Current holes at ASDEX Upgrade Presented by O. Gruber Merkl, J. Hobirk, P.J. McCarthy, E. Strumberger, ASDEX Upgra - hardware upgrades for improved control - integrated advanced scenarios - ion ITB with current hole: equilibrium, current diffusion - electron ITB with current hole - summary T WS (W56) on Physics of Current Holes, Mito, Japan, 3-4 Feb 2004 EURATOM Association
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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.
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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
• 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 sustainmen
t?
Motivation for current hole investigations
(0.8 MA, 2.7 T)
Ion ITB discharge with current hole
barrier extends over the qmin region
ITB driven bootstrap current and shear profile can be aligned
Ion ITB discharge with current hole: MSE results
geometry formula of MSEat ASDEX Upgrade
current hole lostwhen 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 Te 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: IECCD=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 q0 40) and
NEMEC (q0 > 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 (Ti Te 8 - 10 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
H98(y,2)
0.5
1.0
1.5
ne/nGW
0.2 0.4 0.6 0.8 1.0 1.2
Improved H-mode
High N
q95 = 3.3 - 4.3
ne/nGW
0.2 0.4 0.6 0.8 1.0 1.2
N
0
1
4
3
2
Improved H-mode
High N
N = 1.8
q95 = 3.3 - 4.3
Advanced H-modes: performance
* reactor relevant at medium densities : H89-P= 2.8, N= 3.2 (IAEA1998)
optimum exhaust close to Greenwald : H89-P= 2.4, N = 3.5 (H-mode WS 2001)
(at q95 = 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 1020 m-3keV s-1
- QDT(equivalent) 0.2
high Greenwald fraction
best combination of confinement, stability and density
at high > 0.4 and q95 3.5
higher q95 over-compensated
by enhanced performanceN H98-P / q95 = 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 qmin 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
H89-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 qmin region
ITB driven bootstrap current and
shear profile can be aligned
(qmin)
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:
ne/nGW=0.45
No-wall limit reached !? N = 4.0
H89-P = 3.2
Tio = 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: Tio > 20 keV
1.5.1020 m-3keV 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 H89-P= 3.4 and high beta N= 4
- Ti Te 8 - 10 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 H98-P= 1.1 - 1.5 and beta N> 3 (up to no-wall limit)
over substantial operational range of q95 , 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 10-5
- impurity accumulation at improved core confinement suppressed