Transcript PowerPoint プレゼンテーション - uni

Status of LHD high beta Experiments
S. Ohdachi
and
High-beta theme group
2009/7/7 CWGM
Outline of my talk
• Conventional way to achieve highbeta plasma in LHD is to reduce the
shafranof shift so that the heating
efficiency is kept even in low
magnetic field. Improvements in the
last experimental campaign will be
presented.
• Another aproach for high-beta plasma in LHD is introduced. Pellet
induced “high-central-beta” type discharge.(IDB/SDC like discharge in
low magnetic field)
– Selected profile data are to be stored in the ISHPDB.
• MHD instabilities related with two type of high-beta plasma.
– Core density collapse with high-central-beta discharges
High beta experiment
•
•
Reduction of the Shafranov Shift by aspect ratio control(g scan), and
reduction of the magnetic field as far as we can expect heating, we have
increased beta value 5.0%（88611, Gas puffing, 0.41T）、5.1%（87493, pellet
injection, 0.425T）
We can sustain high-beta plasma for more than 100 times of energy
confinement time
Increase of b and ICRF antenna
Mainly by increase
in heating power
Antenna
by
removal
of antenna
Max before 11th
campaign
Operation region is extended
•
•
No ICRF antenna in 12th campaign. We coud do experiment with lower
magnetic field.
From directional probe measuring re-entering particle, there is clear
difference between 11th and 12th.
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
Edge pressure gradient is approaching its limit
• Pressure profile in the
edge region is almost the
same with H-mode
discharges.
m/n = 2/3?
= ½?
• In H-mode discharges,
increase of the edge
pressure gradient is
suppressed by largeamplitude MHD
modes(m/n = 2/3, 1/2, ..).
Standard high-beta / High central Beta
• New approach to the
high-beta plasma with
peaked pressure profile
(high-central-beta
scenario) is tried.
• 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.
Operation Regime of high-beta plasmas
• In Inward shifted
configurations, 2/1,
3/1Sawtooth is concerned.
• In outward shifted cases,
Core Density Collapse is a
important problem to be
avoided.
• From the real-time control
of the magnetic axis, we
try to access the highbeta regime.
2/1 Sawteeth / core MHD modes
m=2 sawteeth and its pre-cursor
Outboard
ISX
m=2, r~0.7±0.1
• m=2 precursor. Strange shape is
caused by the geometric effect.
• Scale of Sawteethies is small.
However, the peaking of plasma is
disturbed by them.
3.5U
Inboard
6.5U
Tims [s]
New PC power supply for Magnetic Axis control
Capacity of PC PS was increased
-
IS, IV coils : < 6. 2 kA / H 45 V, P 213 V (SS H 45 V, P 33 V)
-
Operation with  1.5 T is available / Fixed Bt or Fixed IHC operations
Example of Rax Swing Discharge (2.0 sec)
Reference
Rax = 3.6 m, Bt = -0.425 T, gc = 1.20
NBI#1,3 (Co.,1.3 s~) NBI#2 (Ctr., 1.8s~)
Rax Swing
Rax = 3.6 m  3.5 m for 2 sec
Both Rax and R00 shifts with the preset
Both beta and central pressure are
almost the same at Rax  3.54 m
Central pressure and beta dropped
when Rax shifted to less than 3.54 m
Control of the Magnetic axis
•
•
MHD unstable boundary can be determined.
Optimization of experiments is still needed to get higher beta plasma.
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.
Pressure driven Modes?
•
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
Parameter regime of CDC
CDC
2.5T
•
With lower magnetic
field, achievable
density is lower.
•
In, similar pressure
profile with lower
collisionality plasma,
no CDC is observed.
•
Only low frequency
oscillations are
observed.
1.5T
Small scale crash
Sometimes with precursor
0.75-1.0T
Clear pre-cursor
No crash or very weak crash
Mode structure with low-Bt
ISX
1.375T
Out
73856 1.5T
CDC
Every
In
100ms
p
m/n = 1/1 is dominant
The region affected by the crash is
where we observe MHD modes in low
collisionality condition.
R [m]
Time [s]
m/n = 1/1 structure can be seen 2D SX camera
Out
In
Time [s]
Consistent with the
simulated image
assuming core localized
m/n= 1/1 mode
Exp(-((r-0.2)/0.15)^2)
m=1
Summary
• With standard scenario, reduction of the Shafranov shift, we updated
the beta value. 5.0%（88611, stationary, 0.41T）、5.1%（87493, with
ice-pellet, 0.425T）. From equilibrium calculations(HINT2), achievable
beta value is limited by the heating power not by the equilibrium limit.
• We have tried high-central-beta configurations so that we can avoid
edge MHD modes. Central beta is almost as much as conventional
high-beta approach.
• Two MHD unstable region is important to form high-beta profile.
• Core density collapse(CDC) is more dangerous. Cause of the CDC is
still unknown. Pressure driven MHD modes is possible candidate.
• We start to avoid those regions, using active magnetic axis-shift
experiments.
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
exp(-((r0.2)/0.15)^2)
3.85m
β=0
計算結果をまわすために生じた偽イメージ
左図の分布を仮定
Sin
Cos
m=1
SXアレイの揺動分布
m=3
ありえない
If we rotate the
image, quality
becomes worse
Raw data
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
Equilibrium and beta-limit (Y. Suzuki)
New results with
different boundary
condition
Old Calculation
• Equilibrium beta-limit is determined by the
degree of the ergodization in the edge region.
• In g = 1.254 experiments, from previous HINT2
calculation, beta limit was predicted as 3.5%.
However, experimentally, we obtain 3.5% highbeta plasma with pressure gradient in the edge.
Δn=1
Δn=2
白線は崩壊前のイメージの等高線 m=1の変形から上部へ
CDC の発現領域と、平衡配位
2/3
1/2
• 前置振動は磁気井戸部で観測されている。理想インターチェンジモードは安定。
• 抵抗性の MHD modesかBallooning MHD modes 候補になる。
CDC scale with magnetic field
• With a low magnetic field, the
scale of the CDC is small.
• 1.0T ≦ Bt ≦1.5T, precursors
are observed. The radial
structure of the precursors,
we try to investigate the
cause of the CDC.
• Bt < 1T, no CDC has been
observed.