Download "High density operation (SDC/IDB configuration) in LHD and its

"High density operation (SDC/IDB configuration) in LHD
and its operational limits"
S. Ohdachi
•LHD introduction
•MHD characteristics and high-beta operation
•high-density operation and high-central-beta plasma
Large Helical Device(LHD)
Poloidal coil
L = 2, m=10 Heliotron Type
R = 3.5 - 3.9m, a ~ 0.6m
Bt = 0.5 – 2.8 T, NBI ~ 15MW
Helical coil
• In Heliotron type plasma,
wide range of the
configuration can be made
from combination of the coil
system. Among them, control
of the magnetic axis is very
effective.
• So far, inward shifted
configuration (Rax=3.6m) is
the best for performance.
Rax is a key parameter for high-b
Magnetic axis position is one of configuration
parameters characterizing MHD and transport:
Inward
Rax
outward
Stability:
hill
well
Equilibrium:
weak dependence
Transport:
Increment of helical ripple
Heating:
Prompt loss of NB
Confinement:
(Experiment)
shift
3.6 m
Shafranov shift
Shafranov shift deteriorates transport and heating
efficiency, although it is better for stability
 adjustment of aspect ratio
9-13 June, 2008, EPS, Greece, S. Sakakibara
Plasma Aspect-Ratio
Plasma aspect-ratio can be changed by controlling current center of HC
Increment of Ap leads to a reduction of Shafranov shift
Poloidal Coils
 favorable for heating efficiency, transport and eq. b-limit
 enhanced magnetic hill and reduction of magnetic shear
 optimum Ap for high-beta plasma production
HC-O
HC-M
H-M
HC-I
Helical Coil
Plasma
9-13 June, 2008, EPS, Greece, S. Sakakibara
Configuration Optimization and Achieved b
Transport
FY2006
Magnetic field (Bt)
Optimization
MHD Stability
FY2002, 06
b increases with the
reduction of the
magnetic pressure.
FY2003, 04
Aspect Ratio (Ap)
Optimization
Heating efficiency is
better with reduced
Shafranov shift.
Heating efficiency is
deteriorated by the
reduction of the
magnetic field.
With reduced shift,
magnetic well is
shallower(unstable)
ISS95,04
tE ∝ P-0.6ne0.5Bt0.8
b ∝ P0.4ne0.5Bt-1.2
b ∝ Bt-0.75 (P ∝ Bt0.35 ,ne ∝ Bt0.54 )
Results of High-b Experiments
5% plasma was maintained for more
than 10tE, whereas 4.8 % one was for
85 tE
Beta increases with input power
No disruptive phenomenon
The duration time in gas-puff
discharge is limited by NBI
tduration : duration time where <bdia>  0.9<bdia>max is sustained
9-13 June, 2008, EPS, Greece, S. Sakakibara
/20
Typical Iota profile and well/Hill boundary
• In LHD, pressure
gradient driven
modes are
important; stability
depends on
magnetic well
depth.
magnetic hill
Low beta
m/n = 2/3
1/q
m/n = 1/1
• With increase of
beta, the well
region expands.
• Unstable region
remains in the
edge region.
• Resistive
interchange mode
always observed in
the edge. (slightly
increase
transports)
Edge
m/n = 2/1
Core
magnetic well
magnetic hill
High beta
High-beta Steady State Discharge
<bdia>max ~ 4.8 %, b0 ~ 9.6 %, HISS95 ~ 1.1
Rax = 3.6 m, Bt = -0.425 T
Plasma was maintained for 85tE
Shafranov shift D/aeff ~ 0.25
Peripheral MHD modes are dominantly
observed.
Core modes vanish in high beta region.
9-13 June, 2008, EPS, Greece, S. Sakakibara
/20
Standard high-beta / High central Beta
Inward
• So far, reduction of the
Shafranov-shift is our main
scheme of the optimization.
• Reduction of the heating
efficiency is minimized for low
Bt discharges if shifts are
reduced.
outward
Stability:
hill
well
Equilibrium:
weak dependence
Transport:
Increment of helical ripple
Heating:
Confinement:
(Experiment)
• New approach to the highbeta plasma with peaked
pressure profile (high-centralbeta scenario) is tried.
Rax
Prompt loss of
NB
3.6 m
Shafranov
shift
• There are many advantages.
– the magnetic well is deeper in the core
region and the pressure gradient in the
edge region (magnetic hill)is smaller.
IDB/SDC plasma
• IDB-SDC plasma is …
– Observed with Rax>= 3.7 m by the refueling at the center
region using ice-pellet injection.
– Fairy peaked density/pressure profile is formed
– In the outward shifted case, central electron density reaches
1021 m-3.
-3
10
20
max. ne(0) 10 [m ]
12
8
6
4
2
0
3.6
3.7
3.8
Rax [m]
3.9
4.0
High-cental-beta(IDB)
discharge with CDC
• A peaked profile is formed in the
recovery phase after sequentially
injected hydrogen pellets. In this
recovery phase, the pressure
profile becomes peaked; highcentral-beta plasma is formed by
this.
• Increase of the b0 is disturbed by
so-called core density
collapse(CDC) events. CDC is an
abrupt event where the core
density is collapsed within 1 ms.
(much faster than other MHD
relaxation events in the LHD)
• The cause of the CDC has not
been clarified. Pre-cursor
activities (n=2) is often observed.
Profile changes with CDC events
• Central beta/density
decreases by 40%.
• Time scale of the crash
is about 1ms.
CDC characteristics
•
MHD activities are observed in the steep
pressure gradient region (Outward)
before the event. One of the candidates
for the CDC events.
•
Due to the magnetic well, low-n ideal
MHD instabilities are stable.
Resistive MHD modes /Ballooning MHD
modes are possible candidate.
•
2/3
1/2
Operation Regime of high-beta plasmas
2/1 Sawteeth core modes
•
Two MHD activities should be
avoided in order to form a
high-central-beta plasmas
using pellet injection.
•
The control of the magnetic
axis is the key to avoid CDC.
By vertical elongation and
aspect ratio control,
Shafranov-shift of the plasma
is reduced; they are found to
be effective to avoid CDC.
Real-time control of the
vertical magnetic field, which
controls the magnetic axis is
planned.
MHD instabilities in inward shifted plasmas
• Sawtooth-like activity are often observed inward-shifted plasmas.
• Thought the effect of these events on the confinement is small, the
increase of the central beta (estimated by the magnetic axis position)
is saturated by the sawtooth-like events; the events affect the
peaking speed of the pressure profile.
Conclusion
• Two approaches to make high-beta plasmas are tried in the LHD.
– Standard Scenario (Bt = 0.425T, Raxvac=3.6m )
<b> = 5 %, b0 ~ 10 %, stationary
– IDB/High-Central-Beta Scenario (Bt = 0.75T, Raxvac= 3.65-3.75m
b0 ~ 10 %, transiently by pellet injection.
<b> = 2 %,
• In IDB/High-Central-Beta Scenario, to avoid MHD unstable region is
important to form a peaked pressure profile.
• Especially, CDC phenomena disturb the increase of the central beta.
However, They can be controlled by
– Reduced Shafranov-shift by the vertical elongation and by the larger aspect ratio
is effective . Real-time control of the vertical magnetic field will be applied.
– In low magnetic field (lower electron temperature), CDCs disappear.