IAEA 2006 Chengdu

M. Agostini, C. Alessi, A. Alfier, V. Antoni, L. Apolloni, F. Auriemma, P. Bettini, T. Bolzonella, D. Bonfiglio, F. Bonomo, M. Brombin, A. Buffa, A. Canton, S. Cappello, L. Carraro, R. Cavazzana, M. Cavinato, G. Chitarin, A. Cravotta, S. Dal Bello, A. De Lorenzi, L. De Pasqual, D.F. Escande, A: Fassina, P. Franz, G. Gadani, E. Gaio, L. Garzotti, E. Gazza, L. Giudicotti, F. Gnesotto, M. Gobbin, L. Grando, S.C. Guo, P. Innocente, R. Lorenzini, A. Luchetta, G. Malesani, G. Manduchi, G. Marchiori, D. Marcuzzi, L. Marrelli, P. Martin, E. Martines, S. Martini, A. Masiello, F. Milani, M. Moresco, A. Murari, L. Novello, S. Ortolani, R. Paccagnella, R. Pasqualotto, S. Peruzzo, R. Piovan, P. Piovesan, A. Pizzimenti, N. Pomaro, M.E. Puiatti, G. Rostagni, F. Sattin, P. Scarin, G. Serianni, P. Sonato, E. Spada, A. Soppelsa, G. Spizzo, M. Spolaore, C. Taccon, C. Taliercio, D. Terranova, V. Toigo, M. Valisa, N. Vianello, P. Zaccaria, P. Zanca, B. Zaniol, L. Zanotto, E. Zilli, G. Zollino, M. Zuin

Overview of RFXOverview of RFX --mod Results mod Results with Active MHD Controlwith Active MHD Control

S. Martini

on behalf of the RFX team

Consorzio RFX, Associazione EURATOM_ENEA sulla Fusione, Corso Stati Uniti 4, 35127 Padova, Italy

IAEA 2006 Chengdu

Outline of Talk Outline of Talk

1.1. Introduction: the RFP and its MHD modesIntroduction: the RFP and its MHD modes

2.2. RFXRFX--mod: new features for MHD controlmod: new features for MHD control

3.3. RFXRFX--mod vs RFX benchmarkmod vs RFX benchmark

4.4. Feedback stabilization of RWMsFeedback stabilization of RWMs

5.5. Mode rotation & Confinement @ 1 MA Mode rotation & Confinement @ 1 MA

6.6. Advanced RFP ScenariosAdvanced RFP Scenarios

7.7. Conclusions and future plansConclusions and future plans

IAEA 2006 Chengdu

The RFP configurationThe RFP configurationThe RFP, as the tokamak, is a nearly force-free configuration: the current density J is ∼ // to B

The recepee: • Apply a small toroidal field Bφ (50 mT OK)• Apply a toroidal voltage Vφ• Wait (some ms) for plasma self-organizationHere is Your RFP plasma:

-0,05

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0 0,05 0,1 0,15 0,2 0,25 0,3

-0,100,10,20,30,40,50,60,7

Bφ (a)

<Bφ>

Bθ(a) & I 18500

[T] [MA]

t [s]

force-free magnetic field : µ0J = ∇×B = µB

Main advantages: • Almost no Bφ -> no superconducting coils• Stabilized by shear -> no current limit• High ohmic heating -> ohmic ignition?• Intrinsically high β

Main disadvantages:

• Poor confinement ?

• Requires a close fitting thick stabilizing shell ?

B &J field lines

Vφ

The answers depends on the control of MHD modes.

See results of RFX, TPE-RX, MST (EX/8-2 this afternoon)

See results of EXTRAP T2R andLListenisten to the rest of the Talk!to the rest of the Talk!

