MHD Issues in Low-A RFP Machine RELAXmasamuneUS-J_MHDWS_Austin2008.ppt Author Rob Lahaye Created...
Transcript of MHD Issues in Low-A RFP Machine RELAXmasamuneUS-J_MHDWS_Austin2008.ppt Author Rob Lahaye Created...
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