Development of an Improved H-mode for ITER: ASDEX Upgrade Scenario

EURATOM Association Talk given at 46th APS, Savannah, USA, 15 Nov 2004 Development of an ITER Relevant Advanced Scenario at ASDEX Upgrade: the improved H-mode Otto Gruber A.C. Sips, A. Stäbler, R. Dux, R. Neu, C. Maggi, Y-S. Na, ASDEX Upgrade Team • Aim of improved H-mode • Performance: MHD stability • confinement • Operational range: q95, n*, r* • Exhaust relevant high density regime • Electron heating in core: ICRH • Summary

ASDEX Upgrade program: physics basis for ITER & beyond ASDEX Upgrade R= 1.65 m a = 0.5 m Ip 1.4 MA Bt 3 T PNI  20 MW PICRH 8 MW PECRH 2 MW • Focus on: • performance-related physics in the ELMy H-mode, • including ELM mitigation, • (ii) scenarios and physics of advanced tokamak concepts, • (iii) MHD stability and active stabilisation, • avoidance and mitigation of disruptions • (iv) edge and divertor physics aiming at optimising • power exhaust and particle control • (v) testing of tungsten as alternative first wall materials

Performance beyond H-mode: "advanced" scenarios ●ITER baseline scenario ELMy H-mode, Ip = 15 MA, Bt = 5.7 T, q95 ~ 3, ne/nGW = 0.9 with: H98(y,2) = 1, N = 1.8  Q = 10 ● “Advanced scenarios“aim atintegration of improved confinement, stability, exhaust and towards steady-state ● “Improved H-mode“ on ITER  longer pulses at good performance (Q=10) at lower current (bootstrap current fraction < 50%) “Hybrid“ of steady-state, non-inductive, reversed shear  ITB scenario and baseline scenario, monotonic shear (q0 < 1)  higher performance (Q 10) at full current ● performance measured by figure of merit for fusion gain QN·H98(y,2) / q952 (= 0.20 for baseline scenario) ● “Improved H-mode“ on ASDEX Upgrade (since 1998)H 98(y,2)up to 1.4 & N up to 3.9simultaneously at q95~ 4

Performance beyond H-mode: "advanced" scenarios ●ITER baseline scenario ELMy H-mode, Ip = 15 MA, Bt = 5.7 T, q95 ~ 3, ne/nGW = 0.9 with: H98(y,2) = 1, N = 1.8  Q = 10 ● “Advanced scenarios“aim atintegration of improved confinement, stability, exhaust and towards steady-state ● “Improved H-mode“ on ITER  longer pulses at good performance (Q=10) at lower current (bootstrap current fraction < 50%) “Hybrid“ of steady-state, non-inductive, reversed shear  ITB scenario and baseline scenario, monotonic shear (q0 < 1)  higher performance (Q 10) at full current ● performance measured by figure of merit for fusion gain QN·H98(y,2) / q952 (= 0.20 for baseline scenario) ● “Improved H-mode“ on ASDEX Upgrade (since 1998)Aim to achieve these conditions stationary: - extrapolation to ITER needs n*<<1 - energy and particle exhaust need nenGW and tolerable ELMs Note: low n* and nenGW only achievable in ITER

recent hardware upgrades  improved control W W W W W - extended pulse length to 10 s flattop (= 10tR even at Te(0) =5 keV) - extended PF coil operational window to run <d> = 0.55 discharges - developed ICRH to routinely deliver > 5 MW in ELMy H-mode - ELM pacemaking by shallow pellet injection or vertical wobbling - increased W coverage of inner wall: minimum erosion & low Tritium retention ● ITER: - tungsten baffles in the first phase - full W wall in its reactor like operation? ●AUG stepwise towards C-free interior - in 2004 campaign70% of first wall covered - W-divertor in upper SN C-divertor in lower SN ●even improved confinement scenarii accessible usually W concentration below 10-5 ● machine has been more ‚delicate‘ to run  central RF heating & ELM control by pellets suppresses impurity accumulation W

