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"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.