IAEA 2006 Chengdu

MHD Mode Classification in RFPsMHD Mode Classification in RFPs

q (r) Internally non-resonant

Resistive Wall Modesm=1, n=-7

m=1, n=-8m=1, n=-9

““ DYNAMO MODES”DYNAMO MODES”

•• provide Jprovide Jθθθθθθθθ at reversal at reversal

radiusradius

•• Magnetic Magnetic

Stochasticity in coreStochasticity in core

•• NonNon--AxisymmetricAxisymmetric

perturbation LMperturbation LM

Externally non-resonant Resistive Wall Modes

m=1, n > 0

m=1, n =-5

m=1, n =-6

m=0, all n

internally internally resonating resonating tearing tearing modes modes

r (m)

IAEA 2006 Chengdu

The Phase Locking of many modes results in a The Phase Locking of many modes results in a nonnon--axisymmetric axisymmetric deformationdeformation: the “: the “slinkyslinky” or ” or Locked Mode (LM)Locked Mode (LM)

The “slinky” Locked ModeThe “slinky” Locked Mode

Each mode causes a helical Each mode causes a helical perturbation of the plasmaperturbation of the plasma IφJn

80

40

200 90 180 270 360

t [ms]

toroidal angle Φ (deg)

#12461

60

1001 cm

Braking torque by the vessel and Braking torque by the vessel and field errors cause field errors cause Wall LockingWall Locking of of the the slinkyslinky

Localised plasma-wall interaction ⇒⇒⇒⇒ 100 MW/m2

Plasma surface distortion x 10

IAEA 2006 Chengdu

2. RFX2. RFX--mod: features and goalsmod: features and goals

Standard treatment of MHD in RFPs:

• A thick conducting shell keeps dynamo modes under control and prevents RWMs.

RFX-mod approach:

• A “thin” shell with τshell << τpulse

• Feedback control by saddle coils mimics an IDEAL SHELL over time scales > τshell

IAEA 2006 Chengdu

τshell =0.05 s (RFX 0.450s)

b/a = 1.1 (RFX 1.24)

RFX-mod new features

• Thinner and more closely fitting shell

• Improved design graphite tile first wall armour

• New Bϕ power supplies

• 192 active saddle coils

• Digital feedback system for MHD modes and main coil circuits

Lower power on leading edges

Better dynamo mode control

Highly flexible control of field errors, Dynamo modes and Resistive Wall Modes

Goal of the modifications:

Demonstrate the operation of the largest existing RFP

• without a thick shell

• in the MA range

• with real-time control of:

- Equilibrium

- MHD modes (tearing and RWM)

IAEA 2006 Chengdu

The MHD control system

Saddle coil layout: Full Poloidal and Toroidal coverage

192 coils: 4 poloidal x 48 toroidal

Wide spectrum of Fourier components :Wide spectrum of Fourier components :

(Dictated by Dynamo modes)

• m=0,1,2 (partial)

• n ≤ 24

Frequency response:Frequency response:

• DC < f < 1 kHz

Digital feedback control system:

Allows a variety of schemes for selective control of modes

See paper by Luchetta A., et al., FT/P5-1

IAEA 2006 Chengdu

Many control schemes are possibleMany control schemes are possible

�Intelligent shell (local radial field compensation)

�Selective Virtual Shell (cancels all harmonics but some, e.g. m=1, n=0 equilibrium field)

�Virtual Shell + rotating perturbations (phase and amplitude of selected harmonics set to follow given waveforms)

�Mode Control (regulators applied to modes rather than to single coils)

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3.Comparison RFX 3.Comparison RFX // RFXRFX--mod mod

Benchmark pulses at 600 kA:

• RFX

• RFX-mod “passive” shell

• RFX-mod Virtual Shell

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Ip and Vloop RFX, RFX-mod,VS

0

10

20

30

40

50

0 0,05 0,1 0,15 0,2 0,25 0,3t [s]

Ip [kA]

0

200

400

600

0 0,05 0,1 0,15 0,2 0,25 0,3

9523 RFX16321 RFX-mod + RTFM18500 RFX-mod Virtual Shell

Vφ (0) [V]

Controlled run down

IAEA 2006 Chengdu

Br(a) reduced below 0.5 mT with VS

m= 1

Tm

ie[s

]

0 π 2π

τshell= 0.05 s => Br growing in RFX-mod standard

1 mT

RFX-mod Virtual ShellRFX-mod passive

IAEA 2006 Chengdu

LM is quenched by Virtual ShellLM is quenched by Virtual Shell

Plasma perturbation due to LM with Virtual Shell and Without: • m=1 is < 1 cm • m=0 is ∼ 1.5 cm

m=1m=1 m=0m=0

m=1 (cm)

m=0 (cm)

Φ (deg)-3

-2

-1

0

1

2

3

4

5

0 90 180 270

17262 RFX-mod18645 RFX-mod VS

-3

-2

-1

0

1

2

3

4

5

0 90 180 270

17262 RFX-mod18645 RFX-mod VS

IAEA 2006 Chengdu

What happens in plasma What happens in plasma core?core?

