Tor

oida

l po

sitio

n

1,8

1,7

1,10

1,11

1,12

• Controlling the currents on the toroidal winding sectors (0,1 mode) the control the m=1 tearing mode position has been obtained

• Also a slight reduction of mode amplitude has been evidenced

0

50

100

150

200

250

300

350

400

10 20 30 40 50 60 70 80 90 100

ext

(0,1)

WLM

Time [ms]

shot #12350


The modified RFX

• It has been conceived to extend the non-axisymmetric control of the MHD modes by introducing a direct action of external harmonic m=1 magnetic fields

• The capability to produce m=0 modes has been improved by the new toroidal system power supply to control the toroidal field independently on each of the 12 winding sectors

• Further significant improvements:– Axisymmetric equilibrium control– Poloidal gap field error minimized– Toroidal gap field error minimized– First wall power handling capability– Vessel wall protection– Plasma breakdown


The modified RFX

new vacuum vessel portsfor ISIS feedthroughs

toroidal coil

new toroidalsupport structure

shell clampingbands

shell equatorialgap shortcircuits

vessel-shellinsulated spacers

vacuum vessel

3 mm copper shell

saddle coilsystem


The stabilizing shell

• The first basic choice has been to install a passive stabilising shell as close as possible to the plasma having a (1,0) = 40-50 ms: – to allow a passive stabilisation for instabilities acting on a time scale faster than

the operational frequency of the power supplies/winding systems (~20 ms)– corresponding to a passive stabilization of the characteristic internal resonant

modes of ~10-20 ms for m=1, n=7 to n=18– the shell will be nearly completely penetrated for the m=1,n=1,5 RWMs during

the shot

Welded gap


The stabilizing shell:passive error field minimization

23°overlapped poloidal gap

short-circuited equatorial gap

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

-1 -0,5 0 0,5 1 1,5

10Hz

20Hz

50Hz 100Hz 500Hz 200Hz

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

-1 -0,5 0 0,5 1 1,5

10Hz

20Hz 50Hz 100Hz

Field error through the poloidal gapoverlapped poloidal gap

Butt joint gap


The saddle coils

• The second design choice regards the shape and the discretization of the radial field control coils:

– the presence of an equatorial gap used also as an opening surface to have access to the vessel -> only saddle coils are compatible

– the saddle coils must be designed without any gap in between, to avoid undesired sources of high spectrum error fields and source of sidebands

23

1

)12()12(484192

max

max

maxmax

n

m

nmN totcoil

Turns 60 (4 layers x 15 turns)

Section 3.6 mm2

Inom 400 A (0.3s)

Vnom 650 V

Material Copper

Insulation grade 2 Dacron glass tape, epoxy pre-impregnated, final vacuum impregnation with epoxy resin


The saddle coils: sidebands

• Toroidal and poloidal sidebands at the plasma edge for a single m=1, n=8 harmonic produced

nsb= n1,8 ± k.Nt

k = ±1, ±2, ….Nt = 48


Saddle coil power supply • Each saddle coil is fed with its own

switching dc/dc power supply, which performs independent control of the current

• H-bridge converter topology with standard voltage components (IGBT )

• Total power: 50 MWOutput voltage 650 V

Output current 400 A

DC link voltage 700 V

Switching frequency 10 kHz

Control law Double Pulse Width Modulation

Duty cycle (ON/Cycle) 0.5 s / 600 s

Time: 20 ms/div

Reference(480 A/div)

Current(480 A/div)

Voltage(750 V/div)


Toroidal field power supply

• The system is foreseen to be used also to generate rotating m=0, n=1-5 modes superimposed to the bias reversed B


Vessel braking torque and driving m=0, n=1 torque

m=1, n=8 braking torqueNormalized to 1 mT of mode amplitude

lower amplitude is expected in the modified RFX

Old RFXRotation

frequency limit

50 Hz

20 Hz

10 Hz 5 Hz

old RFX

The new PS will allow an increased m=0, n=1


• saddle probes <Br>

Magnetic measurements: out-vessel probes

• Btor-Bpol biaxial pick up coils

• “Ad hoc” designed for non-axisymmetric control• The system comprises 192 (4x48) measures of <Br> , Bt , Bp

• Bandwidth few kHz (vessel shielding effect)


Magnetic measurements: In-vessel probes

• Designed to measure high frequency, high n components of Bt

• 96 (48 x 2) measures of Bt

• Bandwidth close to 1 MHz


Control system:computer based, distributed system

• The system includes seven VMEbus stations equipped with single board computers, all connected through one real-time network:– Three stations (processors) dedicated to the real time data acquisition– Four stations (controllers) drive the control power amplifiers.

