Effect of tungsten off-axis accumulation on neutral beam deposition in JET rotating plasmas

Abstract

Evidence for low field side accumulation of tungsten is often observed in bolometry and soft x-ray emissivities of highly rotating JET ITER-like wall (ILW) plasmas. Poloidal variation of the density of high-Z impurities, such as tungsten, in the core of NBI heated plasmas is expected from neoclassical theory due to charge displacement and parallel electric field generated by the centrifugal force. We calculate the poloidally asymmetric distribution of tungsten using fluid equations and a 1D transport simulation with the JETTO/SANCO code. Peaking of tungsten on the outboard side of the plasma is found and verified with soft x-ray and bolometry measurements. We then study the effect of a poloidally asymmetric tungsten distribution on the distribution of the NBI heat source by simulations with the Monte Carlo code ASCOT. The simulations show that the poloidally asymmetric tungsten profile redistributes the fast NBI ions radially through shifting their ionization profile and poloidally through enhanced pitch-angle scattering at high energy. The amplitude of the redistribution is in the order of 10% for the highest n W /n e ratios of ∼10 −4 measured in recent JET H-mode plasmas. As a result of the scattering of the beam particles, the core heat deposition is changed less than 10%, which does not have a significant impact to the performance of JET plasmas. The modelling is in qualitative agreement with measurements by the vertical neutron camera that sees a broadening in the 2.5 MeV neutron profile when tungsten peaks on the outboard side of the plasma.

Local profiles of line emission of impurity ions in rotating fusion plasmas

“…For accurate inversion, the contribution of the toroidal rotation on the poloidal location represented by the coordinates (ψ, θ) is required. This transformation was performed by Koskela et al [6] where they assumed a trace amount of impurity, quasi-neutrality (n e = n i ), and temperature equilibrium (T i = T s ) with no parallel temperature gradient, where the subscripts e and i denote thus, the j th pixel is calculated as N θ (k − 1) + l, where N θ is the total number of θ-grid.…”

“…In the case of heavy impurities such as tungsten, which will have a stronger poloidal asymmetry as observed in JET [6,31] and ASDEX-U [5,32] with a core toroidal rotation speed of under 200 km s −1 , the investigation of its poloidal distribution of each charge state is important for analyzing the transport phenomena. This is particularly true in the high collisionality regime where the neoclassical inward convection is dominant, which should be avoided [3].…”

“…Radiation images emitted by heavy impurities in a rotating fusion plasma exhibit a poloidal asymmetry, primarily due to the centrifugal force [1] when the plasma Mach number of the species is of order one [2]. This characteristic has been explored through a number of studies, especially in JET and ASDEX-U [3][4][5][6][7] equipped with a tungsten divertor, with the help of the tomographic reconstruction of soft x-ray measurements and bolometers having multiple overlapped viewing chords on the poloidal cross-section. These tomographic methods deal with the total density of an impurity species.…”

Control of power, torque, and instability drive using in-shot variable neutral beam energy in tokamaks

“…Developing this capability would provide similar benefits and improved plasma control for other magnetic confinement fusion facilities. tungsten impurity density affecting beam deposition in JET [5]; using increased neutral beam heating to enable access to new plasma parameters regimes in the TCV tokamak [6]; and developing the capability to attach neutral beams to the complicated geometry of the Wendelstein 7-X stellarator [7]. Running a tokamak neutral beam system with time-variable energy is counter-intuitive since it will generally result in a reduction of the total power deposited into the plasma.…”

“…Neutral beams are a major auxiliary heating and current drive system for present tokamaks, and the system being built for ITER will be responsible for providing an injected power of 34 MW [3]. A sampling of the most recent worldwide effort to advance neutral beam technology and its application in magnetic confinement fusion includes: the creation of a new, detailed simulation code for the specific ITER beams [4]; modeling and experimental comparison of edge Control of power, torque, and instability drive using in-shot variable neutral beam energy in tokamaks tungsten impurity density affecting beam deposition in JET [5]; using increased neutral beam heating to enable access to new plasma parameters regimes in the TCV tokamak [6]; and developing the capability to attach neutral beams to the complicated geometry of the Wendelstein 7-X stellarator [7]. Running a tokamak neutral beam system with time-variable energy is counter-intuitive since it will generally result in a reduction of the total power deposited into the plasma.…”