TheVirtual Shell TheVirtual Shell quenches edge quenches edge field errorsfield errors

Reduction of dynamo modes in plasma core

RFX-mod standard

RFX-mod VS

Br (a)

Br max (core)

Dynamo modes at edge

Dynamo modes reduced by a factor >2 with Virtual Shell

Paper by R.Paccagnella et al., TH/P3-19

Dynamo modes in core

IAEA 2006 Chengdu

Electron Temperature profiles and χ

r [m]

χe [m2s]

r [m]

Virtual Shell

standard

#17453 : ττττE~ 1.5 ms

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4. Feedback Control of Resistive Wall Modes

IAEA 2006 Chengdu

RWM RWM controlcontrol in in RFXRFX--modmod

• 17287-> full Virtual Shell

• 17301-> Selective Virtual Shell free evolutionm=1, n=-3 ÷ -6.(n=-6 most unstable)

• 17304-> same as 17301, but with VS control on from 150 ms

More results to be presented in a few days at the APS!

Full Virtual shell

n=-3 ÷ -6 control off

n=-3 ÷ -6 control off control on

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5. Mode rotation /Confinement @ 1 MAMode rotation /Confinement @ 1 MA

High current pulses:

• Locked Mode Rotation

• 1 MA discharges

IAEA 2006 Chengdu

LM rotation by RTFMLM rotation by RTFM

external m=0 mode

n-1, m=1 mode goes backward

n+2, m=1 has doubled frequency (harmonic generation)

most restrained m=1 is stationary

n+1, m=1 mode goes forward

harmonic generation

φ0ext

φ1,7LM

φ1,8LM

φ1,9LM

φ1,10LM

φ1,11LM

0

0

90

180

270

0

90

180

270

0

90

180

270

20 40 60 80 100

Time (ms)

# 1

23

50

0

90

180

270

0

90

180

270

0

90

180

270

360

0

0

0

90

180

270

0

90

180

270

0

90

180

270

20 40 60 80 100

Time (ms)

# 1

23

50

0

90

180

270

0

90

180

270

0

90

180

270

360

0

t (ms)

80

40

20

0 90 180 270 360

t [ms]

toroidal angle Φ (deg)

#12350

60

1 cm0

Result: continuous LM rotation for the whole discharge

Bartiromo R., et al., PRL83 (1999)1779.

Rotating Toroidal Field Modulation: a rotating m= 0 perturbation is applied by the 12 sectors of the Bφ coils

IAEA 2006 Chengdu

LM rotation via external m=1 perturbations

Several rotating m=1perturbations are generated at frequency equispaced with n:

• 1,-8 @ -10 Hz

• 1,-10 @ 10 Hz

• 1,-11 @ 20 Hz

• 1,-12 @ 30 Hz

This scheme rotates the LM pattern along the stationary 1,-9mode.

A torque is exerted on the m= 0mode via non-linear coupling.

• RTFM works well also in RFX-mod, but...

• Causes an increase in the loop voltage,

• because its m=0 perturbation adds to m= 0 field error, presently the largest error of the VS.

• Fortunately, we developed LM rotation schemes by m=1perturbations, which directly couple to the modes

IAEA 2006 Chengdu

Well Controlled 1MA pulses with LM rotationWell Controlled 1MA pulses with LM rotation

0200400600800

10001200

0 0,1 0,2 0,3 0,4

0

10

20

30

40

50

0 0,1 0,2 0,3 0,4

0

1

2

3

4

0 0,1 0,2 0,3 0,4

0100200300400500600

0 0,1 0,2 0,3 0,4

-1200

-700

-200

300

800

0 0,1 0,2 0,3 0,4

1967619669196781969919700197011970219761φ L

M (

deg)

Τ e

(eV

)n e

·101

9(m

-3)

