Current holes at ASDEX Upgrade Presented by O. Gruber for D. Merkl, J. Hobirk, P.J. McCarthy, E. Strumberger, ASDEX Upgrade Team - hardware upgrades for improved control - integrated advanced scenarios - ion ITB with current hole: equilibrium, current diffusion - electron ITB with current hole - summary LT WS (W56) on Physics of Current Holes, Mito, Japan, 3-4 Feb 2004 EURATOM Association

ASDEX Upgrade: flexible heating and fuelling systems Pellet Centrifuge: m/s repetition rate 80 Hz Neutral Beam Injection: - perp. heating 15 MW / keV - tang. off-axis beams 5 MW/ 100 keV Ion Cyclotron Resonance Heating: - 8 MW / MHz - variable deposition Electron Cyclotron Resonance Heating: 2 MW / 140 GHz / 2 s  4 MW / GHz / 10 s - on-line steerable mirrors R= 1.65 m a = 0.5 m I p  1.4 MA B t  3 T P NI  20 MW P ICRH  8 MW P ECRH  2 MW ASDEX Upgrade

- extended pulse length to 10 s flattop (= 2-3  R even at T e (0) = 10 keV) - extended PF coil operational window to run = 0.55 discharges - developed ICRH to routinely deliver > 5 MW in ELMy H-mode - increasing W coverage of inner wall First wall materials need minimum erosion & low Tritium retention recent hardware upgrades  improved control ● stepwise towards C-free interior - in 2004 campaign 70% of first wall covered - W-divertor in upper SN C-divertor in lower SN ● Up to now: - all plasma scenarii still accessible usually W concentration below machine has been more ‚delicate‘ to run  central RF heating & ELM control by pellets suppresses impurity accumulation at improved core confinement W W W W W

Towards a C-free first wall: - W coating of LFS poloidal limiters (actively cooled) W coated bottom divertor2005 Off-axis CD - upgrading of ECRH started (4 MW / 10 s / steerable mirrors / 105–140 GHz ) - LHR system (3.7 GHz) in discussion Next hardware extensions

Stabilizing shell for external kinks & active RWM control Next hardware extensions TNBI A9 DCN CX/ LHR NI I MSE,... YAG ECRH shell time constant for n = 1 above 30 – 40 ms (d=3 cm, steel) passive shell current feeders ? 2 sets of 8 active toroidal coils (n = 1, 2)

Stabilizing shell for external kinks & active RWM control Next hardware extensions passive shell current feeders ? TNBIA9 DCN NI I ECRH YAGMSE,... A9 CX/ LHR ECRH CAS-3D (P. Merkel): - shell currents for n=1 kink - extension to resistive wall - benchmarking with resistive 2d-wall code

 More compact pulsed reactor / steady-state operation H 89-P  N / q 95 > 0.8  Aim to achieve these conditions in steady state: - energy and particle exhaust need n e  n GW - tolerable ELMs  bootstrap current fraction > 50% in stationary advanced H-mode: prime candidate for ´hybrid´ ITER scenario  BS fraction > 80% for continuous reactor operation: strong ITBs with reversed shear needed 2 Performance beyond H-mode: integrated "advanced" scenarios NN H 89-P ITER 

ITBs produced in the current ramp-up with strong reversed shear at JET (using LHCD) and JT-60U (using NBI) showed the existence of an extended core region with zero toroidal current: current holes open questions: equilibrium, stability, transport, sustainment influence of size: duration limited by skin effect ? ITB driven bootstrap current sufficient? full non-inductive current drive needed for sustainment? Motivation for current hole investigations (0.8 MA, 2.7 T)

Ion ITB discharge with current hole barrier extends over the q min region  ITB driven bootstrap current and shear profile can be aligned

