Thermal Load on the W7-X Vessel from NBI Losses

For the operation of W7-X, as well as other magnetic confinement devices, it is of great importance to predict whether fast particle losses might be concentrated in certain regions to an extent that the plasma vessel or in-vessel components could be damaged. A first assessment of this issue for W7-X had been carried out by [1]. The ANTS code has been developed for the simulation of plasma processes involving the effects from drift motion and particle collisions. In its current form, the code uses a full-f MC approach. As a first application, the thermal load from NBI losses on the inner vessel of W7-X has been investigated. In this paper, an overview of the code structure is given, the numerical approach is outlined and first numerical results are presented. 1. Introduction Neutral beam injection is being considered as a heating source for W7-X. The current design envisions two beamlines consisting of 4 beams each. The complete NBI system would be capable of delivering a total of 8 MW heating power. Using balanced or unbalanced injection, the NBI can be used to drive or compensate a net toroidal plasma current and so adjust the magnetic equilibrium to a desired configuration. The NBI particles are injected at energies of 20keV, 30keV and 60keV. A first assessment of NBI losses and the resulting load on the vacuum vessel in ‘W7-X had been carried out by [1]. In this work, that investigation is repeated with a larger number of particles and using a recent, more accurate representation of the vacuum vessel and in-vessel components, resulting in better spatial resolution. 2. The ANTS code The ANTS code (plasmA simulatioN with drifT and collisionS) is a full-f MC code. It was created based on existing code originally written for the ONSET [2] and EXTENDER [3] packages. In particluar, the code is able to use the entire range of coil types available from the ONSET package in order to describe the external magnetic field. Moreover, the same range of classes for the representation of mesh fields is used. In contrast to the approach used by [1], no flux surface representation of the magnetic field is used inside the plasma domain. Instead, a mesh field is used in the entire domain of computation, and the integration of the particle orbits is carried out in cartesian coordinates. This approach provides the greatest flexibility and will facilitate a faithful treatment of magnetic fields with islands. At this stage of development, the code assumes stellarator symmetry for all its fields. Extensions allowing for fully 3d fields have begun. ANTS reads CAD data using the PISA library written by [7], allowing it to deal with existing descriptions of W7-X available in the ANSYS format. ANTS can be run with different sets of differential equations describing the drift motion of the particles and, at this point, simulates collisions with an arbitrary number of background plasma components. So far, only Maxwellian background plasmas have been implemented. 3. Equilibrium configuration and plasma profiles In this work, NBI losses are calculated for two configurations based on a VMEC