On tokamak plasma rotation without the neutral beam torque


 Elaine Gray
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1 On tokamak plasma rotation without the neutral beam torque Antti Salmi (VTT) With contributions from T. Tala (VTT), C. Fenzi (CEA) and O. Asunta (Aalto)
2 2 Motivation: Toroidal rotation Plasma rotation and rotation shear have several known beneficial effects on plasma performance The level of turbulence is reduced resulting in improved confinement Resistive Wall Modes (RWM) are stabilised and thus locked modes and disruptions can be avoided NTMs and sawteeth can be mitigated by rotation Rotation helps to lower Hmode threshold and improve ITBs
3 3 Inducing rotation In current tokamaks the main source of rotation comes from NBI Toroidal rotation values of the order of 200 km/s are regularly achieved on JET (~10 4 revolutions/second) In ITER and in future power plants this is no longer possible and other means of rotation generation are needed Some small level of momentum can be injected also via asymmetric ICRH JET from above
4 4 Lack of external torque in ITER Moment of inertia of a ring:
5 5 Is there rotation without NBI?
6 6 Need to study the typically subdominant sources and transport
7 7 DIIID balanced NBI experiments Using balanced NBI it is possible to zero the rotation and obtain information of the residual stress / intrinsic torque W.M. Solomon et al 2009 NF
8 8 Scaling law for intrinsic rotation ( Rice scaling ) The extrapolation to ITER predicts rotation velocities in excess of 300 km/s There are several known deficiencies in the way the database has been set up in (some data taken in the core some at the mid radius etc.) and it is being revised to improve its predictive capabilities J. Rice et al 2007 Nucl. Fusion 47
9 9 Intrinsic Toroidal and Poloidal rotation Database An EDFA task for 2012 to achieve predictive capabilities of the plasma rotation profile Participating tokamaks include TCV, JET, AUG and TS The database is constructed of Ohmic or RF heated plasmas with full profiles instead of single points somewhere Also Meta data such as ripple is included to make it possible to understand the differences between shots and to identify the key physics that govern the rotation F. Nave PRL105, (2010)
10 10 Tore Supra experiments to study ripple induced torque/rotation V=13 m 3 max=0.5% V=28 m 3 max=5.5% Tore Supra is a large tokamak with circular cross section and a large TF ripple (up to 5.5%) No NBI heating (Ohmic plasmas) Ripple scan by adjusting the size of the plasma (q95 fixed) EFDA task in WP11/WP12 to study the torque sources due to ripple using the ASCOT code Torque due to thermal ion ripple torque Torque due to the fast ions from the diagnostics NB
11 11 Tore Supra Experiments to study ripple induced torque/rotation Experimental measurements show that the toroidal rotation becomes more negative with larger ripple Doppler reflectometry is used for these plasma to measure independently the magnitude of the radial electric field ASCOT calculations using the experimental data as input to evaluate the ripple induced torque both from the thermal ions and from the diagnostics NB C. Fenzi
12 12 Tore Supra Experiments to study ripple induced torque/rotation The torque from the diagnostics NB is cocurrent and of the same order of magnitude as the counter current torque from the thermal ions. Both NB ripple torque and thermal ion ripple torque become more negative (counter current) as the level of ripple is increased Thermal ion torque is strongly edge localised (strong density gradient) =0.5%)
13 13 Tore Supra Experiments to study ripple induced torque/rotation The calculated total torques and their profiles are in qualitative agreement with the observed toroidal rotation Negative rotation at the separatrix is a signature of a negative total torque. The positive rotation in the core of the plasma agrees with the positive torque from the diagnostics NB Caveat: thermal ripple torque is sensitive to the radial electric field used
14 14 Rotation changes with added ECRH (AUG) When adding sufficient ECRF power near the magnetic axis in NBI heated AUG discharges the initially peaked toroidal rotation becomes flat or sometimes hollow It is likely that the addition of ECRH changes the underlying turbulence through the modified plasma profiles causing different transport (to yield outward pinch) and also to generate a counter current torque Similar observations also found on e.g. DIIID R. McDermott et al. PPCF 11
15 15 Rotation in ICRH plasmas (JET) L.G. Eriksson et al. PPCF 09 In JET negative rotation has been observed in the core plasma with ICRF heating Under certain assumptions for the transport they can be explained via fast ion losses but similarly as with ECRH a modified transport remains a possibility Low Ip
16 16 The effect of the momentum transport (pinch velocity) Case ITER: taking the NBI torque as the only source for momentum input the effect of transport assumptions is highlighted Torque from ASCOT T i (and χ i ) from GLF23 With sufficient level of pinch quite a reasonable level of rotation is possible with the weak NBI torque alone v pinch 5m/s v pinch 2m/s v pinch 7m/s v pinch 0 T.Tala et al. IAEA 2008
17 17 Conclusions The intrinsic toroidal rotation is under active research (since we still cannot predict the rotation in current experiments let alone ITER) Experimental databases are refined for better and more comprehensive scaling laws Turbulence codes such as GS2, GEM, ELMFIRE, are getting nearer to the ability to explain experiments and to provide understanding of the underlying processes governing the transport of momentum, energy and particles The improving diagnostics capabilities allow better input to codes for quantitative benchmarking of the theory e.g. ASCOT calculated thermal ion ripple torque agrees well qualitatively but uncertainties in transport do not yet allow quantitative benchmarking with errorbars The development of theory and more realistic simulations together with experiments will help to achieve physics understanding and predictive modelling capabilities for ITER and beyond
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