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AXISYMMETRIC CONTROLS IN RFX-mod:
STATUS AND PERSPECTIVES
M. Barp, R. Cavazzana, G. Marchiori, A. Soppelsa, L. Zanotto
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Outline
• Feedback Control of Reversal
Parameter
• Optimization of Toroidal Power Supply
Control System
• Feedback Control of Plasma Current
• MHD Equilibrium Codes for Horizontal
Equilibrium and Shape Control
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Feedback Control of Reversal
Parameter
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Feedback Control of Reversal Parameter
• A reliable control of F was missing in RFX-mod:
– F controlled in “open loop”
– Control relied very much on the experience of the operator
– Plasma events had strong impact on the control
• Reasons for a reliable and precise control:
– Best performance of RFX-mod at shallow F (-0.05≤F<0)
– At high current and shallow F, small deviations of Bt(a) are tolerable
– F positive  loss of RFP configuration and very poor confinement
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
System Modeling
General scheme and models
• Model of the toroidal
power supply (both power
circuit and control
system)
• Model of the coupling
between the toroidal
windings and the plasma
during the flat-top phase
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Closed Control Loop
Feedback controller scheme
vq (t) affected by
plasma reconnection
v* (t )  
F
Bt a 
Bt
0 a
2 *
kvr* (t )
vq
niT (t )  v (t ) / Rv
Bt , a (t )   0
2R0
vq*
The reversal parameter control can be reformulated as a
control of toroidal edge field at the plasma radius
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Results
Reversal loss
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Optimization of Toroidal Power
Supply Control System
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Optimization of Toroidal Power Supply Control
System (1)
• New feed-forward input to the internal controller of the
toroidal PS to improve the dynamic performance of the
system
• Proportional and Integral gain optimization
V ff
K ff
I ref
Kp
+
-
I tor
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Ki s
+
+
+
+
Toroidal
inverters
M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Optimization of Toroidal Power Supply Control
System (2)
• Small current unbalances in the toroidal coils give rise to
wall mode locking
• Significant m=0, n=1 mode/field error not adequately
corrected
• A proper controller necessary to drive independently the
current in the 12 toroidal coils
– reduction of field errors and current unbalances
– m=0 mode control
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Feedback Control of Plasma
Current
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Current Controller Limitations (1)
• Plasma current controller
implemented and tested
• Tuning campaign not
completed
• Controller step response
can be optimized
• Feedback component is a
lag compensator, which is
affected by the actuators
saturations
• Experimental proposal has
been submitted to tune the
regulator
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Current Controller Limitations (2)
• Plasma Current Controller
(PCC) increases plasma
current independently from
plasma resistance
• High energy deposition on a
localized set of tiles
• Debate whether the machine is
protected “by design” against
such occurrence or if the PCC
should take any action
• At present PCC switched off if
a power threshold is exceeded
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Automatic Ip ramping
Plasma Current 29271
PCAT Voltage 29271
Automatic Ip ramping
Vmag 29271
Vmag estimate 29271
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• Plasma current control
based on operator
experience
• Plasma current ramps at
the fastest rate without
control
• PCAT unit automatically
switched on as soon as
their voltage is higher than
the voltage drop on the
transfer resistor (vm)
• vm not available in the realtime system but a good
estimation can be obtained
M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
MHD Equilibrium Codes
for Horizontal Equilibrium and
Shape Control
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Horizontal Equilibrium and Shape Control
Codes (1)
• Reliable MHD Equilibrium code to describe a wide range
of plasma equilibria in RFX-mod is needed for
engineering applications
• Static/evolutionary MHD equilibrium
– free boundary problem (direct problem): determination of the
plasma boundary for an assigned set of external currents and
JT(y) plasma density profile
– fixed boundary problem (inverse problem): determination of
position and amplitude of external currents for an assigned
plasma boundary and JT(y) plasma density profile
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Horizontal Equilibrium and Shape Control
Codes (2)
• Engineering oriented versions (e.g. Proteus, MAXFEA)
including
realistic
description
of
the
conducting
structures are essential:
– Design magnetic winding systems
– Derive linearized model of the plasma response
– Test and to assess the performances of plasma magnetic control
systems
• Modified 2D non linear MHD equilibrium code MAXFEA
used in RFX-mod to solve particular RFP equilibria
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Horizontal Equilibrium and Shape Control
Codes (3)
• Research activity for a renewed code or a different one
to be adapted to RFP discharges, in particular to the
shallow F equilibria.
• Codes needed for:
– Study of different plasma shape in RFP or Tokamak
configuration in RFX-mod
– Conceptual design of new RFP experimental devices
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives
Conclusions
• F-control system has been implemented and tested in
2010. It proved to be a very useful tool in particular at very
high plasma current.
• The optimization of the toroidal power supply control
system for the improvement of the m=0 mode control and
the reduction of field errors and currents unbalances has
been considered.
• Plasma current control has also been developed in 2010
but not fully tested. It is scheduled in 2011 a campaign to
tune the regulator. A new controller for the automatic
ramping of the plasma current is under consideration too.
• The investigation for a new MHD equilibrium code for
horizontal equilibrium and plasma shape to be used with
shallow F is needed.
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M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status
and Perspectives