AXISYMMETRIC CONTROLS IN RFX-mod: STATUS AND PERSPECTIVES

1M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

AXISYMMETRIC CONTROLS IN RFX-mod: STATUS AND PERSPECTIVES

M. Barp, R. Cavazzana, G. Marchiori, A. Soppelsa, L. Zanotto

2M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

• 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

Outline

3M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

Feedback Control of Reversal Parameter

4M. 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

5M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

• 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

General scheme and models

System Modeling

6M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

Closed Control Loop

The reversal parameter control can be reformulated as a control of toroidal edge field at the plasma radius

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tv r

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v(t) affected by plasma reconnection

Feedback controller scheme

v

v

7M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

Reversal loss

Results

8M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

Optimization of Toroidal Power Supply Control System

9M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

• 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

Optimization of Toroidal Power Supply Control System (1)

+

-

pK

sK i

refI +

+

ffK

ffV

++

Toroidalinverters

torI

10M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

• 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

Optimization of Toroidal Power Supply Control System (2)

11M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

Feedback Control of Plasma Current

12M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

• 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

Current Controller Limitations (1)

13M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

• 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

Current Controller Limitations (2)

14M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

• Plasma current control based on operator experience

• Plasma current ramps at the fastest rate without control

Automatic Ip ramping

• 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 real-time system but a good estimation can be obtained

Plasma Current 29271

PCAT Voltage 29271

Vmag 29271Vmag estimate 29271

Automatic Ip ramping

15M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

MHD Equilibrium Codes

for Horizontal Equilibrium and Shape Control

16M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

• 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() plasma density profile

– fixed boundary problem (inverse problem): determination of

position and amplitude of external currents for an assigned

plasma boundary and JT() plasma density profile

Horizontal Equilibrium and Shape Control Codes (1)

17M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

• 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

Horizontal Equilibrium and Shape Control Codes (2)

18M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

• 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

Horizontal Equilibrium and Shape Control Codes (3)

19M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

• 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.

Conclusions