control of mhd instabilities. similarities and differences between tokamak and rfp

RFX workshop /20.01.2009/ Valentin Igochine

Control of MHD instabilities. Similarities and differences between tokamak and RFP

V. Igochine, T. Bolzonella, M. Maraschek, W. Suttrop, D.Yadykin


Outline of the talk

• Control of MHD instabilities in ASDEX Upgrade– Tokamak scenarios and corresponding instabilities – Conventional scenario: Neoclassical tearing mode and sawteeth

• Main stabilizing/destabilizing factors• ECCD as a main active control tool• ITER requirements for active control

– Advanced scenario: Resistive wall mode • External coils design in ASDEX Upgrade• Present status and start of the operation

• Control of the RWMs in RFPs in comparison with tokamaks– Similarities and differences in RWM behavior and drive – Open questions in RWM physics

• Conclusions


Tokamak scenarios and typical safety factor profiles

C. M. Greenfield et. al. ITER scenarios:

1. H-mode2. Improved H-mode3. Advanced tokamak

scenario

RWM

NTM,Sawtooth

ρ

q

2

1.5

Safety factor

1

qI


Control of MHD instabilities is a key issue to obtain a high-performance plasma

MHD instabilities in the core regime• Neoclassical Tearing Modes (NTMs) - appear in a high beta plasma - limit the achievable beta at N<N

ideal

• Sawtooth Oscillations - have smaller effects on global parameters - are able to trigger an NTM at low N values

Active control is important for both!

Control tool: Electron Cyclotron Current Drive (ECCD) - highly localized current drive fill the hole in bootstrap current - flexible ECCD location is necessary

MHD instabilities in conventional scenario

flat pressure in the island↓

no bootstrap current

↓ growth of NTM


EC resonanceThe island position varies during the discharge

We have to match position of the island with current drive position

• ASDEX Upgrade (changes of Btor)Bt changes is not possible in superconductor device like ITER

• DIII-D (moves the plasma radial)No free volume for radial movement in ITER

Both variants are not acceptable for ITER!

• Changes of ECCD deposition • system of mirrors to change position of the depositionForeseen for ITER

(3,2) NTM

Possible variant of NTMs suppression


ASDEX Upgrade enhances its capabilities in this area

EC resonance(3,2) NTM

Currentdrive

4 new gyrotrons (1 MW & 10s each) with movable mirror system each.

Present status: 1 gyrotron is already installed. The others would be installed in 2009-2010.


EC resonance Possible problems in ITER:

• Deposition width is largeSolution: Current drive should be done in O-point only (Maraschek PRL, 2007)

• Locking of NTM to the wallIn line ECE diagnostic to detect the island (F. Volpe)

(3,2) NTM

Possible problem in ITER and their solutions

Now ITER

Current drive is only inside the island


• ECE can detect modulated ECRH (every 0.2s 2ms off)• MSE migrated to realtime-acquisition and transfers data• standard data transfer framework established• 80ms realtime TORBEAM for deposition predictions

Complete realtime-loop for NTM control


Why do we need to control sawteeth?

• Long Sawteeth have been shown to trigger Neo-classical Tearing Modes

– Long Sawteeth NTMs

– Short Sawteeth Avoid NTMs

• NTMs degrade plasma confinement

• Even bigger problem in ITER

Time (s)

ICRH/MW

NBI/MW

termination

Magnetics: #58884 only

Expanded intime: 15-18s

2/1

3/2

4/3

0

15kHz

long sawtooth

SXR/a.u.

[Sauter et al, PRL, 88, 2002]

Fusion born ’s

Long sawtooth periods

More likely to trigger NTMs

JET


Stability of the (1,1) mode strongly depends from shear at q=1 surface.

ECCD is able to destabilize the mode and make more frequent and smaller sawteeth.

Sawtooth control

Example: Change of shear at q=1 with co-ECCD and counter-ECCD in ASDEX Upgrade [A.Mueck, PPCF, 2005]


Complete realtime-loop for NTM and Sawtooth control

The same system can be used for NTMs and Sawteeth control!

NTM

(1,1), Sawteeth


Tokamak scenarios and typical safety factor profiles

C. M. Greenfield et. al. ITER scenarios:

1. H-mode2. Improved H-mode3. Advanced tokamak

scenario

RWM

NTM,Sawtooth

ρ

q

2

1.5

Safety factor

1

qI


Bu - coils

Bl - coils

A - coils

END 2009: Installation of 4 Bu and 4 Bl coils.

Coils system for ELMs and RWMs control in ASDEX-U


Drilling holes for coils in support structures is the most time consuming work which would be done at the end of 2009.

Actively cooled coils

Coil system design for ASDEX Upgrade


ASDEX Upgrade (8x3) ITER (9x3)

The coil system is similar to ITER design

Comparison of coils geometry in ASDEX-U and ITER


Time schedule (from W.Suttrop, Ringberg 2008)


ELM control is one of the main priorities

Time schedule (from W.Suttrop, Ringberg 2008)


RWM: tens of Hz rotation

particles

Plasma flow: kHz rotation

Coupling:(m, n)(m±1,n)(m±2,n)

RWM is the main common issue for tokamaks and RFPs

• RWM is the main common issue for tokamak and RFP

• RWM is static in RFP and slowly rotates in tokamaks

• Static RWM typically destroys the plasma confinement

Study of RWM locking & unlocking & rotation is important for tokamaks and can be studied in RFPs


• The RWM can be unlocked in RFPs.• Successful experiments on active rotation in RFX-mod.

V. Igochine et.al. EPS2008/ T.Bolzonella et.al., PRL, 2008

RWM rotation experiments in RFX-mod


Result is in very good agreement with ideal mode assumption.

The mode rotation depends on phase shift between feedback and RWM

RWM rotation experiments in RFX-mod. Summary.


Result is in very good agreement with ideal mode assumption.

No effect of plasma rotation up to now. The same rotation in both directions

The mode rotation depends on phase shift between feedback and RWM

RWM rotation experiments in RFX-mod. Summary.


Result is in very good agreement with ideal mode representation.

No effect of plasma rotation up to now.

Next step: increase frequency of the rotation by changing Δφ

Any asymmetry?

RWM rotation experiments in RFX-mod. Next step.


Result is in very good agreement with ideal mode representation.

No effect of plasma rotation up to now.

Next step: increase frequency of the rotation by changing Δφ

IF YES, THEN PLASMA ROTATION COULD BE IMPORTANT

Any asymmetry?

RWM rotation experiments in RFX-mod. Next step.


Conclusions

Different tokamak scenarios require different types of control.

RWM physics is a natural common issue for RFPs and Tokamak

In spite of several differences the mode is the same and advanced knowledge about mode control can be moved from RFPs to Tokamaks.

Further experiments with rotation could help better understand the physics of the RWM.

control of mhd instabilities. similarities and differences between tokamak and rfp

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