Characterization of „improved H-modes“ on AUG • stationary q(r) with low magnetic shear in the centre and q0 ≥ 1 • - early moderate heating • - increase of heating at start of current flattop  type I ELMy H-mode • - strong heating up to 20 MW after >tR • - supported by tailored on- / off-axis NI deposition

Characterization of „improved H-modes“ on AUG • stationary q(r) with low magnetic shear in the centre and q0 ≥ 1 • - early moderate heating • - increase of heating at start of current flattop  type I ELMy H-mode • - strong heating up to 20 MW after tR • - supported by tailored on- / off-axis NI deposition • - low m,n modes substitute sawteeth

Role of MHD: support of stationarity and perf. limits ● support of stationary q-profile (q0~ 1) - fishbones (not always present) - small amplitude NTMs (5,4) - bootstrap current and NBCD ? ● benign MHD in high performance phase - no sawteeth  no seeding of (3,2) NTMs - low shear at (3,2) surface and triangularity reduced NTM drive - higher m,n tearings non-linear coupling further reduces (3,2) ●broad pressure profiles allow operation close to no-wall limit up to bN = 3.5 at high d q95=3.8

#18883 MHD Role of MHD: support of stationarity and perf. limits ● support of stationary q-profile (q0~ 1) - fishbones (not always present) - small amplitude NTMs (5,4) - bootstrap current and NBCD ? ● benign MHD in high performance phase - no sawteeth  no seeding of (3,2) NTMs - low shear at (3,2) surface and triangularity reduced NTM drive - higher m,n tearings non-linear coupling further reduces (3,2) ●broad pressure profiles allow operation close to no-wall limit up to bN = 3.5 at high d ●soft -limit due to 3,2 NTMs (degraded confinement; no disruption) ●disruption due to (2,1) mode mode locking

“Improved confinement”: H98-P =1.1-1.4 ●transport studies - heat transport still described by ITG /TEM turb. - threshold in R/LT  “stiff“ temperature profiles also in center: Ti(r=0) Ti(r=0.4)

“Improved confinement”: H98-P =1.1-1.4 ●transport studies - heat transport still described by ITG /TEM turb. - threshold in R/LT  “stiff“ temperature profiles ● confinement improvement explained by - more peaked ne-profiles (correlated with lower collisionality)  account to some extend for higher H-factors - however, for ne0/ne,ped  2: H-factors of improved H-modes still higher - higher pedestal pressure ? indications, but needs detailed measurements - ITER-H98(y,2) scaling  N-1 but N0 dependence found in standard H-modes - reduced H-factor for Ti→Te (ITG / TEM turb.)

tailoring of density profile with on- / off-axis heating • With q0 close to 1 • Main MHD limitation: NTM’s • Even without sawteeth. • n can be large enough to destabilise NTM’s. d d Adding central ICRH: - flattening of density profile - increasesan (stiff T profile) & D - still peaked enough to get H98-P>1 • off-axis NI heating: • peaking of density profile • due totransport, not core fuelling • - turbulent D + enhanced inward pinch • (GLF23 model: ITG/TEM) • - reduces to Ware pinch at high densities / collisionality

Impurity Control Peaked density profiles, no sawteeth  high central impurity concentration can be severe for NBI only heating Impurity control by central wave heating (divertor configuration): low level central ECRH (1-1.5 MW) or central ICRH (PICRH 0.5 · PNI) • core W concentration strongly reduced • density peaking reduced too  trade-off • minor penalty on H98·N •central C concentration reduced as well

Improved H-mode compatible with W walls and targets • - upper SN configuration with W coated targets •  N =2.8, H98(y,2) =1 • impurity control by central wave heating • - feasible at low W concentrations

Improved H-Mode: operational domain Valid scenario for ITER ? → documentation of dimensionless parameter range q95 scan / high denities up to nGW/ * scan 3.2 – 4.4 0.85ne/nGW 8 -13 10-3

Operational domain: full q95~ 3.2 - 4.5 range q95 scan at fixed Ip,  ~ 0.2, ne/nGW ~ 0.4, n*/n*ITER~ 1.5 - power ramps up to bN-limit of ~3.0 (close to no-wall limit) - stationary discharge at slightly lower b (duration techn. limited) - H98(y,2) up to 1.4 Parameters scans for improved H-modes q95 scan / high-ne/ * scan3.25 - 4.4/0.85ne/nGW/ 8 - 13E-03