Vφ(V

)Ι p

(kA

)

Start of controlled run down

a

b

c

d

e

t (s)

0200400600800

10001200

0 0,1 0,2 0,3 0,4

0

10

20

30

40

50

0 0,1 0,2 0,3 0,4

0

1

2

3

4

0 0,1 0,2 0,3 0,4

0100200300400500600

0 0,1 0,2 0,3 0,4

-1200

-700

-200

300

800

0 0,1 0,2 0,3 0,4

1967619669196781969919700197011970219761φ L

M (

deg)

Τ e

(eV

)n e

·101

9(m

-3)

Vφ(V

)Ι p

(kA

)

Start of controlled run down

a

b

c

d

e

t (s)

0

50

100

150

200

250

300

350

400

450

500

-0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5

Te (eV)

r (m)

LM rotation with Virtual Shell + rotating m = 1 perturbation

achieves very reliable LM rotation

This results in long and This results in long and reproducible 1MA pulsesreproducible 1MA pulses

Where density control is not Where density control is not lost from pulse to pulselost from pulse to pulse

And current, density and And current, density and temperature can be temperature can be

“superimposed”“superimposed”

temperature profiles are temperature profiles are broad and very similar toobroad and very similar too

IAEA 2006 Chengdu

1MA pulses RFX1MA pulses RFX--mod vs RFXmod vs RFX

10

20

30

40

50

60

70

80

0 5 10 15

RFX

RFX-mod

Power (RFX)

Power (RFX-mod)

Vφ (0) [V]

I/N·10-14 (Am)

10

20

30

40

50

60

70

80

0 5 10 15

RFX

RFX-mod

Power (RFX)

Power (RFX-mod)

Vφ (0) [V]

I/N·10-14 (Am)

0

50

100

150

200

250

300

350

400

450

500

0 200 400 600 800 1000 1200Ip [kA]

Te0 [eV]

Toroidal Loop Voltage on RFX-mod is more than 10V lower than in RFX

Te increases linearly with current in RFX-mod , with no saturation at 1MA as in RFX

nG/n

For a discussion of Greenwald density in RFPs , nG see:

Valisa M. et al. EX/P3-17

IAEA 2006 Chengdu

Energy confinement in RFXEnergy confinement in RFX--modmod

0

0.5

1

1.5

0 0.5 1 1.5

RFX-mod VSRFX-mod NVS

ττ ττ E (

ms)

1.1 10-13 I 1.17 (I/N)-0.35 b8-15

-0.61 (A*Am*T)

• Energy confinement time has a clear scaling with plasma current

• Also the dependence on field error amplitude is very encouraging

more in Innocente P. et al., Paper EX/P3-10

IAEA 2006 Chengdu

6. Advanced RFP scenarios

• Quasi Single Helicity States

• Oscillating Poloidal Current Drive

IAEA 2006 Chengdu

Long lasting QSH in Virtual Shell dischargesLong lasting QSH in Virtual Shell discharges

SXR1/SXR2

long lasting QSH are more likely long lasting QSH are more likely -- at high current andat high current and-- shallow reversal (F shallow reversal (F ~ ~ --0.1)0.1)

Island

a hot helical core is presenta hot helical core is presentfor time periods >for time periods >ττττττττEE

Shafranovshift

SXR1SXR2

Mode m=1 n=Mode m=1 n=--77 (innermost (innermost resonating) grows much larger resonating) grows much larger than the than the other dynamo modesother dynamo modes

Quasi Single Helicty states Quasi Single Helicty states were discovered on RFXwere discovered on RFXESCANDE D. F., et al. Phys. ESCANDE D. F., et al. Phys. Rev. Rev. LettLett. 85 (2000) 1662. 85 (2000) 1662

IAEA 2006 Chengdu

Oscillating Poloidal Current Drive

0

2

4

6

8

10 19530 (std)

19615 (OPCD)

b t(1,-

7) (

mT

)

(a)

0

1

2

3

4

5

b t(1,-

8--1

1) (

mT

)

(b)

300

400

500

600

Te (

eV)

(c)

-0,15

-0,12

-0,09

-0,06

-0,03

0,08 0,09 0,1 0,11 0,12 0,13 0,14

F

t (ms)