• The performance was measured: – latency time worst case 300 s

Integrated Control

CT-A903 AL CTS 20-Nov-2003 Radial Field Detection Coils (4x48 ch’s), Toroida l Pick-up Coils ( 4x48 ch’s)

Data Acquisition& Pre-processing

bm,n

LHD

References

Local Contro l

r, Ip

Electromagnetic Measurements + Electric Machine Measurements

MHD Sector Controller

MHD ModeSector Contro ller

Radial Field Processor

Toroidal FieldProcessor

ToroidalContro ller

r, Ip ,

<bwall>

brm,n

AxisymmetricProcessor

S a d d le Co ilAmp lifier

01 - 96

brm,n

bm,n

Vertica l FieldAmp lifier

01 - 08


01 - 08


01 - 08


01 - 08


01 - 08

Ma g n etizin gAmp lifier

01 - 04


01 - 08


01 - 08

To ro id a lAmp lifier

01 - 04


01 - 08


01 - 08

Po lo id a l Fla t-To pAmp lifier

01 - 04

To ro id a l S ecto rAmp lifier

01


01


01


01 - 12

AxisymmetricContro ller

brm,n

bm,n

Real-timeNetwork

References

References

References

Power Amplifiers

S a d d le Co ilAmp lifier

01 - 96

b0,n

LHD<bwall>

MHD ModeSector Contro ller


Control system: MHD mode control scheme

• It consists of a lumped parameter electromagnetic model of the Saddle Coil (SC) system integrated with a linear model of the evolution of RWMs in a RFP plasma

- 0+KApplied voltages

Actuator: SC

X’= Ax + Buy = Cx + Du

Coil currentsDynamic & FFT

Field harmonicsGenerated by SC X’= Fx + Gu

y = Hx + Pu

Plasma dynamic model

Reference

Measured fieldharmonics


Control system: MHD mode control scheme applied to T2R

• Recently in T2R a saddle coils system has been installed:– Total SC= 64– Poloidal = 4– Toroidal = 16– Not covering completely the

plasma surface• The RFX MHD mode control

system has been tested• The RWMs multi mode control

has been demonstrated

NO FEEDBACKFeedback on n=+5,+6Feedback on n=+5,+6,+7,+8


Control strategies

• MHD mode control– stabilisation of RWMs having m=1, n=2-5– interaction with internally resonant tearing modes, to either mitigate or excite their

amplitudes or control their phases

• “Virtual ideal shell” close to the plasma.

• “Wise virtual shell” is similar to the “virtual ideal shell”, but the components of the radial magnetic field are minimised, except for the equilibrium m=1,n=0 component

• Phase control of m=0, n=1-5 modes. Action on the dynamic current unbalance on the toroidal winding sectors to produce m=0, n=1-5 rotating modes able to drag the m=1 phase & wall locked modes


Conclusions

• The new RFX device is the most versatile experiment to test the interaction of external harmonic fields with MHD modes

• The experiments will allow to investigate: – the RWMs stabilization and tearing mode interaction– the error field control, including the effect of the sidebands

• All of these features are of common interest for:– Present tokamak and RFP experiments– For the implementation of similar systems in ITER

23 rd SOFT 20-24 September 20041/31 Control of non-axisymmetric magnetic fields for plasma enhanced performances: the RFX contribution P. Sonato, R.Piovan,

Documents

rd soft

mhd instabilities

mhd plasma instabilities

rfx control

sources of error fields

tokamak error fields

tokamaks error field

sources of instabilities