Operational domain: no degradation at low r* r* scan at fixed q95 = 5, bN ~ 2.8, ne/nGW ~ 0.5, H98(y,2) 1 - standard H-modes: onset of NTMs scales with r* - stationary improved H-mode discharges by bp feedback - r* varied by a factor of 1.5 ri*= 0.00646 <Ti> /(Bt a)

Improved H-Mode: operational domain Documentation of dimensionless parameter range: q95 scan / high ne/nGW / *scan q95 full range 3.2 – 4.5 accessible at high performance * close to ITER value at moderate ne i* 4 – 6 times i*-ITER (~ 2 ·10-3): no i*-dependence of performance ne/nGW high ne possible  reactor relevant edge & div conditions

reactor-relevant performance at high density Fully integrated scenario - N = 3.5 (at q95=3.6, d=0.43) - HH98-P = 1.15 - particle density close to Greenwald - up to 40 energy confinement times type-II ELMs NH98(y,2) / q952 = 0.31 - combined with type II ELMs close to double null - steady target power loads  in average 2.5 MW power to outer bottom target (10 MW heating)  1.3 MW to upper targets

Improved H-Mode: operational domain Data base 2003/04: different time slices, also at low heating power; earlier high-ne plasmas included Performance vs. measure of bootstrap fraction (~ 0.5p): q95~ 3 – 4: H98·N/q952 0.3 →beyond Q =10 q95~ 4 – 5: H98·N/q952 ~ 0.2 → long pulse lengths at standard ITER performance

#19314 Compatibility w. significant e-heating: core ICRH So far: improved H-mode results obtained with dominant NBI heating dominant ion heating (Ti >Te) & input of particles and momentum, in contrast to -heating in reactor-type plasma Demonstration of improved H-mode with PICRH PNBI (ICRH dominates core) •Ti~ Te •N ~ 2.6 / H98(y,2) ~ 1.2  N · H98(y,2) / q952 = 0.24 • dominant central electron heating not yet achieved: - ICRH still heats ions - but inside   0.3: Pel enhanced by ~2.5 • during ICRH central W-concentration strongly depressed

Summary: Improved H-mode at ASDEX Upgrade • demonstrates that advanced requirements for stability (N 3.5), confinement (H98 >1) and exhaust (ne/nGW 0.9)can be simultaneously met: stationary (>10 resistive times) and over wide range in q95~3.2 – 4.5 • specific q-profile w. low central shear (q0 close to 1): avoids sawteeth, allows benign NTMs during high performance • core transport ITG/TEM dominated - density peaking increases with off-axis heat dep. and low collisionality - contributes to H98(y,2) >1 (trade-off w. impurity accumulation) - higher pedestal pressure may contribute • widens performance well beyond ITER baseline scenario q95 < 4: N H98(y,2) / q952 up to 0.31 q95 ~ 4-4.5: N H98(y,2) / q952 ~ 0.2 with non-inductive CD above 50% • dimensionless parameter scans towards ITER - * / *ITER close to 1 at low densities - no performance dependence on i* • integration with type II ELMs at high densities and full performance • obtained also with dominant core RF heating

Summary: Improved H-mode at ASDEX Upgrade 1000 Fusion Power (MW) ( n2T2 β2 bN2 Ip2) 800 b limit ? bN=2.5 600 high density 400 bN=2 200 low density 0 0.85 0.90 0.95 1.00 1.05 1.10 1.15 H H98(y,2) → strong candidate for ITER beyond baseline scenario: - long pulse “hybrid“ scenario at lower current above 1000 s or - operation close to ignition at full current (Q > 20) Improved H-mode scenario is investigated at other devices (DIII-D, JT60-U, JET) → broad extension of ITER relevant database (see A.C. Sips, IAEA 2004)

Development of an Improved H-mode for ITER: ASDEX Upgrade Scenario