Control of non-axisymmetric magnetic fields for plasma enhanced performances: the RFX contribution
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Transcript of Control of non-axisymmetric magnetic fields for plasma enhanced performances: the RFX contribution
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
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
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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
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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
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Introduction: MHD resonance
• Tokamak:– Internally resonant modes– Externally resonant modes
Resonance for rational q
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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
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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
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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
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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
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Error fields & RWM extrapolation to ITER
• The RWMs can be stabilized by feedback control acting on the outer correction coils
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MHD instabilities in the RFP experiments:mode classification
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RWMs in the RFP experiments
EXTRAP-T2R
HBTX-1C
internally nonresonant on-axisRWM, m=1, n=-10
Internally resonanttearing mode m=1, n=-12
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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
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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
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m=1 mode control through m=0 mode coupling in RFX
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1,7
1,10
1,11
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• 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
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10 20 30 40 50 60 70 80 90 100
ext
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Time [ms]
shot #12350
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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
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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
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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
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The stabilizing shell:passive error field minimization
23°overlapped poloidal gap
short-circuited equatorial gap
0,0
0,2
0,4
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0,8
1,0
1,2
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-1 -0,5 0 0,5 1 1,5
10Hz
20Hz
50Hz 100Hz 500Hz 200Hz
0,0
0,2
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0,8
1,0
1,2
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-1 -0,5 0 0,5 1 1,5
10Hz
20Hz 50Hz 100Hz
Field error through the poloidal gapoverlapped poloidal gap
Butt joint gap
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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
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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
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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
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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)
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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
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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
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• 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)
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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
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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
Vertica l FieldAmp lifier
01 - 08
Vertica l FieldAmp lifier
01 - 08
Vertica l FieldAmp lifier
01 - 08
Vertica l FieldAmp lifier
01 - 08
Ma g n etizin gAmp lifier
01 - 04
Vertica l FieldAmp lifier
01 - 08
Vertica l FieldAmp lifier
01 - 08
To ro id a lAmp lifier
01 - 04
Vertica l FieldAmp lifier
01 - 08
Vertica l FieldAmp lifier
01 - 08
Po lo id a l Fla t-To pAmp lifier
01 - 04
To ro id a l S ecto rAmp lifier
01
To ro id a l S ecto rAmp lifier
01
To ro id a l S ecto rAmp lifier
01
To ro id a l S ecto rAmp lifier
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
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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
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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
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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
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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