Ion ITB discharge with current hole: MSE results geometry formula of MSE at ASDEX Upgrade current hole lost when third source switched off (ITB lasts longer) DTM magnetic axis

current hole: equilibrium reconstruction Cliste: - solves Grad-Shafranov equation using external magnetics and MSE data - cubic spines for basic functions  prevents sharp „current hole“ edge - uses poloidal flux  as main coordinate - for very low central current densities,  pol =  as a function of spacial coordinates is poorly defined  convergence problems - new version:  mid   tor weighted sum of previous solutions for  (R,z) and j(R,z) during iteration to improve convergence (successive ´over-relaxation´) NEMEC: - modified 3-d stellerator equilibrium code (S.P. Hirshman) - energy minimizing fixed / free boundary code assuming nested flux surfaces - uses toroidal flux as main coordinate

current hole: equilibrium reconstruction - good agreement of the q-profiles except of the current hole edge - measured position of the (2,1) DTM from SXR & ECE

current hole: equilibrium reconstruction - colored points are the MSE observation points - shaded area labels the current hole Comparison of equilibrium reconstruction with MSE measurements

current hole: current diffusion - ITB driven off-axis bootstrap current not sufficient to maintain current hole - initial current hole taken from CLISTE at 0.3 s vanishes within 100 ms - fast diffusion of beam current density ? - high fast particle content may contribute to BS current

current hole: confinement - reversed magnetic and velocity shear improve heat insulation in core   T driven transport suppressed  internal transport barriers (ITBs) - stored energy of ion ITBs increases linearly with heating power

ITB scenario with counter-ECCD pre-heating # 17542

Electron and ion ITB - no MSE available - sawtooth-like crasches in T e due to collapsing electron ITBs during ctr-ECCD: indicates strong reversed shear (previous AUG results) or current hole (JET) Ctr-ECCD

Combined electron and ion ITB early ctr-ECCD produces electron ITB at delayed NBI onset, ion ITB develops  combination of electron and ion ITB foot of electron ITB sits at smaller radius

Small current hole during on-axis ctr-ECCD (ASTRA) TORBEAM: I ECCD =70 KA

Summary  current holes in ion ITB discarges (early NI heating) observed : - current hole diameter up to 25% of minor radius - equilibrium reconstruction with CLISTE (convergence up to q 0  40) and NEMEC (q 0 > 1000) possible - ASTRA current diffusion simulations show no sustainment by off-axis BS current - anormal beam driven current diffusion & fast particle BS needed: - off-axis co-CD supports: current hole lost with switch-off of tangential off-axis beam current holes with on-axis ctr-ECCD: - electron ITBs - combined electron and ion ITBs with both ECCD and NBI (T i  T e  keV) - ASTRA simulations indicate small current hole during central ctr-ECCD  extended control tools for all scenarii: - operation at high shaping - variable schemes for profile control (pressure, momentum, density, j, impurities) - variety of methods for NTM suppression - ELM control via shaping (type II ELMs), QH-mode and pellet pacemaking - kink and RWM control envisaged

Ion ITB discharge with current hole: SXR results

H 98 (y,2) n e /n GW Improved H-mode High  N q 95 = n e /n GW NN Improved H-mode High  N  N = 1.8 q 95 = Advanced H-modes: performance  * reactor relevant at medium densities : H 89-P = 2.8,  N = 3.2 (IAEA1998)  optimum exhaust close to Greenwald : H 89-P = 2.4,  N = 3.5 ( H-mode WS 2001) (at q 95 = 3.5) - continuous transition

ITER advanced ITER advanced Advanced H-modes: progress towards steady state & adv. performance Steady conditions for many current redistribution times:  low * - tripple product m -3 keV s -1 - Q DT (equivalent)  0.2  high Greenwald fraction  best combination of confinement, stability and density at high  > 0.4 and q 95  3.5  higher q 95 over-compensated by enhanced performance  N H 98-P / q 95 = 0.35 (0.2 in conv. ITER) 2

