Modelling of JET Hybrid Scenarios with GLF23 Transport Model: ExB Shear Stabilization

1. Modelling of JET hybrid scenarios with GLF23 transport model: ExB shearstabilisation of anomalous transportI. Voitsekhovitch, P. Belo, J. Citrin, E. Fable, J. Ferreira, J. Garcia, L. Garzotti, J. Hobirk, G. M. D. Hogeweij, E. Joffrin, F. Kochl, X. Litaudon, S. Moradi, F. Nabais, JET-EFDA contributors, and the EU-ITM ITER Scenario Modelling group

2. Motivation for validation/use of GLF23: reasonable compromise between physics complexity and computational time modelling of the whole scenario, test of plasma control algorithms. • Outline • Experimental database • Modelling of JET hybrids: • - ExB shear suppression of anomalous transport with • electrostatic GLF23 model • - estimation of ExB shear effect in presence of linear βe • stabilisation • Projection to ITER hybrid scenario • Discussion: role of fast ions, PVG turbulence

3. JET discharges (C wall) selected for GLF23 validation J. Hobirk et al, PPCF 2013 74826, 75225 Pnbi/5, MW Ipl, MA - Low triangularity: 1.7 MA / 2T - High triangularity: 0.8MA/1.1 T (79635), 1.3MA/1.7 T (75590), 1.7MA/2.3T (77922) - NTMs: 74641 (weak 3/2, 4/3, 2/1), 74634 (weak 2/1), 74637 (4/3, 5/4 during last half of selected t), others are NTM-free during selected time interval n=3 Wdia, MJ n=2

4. Simulation framework • Four-field simulations: Te, Ti, nD and toroidal rotation velocity (GLF23, NCLASS) (ASTRA) • Prescribed (CX) impurity density and q profile (EFIT with MSE + polarimetry). Prescribed pedestal for Te, Ti, nD, Vtor. • NBI heat, particle and momentum sources from TRANSP • Wall particle source: iterative TRANSP-EDGE2D simulations TRANSP (core region: NBI CX losses, D wall source, p) EDGE2D simulations by Paula Belo JET 77922 JET 77922, 7.95 s Heat & NBI D fluxes actual gas puff SNBI (NUBEAM) D neutral influx Swall (FRANTIC) Neutral influx from EDGE2D at 7.95s D source/1019, 1/m3/s Gas puff / 1022, 1/s EDGE2D (pedestal-SOL region: match of pedestal Te, Ti, ne and divertor measurements)  Time, s

5. Four-field simulations of low & medium density JET hybrid discharges GLF23/ASTRA (curves), TRANSP data fit (symbols)    • ExB shear stabilisation in GLF23: net = max - EExB (0.5 < E <1.5) [Waltz et al, PoP 1997] • E determined in gyrofluid & gyrokinetic turbulence simulations (large range depending on physics assumptions and plasma conditions) • E adjusted to match experiments: E =1.37 (DIII-D), E =1 (JET high N, Te and Ti modelling) • for discharges selected here: •  low ne & low Vtor: E 0.5 (weak ExB shear stabilisation • low ne & high Vtor: E 1 (strong ExB shear stabilisation) • medium density & high Vtor: E 0.6 (weak ExB shear stabilisation)  = Pr·iGLF23 

6. Parametric dependencies of ExB shear stabilisation in electrostatic GLF23: toroidal rotation and Mach number E uncertainty determined by 15% deviation of Te, Ti, nD and Vtor from their measured values • E increases with rotation and Mach number (till M0.4) • Pr=0.3 with small uncertainties in discharges with strong ExB shear stabilisation (Vtor is strongly coupled to other quantities) • Large Pr uncertainty (0.4 - 0.95) in discharges with weak ExB shear stabilisation (low Vtor; medium ne & large Vtor) • ExB shear suppresses thermal transport more efficiently than particle transport in discharges with good core confinement: optimised E ~ 0.5 for particle transport, E ~ 1 - 1.3 for thermal transport

7. Parametric dependencies of ExB shear stabilisation in electrostatic GLF23: ExB and Ti/Te E uncertainty determined by 15% deviation of Te, Ti, nD and Vtor from their measured values • Ti / Te affects the ITG threshold ~ (1 + Ti / Te) in flat density limit • observed correlation may be the consequence of the ExB shear stabilisation • ExB shearing rate estimated using the measured Te, Ti, ne and Vtor • In general, E increases with ExB and then remains constant (as assumed in GLF23)

