High-beta Program 3 June 2002 Objective: Investigate effect of toroidal currents on high-b performance: 1. Low-iota: discharges with current ramp and constant density Recover #54156 (B = -1.25 T, iota = 0.30, Bz = -190 G, Icc = - 2 kA) but use 900 MHz target a) apply proper gaspuffing to heal collapse of energy and density at iota(a) = 0.5 b) get profile- and MHD-data just before collapse c) keep OH-current just below critical value and increase density (Murakami-limit ?) 2. High-iota: optimize discharge by dynamic Bz- and Icc-program without toroidal current drive Recover #54023 (B = -1.25 T, iota = 0.52, Bz = -220 G, Icc = - 2.5 kA), 900 MHz target a) extend time of beta increase to 150 ms by proper stacking of NBI power b) apply Bz - ramp 100...220 G during increase of beta ! (constant Icc = - 2.5 kA) c) apply Icc - ramp -0.7...-2.5 kA G using optimized Bz - program 100 200 300 400 500 ms NBI <b> Bz Icc

High-beta Program 3 June 2002 - cont‘d 3. High-iota: OH current ramp during increase of beta B = -1.25 T, iota = 0.52, optimized Bz , Icc program (see 2.), 900 MHz target a) apply current ramp of ≈ 0...-6-8 kA to maintain iota(a) > 0.5 (≈ 0.52) during increase of beta (compensate beta induced decrease of edge iota, avoid iota = 1/2) b) optimize Bz - ramp (≈ 4 different flat top values) c) optimize Icc - ramp (with optimized Bz) d) Scan Ip with optimized Bz and Icc (0...-20 kA , 20 kA  iota(0) ≈ 1.25, iota(a) ≈ 0.71) 100 200 300 400 500 ms NBI <b> IOH

High-beta Program 3 June 2002 - cont‘d 4. High-iota: OH current ramp during flat top of beta B = -1.25 T, iota = 0.52, optimized Bz , Icc program (see 2.), 900 MHz target a) fast beta increase by proper stacking of NBI power d) Scan Ip 0...-20 kA (20 kA  iota(0) ≈ 1.25, iota(a) ≈ 0.71) 100 200 300 400 500 ms NBI <b> IOH

High-beta Program 3 June 2002 - cont‘d 5. Very high-iota: reversed OH current B = -1.25 T, iota = 0.72, optimized Bz , Icc program , 900 MHz target a) apply current ramp of ≈ 0...+7 kA to achieve shearless high-b configuration (beta-ramp during Ip flat top phase) , MHD limit decreased (no shear stabilization) ? b) increase reversed current to achieve cross iota = 0.5 tearing modes, pressure driven modes in case of reversed shear ? 6. Very high-iota: reversed OH current, low-b configurations B ≥ 1.25 T, iota = 0.72, reduced NBI power a) Scan Ip , reversed shear  neoclassical stabilization ?

1. Recover high-b current ramp shot #54156 B = -1. 25 T, iotaext = 0 1. Recover high-b current ramp shot #54156 B = -1.25 T, iotaext = 0.30, Bz = 190 G , Icc = -2 kA

1. Recover high-b current ramp shot #54156 (70 GHz startup) B = -1 1. Recover high-b current ramp shot #54156 (70 GHz startup) B = -1.25 T, iotaext = 0.30, Bz = 190 G , Icc = -2 kA (900 MHz startup) ruby Thomson

Healing of energy (density) collapse by gas puffing possible ?

Healing of energy (density) collapse by gas puffing possible Healing of energy (density) collapse by gas puffing possible ? - influence of iota ? -

MHD in #55846 Fast crash (≈ 20 ms) during Ip = 0 phase  interchange/ballooning resistive ? (low Te) Tearing mode activity at iota = 1/2 sometimes after start of radiative collapse causing contraction of current profile

Plasma current kept just below critical value (iota(a) < 1/2) - collapse avoided -

Plasma current kept just below critical value (iota(a) < 1/2) - collapse triggered by gas puff (contraction of current profile detabilizes tearing modes) -

2. Recover high-iota, high-b currentless shot #54023 B = -1 2. Recover high-iota, high-b currentless shot #54023 B = -1.25 T, iotaext = 0.52, Bz = 220 G , Icc = -2.5 kA, 900 MHz startup

Apply Bz-ramp (100. 220 G) and Icc-ramp (-0. 7. -2 Apply Bz-ramp (100...220 G) and Icc-ramp (-0.7...-2.5 kA) - improved start-up, improved low-b performance ? -