RFX-mod Programme Workshop, 20-22/01/09, Padova - T. Bolzonella1 Tommaso Bolzonella on behalf of RFX-mod team Consorzio RFX- Associazione Euratom-ENEA

RFX-mod Programme Workshop, 20-22/01/09, Padova - T. Bolzonella 1

Tommaso Bolzonellaon behalf of RFX-mod team

Consorzio RFX- Associazione Euratom-ENEA sulla fusione, Padova, Italy

RFX-mod Programme Meeting 2009 – Padova - 20-22/01/2009

Resistive Wall Modes : Resistive Wall Modes :

present results and strategiespresent results and strategies


Summary

A RWM primer: Five Ws (and one H) on Resistive Wall Modes

Recent results on tokamak RWMs and future hot issues

RFP possible contributions on RWM common issues

RFP strategies to increase the impact on RWM research


What are RWMs?

Resistive Wall Modes are MHD instabilities that originates from the ideal kink when a resistive wall is surrounding the plasma.

Their main characteristic is that their growth rate depends on the typical penetration time of the passive boundaries, i.e. it is much slower than other typical MHD or hybrid timscales ( control!).

(B. Alper, et al., PPCF, 1989) (P.R. Brunsell, et al., Phys. Plasmas, 2003)


When were they found and Who is afraid of RWMs?

After their first experimental identification done on a RFP (HBTX1C in Culham, 1989, i.e. 20 years ago: happy anniversary RWMs!), since early 90’s their study became very popular also among the tokamak community as beta limiting instabilities.

On the RFP side instead, the interest remained “quiescent” till the new millennium, when data and experiments from T2R in Stockholm triggered a cascade of new important results on RWM physics and, in particular, on their active control.

(A. Garofalo, et al., Phys. Plasmas, 1999)

(P.R. Brunsell, et al., PRL, 2004)


Why are we concerned about RWMs?

In the tokamak case, RWMs represent one of the major limiting MHD phenomena in Advanced Tokamak regimes.

In the RFP case a set RWMs is always unstable as soon as the discharge duration exceeds the penetration time of the passive boundary. This means that any present of future RFP willing to explore long discharge scenarios has to cope with RWMs. At present RWMs are the most dangerous disruptive instability for a configuration otherwise robustly disruption free.

RFX-mod discharge #18542 is terminated by a non-controlled (on purpose) RWM.


Where can we find RWMs?

Even if RWMs can be found in both tokamaks and RFPs, they appear with some important differences:

– In the tokamak they grow as pressure driven, resonant instabilities.

– In the RFP they grow as current driven, non-resonant instabilities.

In tokmaks RWM appear mainly as n=1 modes, with different m components.

In RFPs they appear as m=1 modes, with an n spectrum that depends on the specific q profile.

As a result of their importance, many facilities (both tokamaks and RFPs) have or are developping active tools for RWM control. This is in particular true for the next tokamak generation, see for example KSTAR and JT-60SA.


How can we control RWMs?

This is of course the basic question…

There are basically two main ways:

– Indirect control: since plasma rotation has a stabilizing effect on RWM, by sustaining it (error field control, momentum injection, …) also RWM can be controlled. This works mainly on tokamaks.

– Direct control: a suitable set of active coils coupled to a set of sensors can feedback control the RWM amplitude. This is the preferred way on RFPs and will be the only control mechanism on slow rotating tokamaks.

(E.J. Strait, et al., Nucl. Fusion, 2003)

DIII-D

RFX-mod


Major present results on RWM in tokamaks

Stabilisation in slow rotating plasmas: error fields vs balanced momentum injection. New, more favourable threshods found, but extrapolation to ITER still uncertain.

Importance of kinetic dissipation mechanisms

Development of new numerical tools (including 3D effects)

Development of more efficient control softwares (M. Takechi, et al., PRL, 2007)

(H. Reimerdes, et al., PRL, 2007)

Similar results in


Major remaining issues on RWM in tokamaks

ITER (DEMO) flow rotation?

RWM stability threshold and 3D error fields

Mode rigidity

Kinetic effects on RWM stability

Optimal active coil system geometry

Optimal sensor type and location

Optimal control algorithms

RWM identification (interplay with NTMs)

RWM triggering by other MHD events (ELMs, fishbones, fast particles instabilities, …)

(M. Okabayashi, et al., Geneva, IAEA-FEC, 2008)

(P. Thomas, et al., Geneva, IAEA-FEC, 2008)


Well established and diagnosed phenomenology

Clear data for code benchmarking

Experimental verification of plasma rotation influence on RWM growth rate, search for possible dissipation mechanisms

Advanced control algorithms implementation and test

Resonant Field Amplification studies (influence of error fields and of non-optimized control)

Mode rigidity

Major present directions on RWM in RFPs

(M. Okabayashi, et al., PPCF, 2002)

Many subjects common to RFPs and tokamaks!

(H. Reimerdes, et al., PRL, 2004)

RFX-mod Programme Workshop, 20-22/01/09, Padova - T. Bolzonella 112008 IAEA Fusion Energy Conference, Geneva - P. Martin

n=4

2D codeCARMA 3D

RFX-mod data (n=5 RWM)used to validate the code

Benchmarking ITER stability code against RFX measurements

ITER prediction codes need benchmarking against experimental data

CARMA (MARSF + CARIDDI) is a MHD ideal code coupled with an arbitrary 3D magnetic boundary

Used to assess role of 3D effects for stability predictions (holes, extensions..) and compare with 2D predictions

F parameter

(1/s)

(F. Villone et al., PRL, 2008)


Advanced RWM control and mode un-locking

Active rotation of non-resonant wall-locked RWM is induced by applying complex gains (keeping the mode at the desired constant amplitude)

RWM amplitude

RWM phase

(T. Bolzonella, V. Igochine et al., PRL, 2008)


One more thing…


How can we study mode rigidity?

The MHD feedback control system can be on purpose “downgraded” both in power and number of coils, to study tokamak-relevant problems

– The effect of reduced available power

– Effects of mode non-rigidity with a reduced set of coils

– Interaction between RWMs and Tearing Modes (TM)


How can we study mode rigidity?

The MHD feedback control system can be on purpose “downgraded” both in power and number of coils, to study tokamak-relevant problems

– The effect of reduced available power

– Effects of mode non-rigidity with a reduced set of coils

– Interaction between RWMs and Tearing Modes (TM)


With full control (192 active coils) on all the fourier components RFX-mod can operate up to 1.8 MA, handling something like 30MW of ohmic power without limiters or divertors. What woud happen downgrading the control system?

Can it work?


Full control (192 active coils) on all the fourier components but one RWM

Different action on different MHD instablities

Reduced control (downgraded active coil array) only on the target RWM. The selection is done by the software control inside the cycle time.










Comments and conclusions

RWM studies are common field for tokamak and RFPs and coordinated projects represent a great opportunity for reciprocal benefits.

Several areas of common interest can be identified and in some of them RFPs can contribute to ITER relevant topics.

Future improvements could be useful, e.g. in the diagnostic area (plasma flow, current profiles, …) and in the numerical modelling. Important steps on those directions are already progressing.

RFX-mod capabilities are perfectly suited to study many of the hottest issues on RWMs!

Documents

RFX-mod Programme Workshop, 20-22/01/09, Padova - T. Bolzonella1 Tommaso Bolzonella on behalf of RFX-mod team Consorzio RFX- Associazione Euratom-ENEA