Status of LHD high beta Experiments

1. Status of LHD highbeta Experiments S. Ohdachi and High-beta theme group 2009/7/7 CWGM

2. Outline of my talk • Conventional way to achieve high-beta 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

3. 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

4. Increase of b and ICRF antenna Mainly by increase in heating power • 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. by removal of antenna Antenna Max before 11th campaign Operation region is extended

5. High-beta Steady State Discharge 9-13 June, 2008, EPS, Greece, S. Sakakibara • <bdia>max ~ 4.8 %, b0 ~ 9.6 %, HISS95 ~ 1.1 • Plasma was maintained for 85tE • Shafranov shift D/aeff ~ 0.25 • Peripheral MHD modes are dominantly observed. Rax = 3.6 m, Bt = -0.425 T Core modes vanish in high beta region. /20

6. m/n = 2/3? =½? Edge pressure gradient is approaching its limit • Pressure profile in the edge region is almost the same with H-mode discharges. • In H-mode discharges, increase of the edge pressure gradient is suppressed by large-amplitude MHD modes(m/n = 2/3, 1/2, ..).

7. 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. Standard high-beta / High central Beta • New approach to the high-beta plasma with peaked pressure profile (high-central-beta scenario) is tried.

8. 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 high-beta regime. 2/1 Sawteeth / core MHD modes

9. m=2 sawteeth and its pre-cursor ISX Outboard • m=2 precursor. Strange shape is caused by the geometric effect. m=2, r~0.7±0.1 3.5U • Scale of Sawteethies is small. However, the peaking of plasma is disturbed by them. Inboard 6.5U Tims [s]

10. 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

11. 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 Raxshifted to less than 3.54 m

12. Control of the Magnetic axis • MHD unstable boundary can be determined. • Optimization of experiments is still needed to get higher beta plasma.

13. 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; high-central-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.

14. Profile changes with CDC events • Central beta/density decreases by 40%. • Time scale of the crash is about 1ms.

15. 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

16. Parameter regime of CDC • 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. CDC 2.5T 1.5T Small scale crash Sometimes with precursor 0.75-1.0T Clear pre-cursor No crash or very weak crash

17. ISX Mode structure with low-Bt 1.375T Out 73856 1.5T CDC m/n = 1/1 is dominant The region affected by the crash is where we observe MHD modes in low collisionality condition. Every 100ms In p R [m] Time [s]

18. 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

19. 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.

20. Typical Iota profile and well/Hill boundary magnetic hill Low beta m/n = 2/3 1/q • In LHD, pressure gradient driven modes are important; stability depends on magnetic well depth. • 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) m/n = 1/1 Edge m/n = 2/1 Core magnetic well magnetic hill High beta

21. 3.85m β = 0 exp(-((r-0.2)/0.15)^2) 左図の分布を仮定 Sin Cos 計算結果をまわすために生じた偽イメージ m=1 SXアレイの揺動分布 m=3 ありえない

22. If we rotate theimage, quality becomes worse Raw data

23. Plasma Aspect-Ratio HC-O HC-M H -M HC-I Plasma 9-13 June, 2008, EPS, Greece, S. Sakakibara Plasma aspect-ratio can be changed by controlling current center of HC Increment of Ap leads to a reduction of Shafranov shift  favorable for heating efficiency, transport and eq. b-limit  enhanced magnetic hill and reduction of magnetic shear  optimum Ap for high-beta plasma production Poloidal Coils Helical Coil

24. Equilibrium and beta-limit (Y. Suzuki) New results with different boundary condition • 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% high-beta plasma with pressure gradient in the edge. 。 Old Calculation

25. 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

26. CDC の発現領域と、平衡配位 • 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。 2/3 1/2

27. 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.