ITBs: missing stationarity due to MHD events - ITBs with early heating and RS - limited by coupling of infernal (at q min  2) and extrnal kinks to  N < 2 ITBs with delayed heating - highest performance achievable - high performance terminated by ELMs Combined electron and ion ITBs - high performance terminated by central 2/1 MHD Decisive influence of scenario:  sustained only with L-mode edge or poor H-mode edge  at better performance discharges short compared with current diffusion time  high control efforts required: p, j, MHD modes  a self-consistent scenario with reduced control requirements exists NN H 89-P ITER 

high pressure gradient needed to get 80% bootstrap current fraction (Q >30) reversed magnetic and velocity shear improve heat insulation in core   T driven transport suppressed  internal transport barriers (ITBs) ITB driven bootstrap current and reversed shear profile can be aligned optimise MHD stability – high p-gradient at q(min) leads to global MHD modes combination of electron and ion ITB scenarii needed Can tokamaks be optimised towards continuous reactor? - foot of ITB at  = 0.6

reversed magnetic and velocity shear improve heat insulation in core   T driven transport suppressed  internal transport barriers (ITBs) high pressure gradient needed to get 80% bootstrap current fraction (Q > 30) ITB driven bootstrap current and reversed shear profile can be aligned optimise MHD stability: high p-gradient at q(min) leads to global MHD modes combination of electron and ion ITB scenarii needed Can tokamaks be optimised towards continuous reactor? early ctr-ECCD produces electron ITB at delayed NBI onset, ion ITB develops foot of electron ITB sits at smaller radius

Ion ITBs: barrier position and q profile aligned - MHD modes trigger ITBs  relation with rational q values - strong barriers only in connection with reversed magnetic shear barrier extends over the q min region  ITB driven bootstrap current and shear profile can be aligned  (q min )  pol

Ion ITBs: route to very high bootstrap fractions ITB scenario with delayed heating: - heating of 15 MW late in the current ramp - lower SN with high triangularity - transition to H-mode 0 1 t(s)

Ion ITBs: route to very high bootstrap fractions 800 kA: n e /n GW =0.45 No-wall limit reached !?  N = 4.0 H 89-P = 3.2 T io = 14 keV first large ELM destroys ITB ! ITB scenario with delayed heating: - heating of 15 MW late in the current ramp - lower SN with high triangularity - transition to H-mode 1 MA: T io > 20 keV m -3 keV s -1 ≥ 60 % BS current

Motivation

Can tokamaks be optimised towards continuous reactor? early ctr-ECCD produces electron ITB at delayed NBI onset, ion ITB develops foot of electron ITB sits at smaller radius

 Highest performance achieved in Ion ITBs with reversed shear - scenario extended to high confinement H 89-P = 3.4 and high beta  N = 4 - T i  T e  keV with ctr-ECCD and NI - duration limited by strong ELMs, core and edge MHD modes - up to now transient max performance not sustainable - benchmark is advanced H-mode scenario Summary (1)  Extended control tools for all scenarii: - 10 s flat-top pulses allow current profile relaxation - operation at high triangularities close to DN (  = 0.55 achieved) - variable heating / CD schemes for profile control (p, momentum, density, j, impurities) Active MHD control: - variety of methods for NTM suppression - ELM control via shaping (type II ELMs), QH-mode and pellet pacemaking  reduced target loads, impurity control - kink and RWM control envisaged - disruption mitigation (not covered)

Summary (2)  Advanced H-mode scenario: a basis for ITER hybrid operation (even steady-state or ignition possible) - relaxed low shear q-profile (long sustainment compared to res. diffusion) - control of density peaking & impurity accumulation with tailored heat dep. - enhanced confinement H 98-P = and beta  N > 3 (up to no-wall limit) over substantial operational range of q 95,  and density - integration of type II ELMs close to Greenwald density and double null - despite high densities, > 60% non-inductive current drive achieved stepwise towards C-free interior (reduced erosion, T retention) - all advanced plasma scenarii accessible with W concentration below impurity accumulation at improved core confinement suppressed with central RF heating & ELM control by pellets