Tailoring the charged particle fluxes across the target surface of Magnum-PSI

Abstract

Linear plasma generators are plasma devices designed to study fusion-relevant plasma-surface interactions. The first requirement for such devices is to operate with adjustable and well characterized plasma parameters. In the linear plasma device Magnum-PSI, the distribution of the charged particle flux across the target surface can be tailored by the target bias. The process is based on the radial inhomogeneity of the plasma column and it is evidenced by electrical measurements via a 2D multi-probe system installed as target. Typical results are reported for a hydrogen discharge operated at 125 A and confined by a magnetic field strength of 0.95 T in the middle of the coils. The probes were biased in the range of -80 to -30 V, while the floating potential of the target was about -35 V. The results were obtained in steady-state regime of Magnum-PSI, being time-averaged over any type of fluctuations. Depending on the relative value of the target bias voltage with respect to the local floating potential in the plasma column, the entire target surface can be exposed to ion or electron dominated flux, respectively, or it can be divided into two adjacent zones: one exposed to electron flux and the other to ion flux. As a consequence of this effect, a floating conductive surface that interacts with an inhomogeneous plasma is exposed to non-zero local currents despite its overall null current and it is subjected to internal current flows.

Secondary electron emission under magnetic constraint: from Monte Carlo simulations to analytical solution

“…The magnetic field range between magnetrons and tokamaks, , is covered by magnetically confined linear plasma generators (LPG) 54 , 55 . Such devices are specially designed to investigate plasma-surface interactions which are relevant for edge regions of fusion reactors 56 . Whilst the magnetic field is externally imposed, the electric field in front of the surface depends on plasma parameters.…”

Oscillatory vapour shielding of liquid metal walls in nuclear fusion devices

“…Radial currents are naturally occurring in the plasma column of Pilot-PSI as a result of its source potential configuration 16 , 17 . It is clear from this work that there exists a net current carried by electrons in the centre of a floating target while the edge receives a net ion current.…”

Thermalized collisional pre-sheath detected in dense plasma with coherent and incoherent Thomson scattering

“…Langmuir probes are widely used to measure various plasma properties [25], and have also previously been employed in Magnum-PSI [26,27].…”

“…The probe pin is facing the plasma in a 2 mm-diameter hole in the clamping ring and is mounted flush to the surface. Due to radial electric fields from the probe under bias in combination with a finite ion Larmor radius (0.16 mm at 1.8 eV , 1.2 T ), the effective collection area A ef f exceeds its front surface area [26]. In this work, we assumed A ef f = 12.6 mm 2 , equal to the surface of the clamping ring hole.…”

“…Following the Bohm criterion, ion collection does not depend on bias, while the electrons do not reach the surface if it is sufficiently negatively charged. If voltage sweeping is not feasible, for example due to setup limitations or during transients, I sat is often assumed to equal I p at a bias that is large compared to eT e [26]. However, radial electric fields emanating from the probe cause increased radial transport and therewith an increase of A ef f .…”