8. Uncertainty in Prandtl number and E with electrostatic and electromagnetic GLF23 • Lower E in EM GLF23 due to e stabilisation • 75225 (strong ExB shear stabilisation): four simulated quantities are strongly coupled limiting the choice of Pr and E • 77922 (weak ExB shear stabilisation): density and temperatures weakly depend on Vtor, more freedom on the choice of Pr and E • the influence of EM effects on the choice of E is stronger at medium density (77922) E uncertainty determined by 15% deviation of Te, Ti, nD and Vtor from their measured values ES EM

9. EM GLF23: linear e stabilisation and MHD onset with fixed and simulated profiles • e stabilisation in GLF23 is artificially enhanced by changing Ce • Calculation of growth rate at =0.4 using fixed plasma profiles (open symbols): • e stabilisation is important in 77922 and 75225 • 77922 is closer to MHD onset than 75225 • GYRO shows KBM onset at e=0.011 • Stability analysis in self-consistent four-field simulations (closed symbols): • growth rate increases with Ce reflecting confinement improvement • MHD onset in 77922 is close to GYRO KBM onset • strong stiffness with respect to e: Te, Ti, Vtor increase with Ce while density reduces in the ITG region. Opposite behaviour in the MHD-dominant region

10. Projection to ITER hybrid scenario with uncertainties based on JET hybrids • ITER HS with optimised heat mix: 33 MW of NBI + 20 MW of ECCD [Citrin et al NF 2010] • self-consistent simulations of Te, Ti, nD, Vtor, current density, NBI • impurity: Be 2%, Ar 0.12% Stationary profiles obtained for ITER HS with optimised heat mix • Extrapolation based on valided GLF23 settings: • Q=4.2 with Vtor=0 • Q=5.8 with Pr=0.3 E=0.9(JET 75225, low ne) • Q=4.8 with Pr=0.6, E=0.6 (JET 77922, medium ne) • Q increases by 12% due to e stabilisation • Extrapolation based on dimensionless parameters: • M = 0.11 with Pr = 0.3 and E = 0.9  E  0, Q  4 • Ti / Te ~ 1  E  0, Q  4

11. Summary • GLF23 can predict accurately thermal, particle and toroidal momentum (=iGLF23) transport in selected low density JET hybrid discharges assuming that ExB shear stabilisation increases with toroidal rotation or Mach number • ExB shear is less efficient in medium density discharge where H98 is high. Other stabilising effects? • JET-based projection to ITER HS with GLF23 model: toroidal rotation is important for achieving Q > 5 • ExB shear stabilisation used here for improving the GLF23 predictive capability may hide other stabilising effects not included in GLF23

12. Parallel velocity gradient (PVG) turbulence may reduce ExB shear effects • Non-linear fast ion stabilisation: J. Citrin et al, PRL 2013, J. Garcia et al (submitted to JET Pinboard), B. Baiocchi et al, (paper in preparation): toroidal rotation correlates with fast ion pressure in JET NBI heated plasmas • KBM-dominant central region, ITG is dominant around mid-radius: ExB shear stabilisation is efficient in the ITG-dominant domain [S. Moradi et al, paper in preparation]

13. ExB shear stabilisation and PVG driven turbulence • GS2 local non-linear gyro-kinetic electrostatic simulations • Sheared toroidal flow is included using expansion: i/R << M << 1, E ~ VTi/R Highcock et al PRL 2010 (s=0) Highcock et al PRL 2010, PoP 2011 PVG • exponential growth with zero E • transient growth with any nonzero E (if s=0) or above certain E (if s>0) ITG • linear instability is not necessary to sustain nonlinear turbulence in rotating plasmas • transient growth (ITG, PVG) + suitable initial conditions (large-amplitude noise or fully developed turbulence)  subcritical turbulence There is wide range of parameter space where flow shear is large enough to quench the ITG turbulence, but not large enough to drive PVG turbulence

14. ExB shear stabilisation and PVG driven turbulence Both threshold and stiffness are affected by ExB shear [Highcock et al PRL 2010, PoP 2011] Parra et al, PRL 2011: R0/LT threshold at different s and E (left) and torque range where turbulence is suppressed (right) Highcock et al, PRL 2012: effect of q/ E/PVG ~ qR0/r PVG threshold: MLn/L//> 1 [Garbet et al PoP 1999 and refs. therein]