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Resistive Wall Modes : present results and strategies. Tommaso Bolzonella on behalf of RFX-mod team Consorzio RFX- Associazione Euratom-ENEA sulla fusione, Padova, Italy. RFX-mod Programme Meeting 2009 – Padova - 20-22/01/2009. Summary.

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Resistive Wall Modes :

present results and strategies

Tommaso Bolzonella

on behalf of RFX-mod team

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

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

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


Summary l.jpg
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

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


What are rwms l.jpg
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)

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


When were they found and who is afraid of rwms l.jpg
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)

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


Why are we concerned about rwms l.jpg
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.

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


Where can we find rwms l.jpg
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.

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


How can we control rwms l.jpg
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

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


Major present results on rwm in tokamaks l.jpg
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)

Similar results in

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

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


Major remaining issues on rwm in tokamaks l.jpg
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, …)

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

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

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


Major present directions on rwm in rfps l.jpg
Major present directions on RWM in RFPs

Many subjects common to RFPs and tokamaks!

  • 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

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

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

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


Benchmarking iter stability code against rfx measurements l.jpg
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

RFX-mod data (n=5 RWM)

used to validate the code

g

(1/s)

CARMA 3D

2D code

n=4

F parameter

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

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

2008 IAEA Fusion Energy Conference, Geneva - P. Martin


Advanced rwm control and mode un locking l.jpg
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)

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


One more thing l.jpg
One more thing…

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

2008 IAEA Fusion Energy Conference, Geneva - P. Martin


How can we study mode rigidity l.jpg
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)

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

2008 IAEA Fusion Energy Conference, Geneva - P. Martin


How can we study mode rigidity15 l.jpg
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)

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

2008 IAEA Fusion Energy Conference, Geneva - P. Martin


Can it work l.jpg
Can it work?

  • 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?

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


Different action on different mhd instablities l.jpg
Different action on different MHD instablities

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

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

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


Different action on different mhd instablities18 l.jpg
Different action on different MHD instablities

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

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

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


Different action on different mhd instablities19 l.jpg
Different action on different MHD instablities

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

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

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


Comments and conclusions l.jpg
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!

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


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