(d)

BLACK is a standard VS pulse

RED is a VS +OPCD pulse

An Oscillating Poloidal Voltage is applied to the plasma by the

the Toroidal Field Coils

during each Toroidal Field Reversal enhancement fase

The MHD Dynamo modes are suppressed

But for the m=1, n=-7

i.e. a QSH is excited

Which reflects in a higher core Te

0

100

200

300

400

500

600

700

800

-0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5

ensemble av. 1 MAfit 1MA19531 OPCD t=0,095 s

Te (eV)

r (m)

plasma

VθJθ

plasma

VθJθ

It’s based on the Pulse Poloidal Current Drive of MST: Sarff J. S., et al., PRL 72(1994) 3670

IAEA 2006 Chengdu

Conclusions: RFX-modAccomplishments

� Proven operation with τshell =0.05 s (0.45 s in RFX)

� Active Equilibrium + MHD Control (Virtual Shell)

� pulse length = 0.37s (x 3 RFX, x 7 τ shell)

� -25% Vloop

� + 30% Te,Ti

� > 2 x ττττE

� Feedback stabilization of RWMs

� Tearing modes control (edge amplitudes and phases) led to LM rotation & reproducible high performance 1MA pulses

� Improved Dynamo modes control for advanced RFP scenarios (Quasi Single Helicity and Oscillating Polidal Current Drive)

� And many more things that did not fit in 17’

Outperformed Outperformed previous thick previous thick

shellshell

IAEA 2006 Chengdu

Future plansFuture plans

• Operation at higher currents, up to 2 MA design value.

• Full exploitation of the MHD control (THERE IS ROOM T O IMPROVE):

– Virtual Shell algorithm to include the shell delay time.

– Closer Virtual Shell: control system fed with error fields computed at the plasma edge

– Integratedm = 0 mode control, supplemented by the Bφ coils.

• Further development of advanced RFP scenarios:

– Combined OPCD/pellet injection, QSH /pellet injection

– Reversed Field Tokamak

– Oscillating Field Current Drive

• RWM stabilization experiments (also in support of Tokamak program):

– Different equilibria

– Utilize subsets of coils

– RFA studies

IAEA 2006 Chengdu

RFXRFX--modmod

12 toroidal coils sectors with independent power supplies

Closed shell equatorial gapvacuum vessel with

improved graphite tiles 3 mm Cu shell

192 active saddle coils for MHD

control

R 2 m

a 0.46 m

I 0.3-1.1 (2) MA

τpulse 0.37 s

τshell 0.05 s

• A flexible device to study confinement at high current and in advanced scenarios for the RFP

• A powerful tool for MHD studies in support of the other programs

IAEA 2006 Chengdu

The RFX-mod thin shell

Shortcircuitedgap: 50 bolted copper plates

• 3 mm Cu (RFX 65 mm Al)

Welded gap

• 1 overlapped poloidal gap

• 1 toroidal gap ( high field side)

• τshell =0.05 s (RFX 0.450 s)

• Shell prox. b/a = 1.1 (RFX 1.24)

IAEA 2006 Chengdu

First wall• Optimized tile shape:

– Tapered poloidal shape

– Thickness reduced to 17 mm (a = 0.459 m)

– Reduced poloidal / toroidal gaps between adjacent tiles to < 3 mm

• Factor ≈ 2 reduction of Pwmax

IAEA 2006 Chengdu

RFX-mod: MHD Control System

latency time: 330 µsm modes: 0, 1, -1, 2 (partial)n modes: 0-23

IAEA 2006 Chengdu

Field lines stochasticity vs Dynamo modes

m= 0 only

m= 1 only

m = 0 & m = 1

PRL 96, 025001 (2006)

IAEA 2006 Chengdu

IAEA 2006 Chengdu

0

200

400

600

800

1000

1200

0 0,1 0,2 0,3 0,4

0

200

400

600

800

1000

1200

0 0,1 0,2 0,3 0,4

0

200

400

600

800

1000

1200

0 0,1 0,2 0,3 0,4

0

200

400

600

800

1000

1200

0 0,1 0,2 0,3 0,4

0

200

400

600

800

1000

1200

0 0,1 0,2 0,3 0,4

0

200

400

600

800

1000

1200

0 0,1 0,2 0,3 0,4

0

200

400

600

800

1000

1200

0 0,1 0,2 0,3 0,4

0

200

400

600

800

1000

1200

0 0,1 0,2 0,3 0,4

Well Controlled 1MA pulses with LM rotationWell Controlled 1MA pulses with LM rotation

