MHD Issues in Low-A RFPMachine RELAX

S.Masamune, A.Sanpei, R.Ikezoe, T. Onchi,K.Oki, T.Yamashita, H.Shimazu, H. Himura,

N.Nishino1), R.Paccagnella2)

Kyoto Institute of technology, Kyoto, Japan1)Hiroshima University, Higashi-hiroshima, Japan

2)Consorzio RFX, Padova, Italy

US-Japan Workshop on MHD control, MagneticIsland and Rotation, 11/23-25, 2008　（UT atAustin).

REversed field pinch of Low-Aspect-ratio eXperiment

• R/a = A = 2(51 cm/25 cm)

• Optimization inprogress

Kyoto Institute of Technology

Normal RFP discharges established

0 0.5 1.0 1.5 2.0Time (ms)

Ip (

kA)

Voo

p (V

)B

t-w

all

(mT

)<B

t> (

G)

0

20

40

020

0100200

20

60100

~ 30 V

Goal of RELAX experiment

• Experimental study on advantages of low-A RFPconfiguration

- Improved confinement with QSH for achievinghigh beta

- Experimental identification of bootstrap current (targer parameters: Te~300eV, ne~4x1019m-3 at

Ipp~100kA)

Tearing and RWM play important roles in the RFP

- formation and sustainment of the configuration through nonlinear MHD- of general interest as a control problem of highly nonlinear system

Nonlinear MHD Phenomena(relaxation, dynamo, magnetic reconnection, magneticchaos, ion heating, momentum transport, etc.)

RFP Configuration(Global structure) MHD Instabilities

ControlInput

Output

Lower A means lower n for dominant m = 1 modes

-0.1

0

0.1

0.2

0.3

0.4

0 0.2 0.4 0.6 0.8 1

A = 2

A = 3

q

r/a

1/4

1/51/6

...

Quasi-periodic growth of a single dominant helical mode(m=1/n=4)

12

,12

,12

s

max

min

!

= """

#

$

%%%

&

'

((

)

*

++

,

-= .

.

n

nn nn

n

b

bN

Spectral index Ns Characteristic of the QSH RFP state:lower dominant mode number (mostlyn = 4) and higher amplitudes than inother RFPs.

Dominant helical structure observed with high-speed camera

Filament structure indicates simple structure ofplasma parameters →　effect of lowering A

t

Simulated helixwith m=1/n=4

Possibility of rotating Helical Ohmic Equilibrium state- A large-scale magnetic field profile change -

Quasi-periodic oscillation between reversed and non-reversed states

Similar large-scale oscillatory behavior in Br and Bθ

Bφ (m

T)

Ip (kA

)

Measured field profile agrees well with Helical Ohmic Equilibrium statewith closed helical flux surfaces

R. Paccagnella, IEA / RFP Workshop 2000

Numerical solution of Helicallysymmetric RFP equilibrium

: Low-frequency (f<2kHz) component

bBB +=)0,0(

Theory

)0,0(B

Experiments

b)0,0(

BBb !=

Excellent agreement may be an indication ofrotating Helical Ohmic Equilibrium state

)0,0(

!B

)0,0(

zB

0.44

br

bz

bθ

Flux surfacesrecovered!

Radial profiles (0.6<r/a<1.0)

:

3D MHD simulation can reproducemajor MHD characteristics of RELAX plasmas

MHD simulation reproducesquasi-periodic oscillation of the dominant m=1/n=4 mode

Oscillating behavior of the tearingpart of the spectrum(m=1/n=4,5,..) in RELAX can becompared with 3D MHDsimulation with ideal wallboundary condition.- due to mode rotation- due to short discharge duration

MHD simulation predicts RWM will be problematicfor longer pulse operation

Experimental implication of RWM in RELAX

=> Ip starts decreasing

Br (m=1/n=2) measuredon the outer surface ofthe vacuum vessel

%5.11)(/)(~

!"aBaB pr

Growth rate vs. na/R of externalkink modes for α-Θ0 model profiles

MHD control plans in RELAX

• Discharge performance improvement: - Ip~100kA, τ: ~2ms => 5ms (within present

capability) - Static helical perturbation Further improvement will require improved

magnetic boundary: - feedback control systemAnother means for confinement improvement

(current profile control, e.g. ) may be necessary

Conclusion

• RFP plasma with MHD properties characteristicto low-A configuration attained in RELAX

• Dominant mode with lower n realized• Simple helical structure observed• Possible Helical Ohmic Equilibrium state

demonstrated• 3D MHD simulation could reproduce most of

the characteristics

Growth of dominant mode is related to mode rotation

m=1/n=4 mode behavior:Longer QSH period for slower rotation

Shorter QSH period with higher spectralindex Ns for faster rotation