| OT02 · Modelling nonperturbative frequency sweeping
R. G. L. Vann1,2, R.O. Dendy3,2, M. P. Gryaznevich3, S. E. Sharapov3
1Department of Physics, University of York, Heslington, York YO10 5DD, U.K.
2Department of Physics and Centre for Scientific Computing, University of Warwick,
Coventry CV4 7AL, U.K.
3Euratom/UKAEA Fusion Association, Culham Science Centre,
Abingdon, Oxfordshire OX14 3DB, U.K.
Abstract: High frequency
magnetohydrodynamic (MHD) activity in tokamak plasmas may be driven by
energetic ion populations. The time evolution of this MHD behaviour, notably
frequency sweeping, provides insights into the changing properties of both the
energetic particle population and the ambient plasma. To extract this
information, it is necessary to model the self-consistent nonlinear interaction
of the energetic particles with the MHD modes, incorporating energetic particle
resonance and background damping; this is done by applying a fully nonlinear
self-consistent numerical implementation [2] of the Berk-Breizman augmentation
of the Vlasov-Maxwell system (hereafter “VM(BB)”). Perturbative frequency
sweeping observations [3,4] have already been successfully modelled using the
VM(BB) system. In this paper we report new results on modelling nonperturbative
frequency sweeping. 
Fig. 1 MAST shot 11005 Fig. 2:
VM(BB) simulation Plasma waves driven by energetic
particles are said to be nonperturbative if their dispersion properties are
modified significantly by the energetic particle population [1]. Figure 1 shows
data from the Mega-Amp Spherical Tokamak that is believed to reflect this
effect. In this paper we present self-consistent nonlinear modelling of
nonperturbative frequency sweeping using VM(BB) (Fig. 2). In both Fig. 1 and
Fig. 2 we note that: the sweeping rate is approximately constant until it
begins to die; modes are born at different frequencies; the sweeping is
predominantly in one direction only. The nonperturbative frequency sweeping
shown in Fig. 2 occurs for fixed model parameters, suggesting that Fig. 1 need
not imply evolving plasma properties. In this respect it resembles recent
modelling [4] of perturbative frequency sweeping.
[1] F. Zonca and L. Chen, Phys. Plasmas 3, 323 (1996)
[2] R. G. L. Vann et al., Phys. Plasmas 10, 623 (2003)
[3] S. D. Pinches et al., Plasma Phys. Control. Fusion 46, S47
(2004)
[4] R. G. L. Vann, R. O. Dendy, and M. P. Gryaznevich, Phys. Plasmas 12(3)
(2005)
This work was funded
in part by Euratom and the UK EPSRC.
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