Ip [kA]

IAEA 2006 Chengdu

Ti from Doppler broadening of O VII 1623Å line ( r/a ~ 0.4)

Te from double filter

Ti,e (VS) ~ 1.3xTi,e standard

Ti

Te

17082, standard

17092,VS

IAEA 2006 Chengdu

Rotating Toroidal Field ModulationRotating Toroidal Field Modulation

t [s]

Current in the sectors

A rotating m=0 perturbation by the 12 Bφ coil sectors exerts a torque on the q=0 island:

Tz0,1 ∝ br

0,1Br0,1(r,1)sin(∆φ0,1)

Tviscm ,n ∝ (br

m,n ) 2 ωRotation is opposed by drag of eddy currents in vessel

A sufficiently high external Br0,1 overcomes the drag and lock

in phase the 0,1 mode

1

23

6

45

1

23

6

45

1

23

6

45

IAEA 2006 Chengdu

Three mode interactionThree mode interaction

ω 1,n+1 − ω 1,n = ω 0,1

• high n modes will co-rotate with (0,1) ext. perturbation

• low n modes will counter-rotate

• in general =>

Tz1,n ∝Cnbr

1,nbr1,n+1br

0,1sin(φ1,n+1 −φ1,n −φ0,1)+ Cn−1br

1,nbr1,n−1br

0,1sin(φ1,n−1 −φ1,n +φ0,1)

Due to non-linear coupling the rotating m=0 mode exerts a torque on each couple of m=1 n, n+1 modes :

IAEA 2006 Chengdu

Comparison between RFX-mod and old RFX

Virtual shell is better than passive thick shell for proximity and control of plasma perturbations and error fields @ gaps and portholes.Requires acceptable power:

in RFX-mod ~ 1 MW (~ 20 MW ohmic) @ ~ 1 MA

P5.088

0

2

4

6

8

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 1 3 5 7 9 11 13 15 17 19

br_max m=1 RFX-mod w/o VS

RFX-mod with VS

old RFX with thick shell

mT

n

IAEA 2006 Chengdu

B &J field lines

Vφ

The RFP configurationThe RFP configurationforce-free magnetic field: µ0J = ∇×B = µB

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Jθ

Jφ

r/a

0

0.2

0.4

0.6

0.8

1

Bθ

Bφ

A ‘dynamo’ mechanism A ‘dynamo’ mechanism drives Jdrives Jθθ at the Bat the Bφφ reversal reversal surfacesurface

IAEA 2006 Chengdu

0

0,5

1

1,5

2

2,5

3

3,5

4

0 60 120 180 240 300 360

∆(ϕ)∆(ϕ)∆(ϕ)∆(ϕ)#17079 (VS)#17082

(cm)

ϕ (deg)

t=40ms

Non-axisymmetric shift with and w/o VS

IAEA 2006 Chengdu

The RFP DynamoThe RFP DynamoB &J field lines

Vφ

A ‘dynamo’ mechanism A ‘dynamo’ mechanism drives Jdrives Jθθ at the Bat the Bφφ reversal reversal surface and beyondsurface and beyond

This can be otained only by breaking the toroidal This can be otained only by breaking the toroidal symmetry (Cowling theorem).symmetry (Cowling theorem).

Here is where the RFP “dynamo” modes come into play...Here is where the RFP “dynamo” modes come into play...

IAEA 2006 Chengdu

Plasma wall interaction with new wall

•The penetration of the mode radial field component through the thin shell is fast, which leads to enhanced plasma wall interaction.

•Improved graphite tiles design counters this effect by spreading the power deposition over the tile center rather than on edges.

•This is confirmed by CCD images of plasma-wall interaction taken during pulses.

•Periodic in-vessel inspection showed that, contrary to the past, after several hundred pulses no damage has been done to the tile edges.