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 inst… Show more
“…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.…”
Section: Analytical Solution and Resultsmentioning
The secondary electron emission process is essential for the optimal operation of a wide range of applications, including fusion reactors, high-energy accelerators, or spacecraft. The process can be influenced and controlled by the use of a magnetic field. An analytical solution is proposed to describe the secondary electron emission process in an oblique magnetic field. It was derived from Monte Carlo simulations. The analytical formula captures the influence of the magnetic field magnitude and tilt, electron emission energy, electron reflection on the surface, and electric field intensity on the secondary emission process. The last two parameters increase the effective emission while the others act the opposite. The electric field effect is equivalent to a reduction of the magnetic field tilt. A very good agreement is shown between the analytical and numerical results for a wide range of parameters. The analytical solution is a convenient tool for the theoretical study and design of magnetically assisted applications, providing realistic input for subsequent simulations.
“…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.…”
Section: Analytical Solution and Resultsmentioning
The secondary electron emission process is essential for the optimal operation of a wide range of applications, including fusion reactors, high-energy accelerators, or spacecraft. The process can be influenced and controlled by the use of a magnetic field. An analytical solution is proposed to describe the secondary electron emission process in an oblique magnetic field. It was derived from Monte Carlo simulations. The analytical formula captures the influence of the magnetic field magnitude and tilt, electron emission energy, electron reflection on the surface, and electric field intensity on the secondary emission process. The last two parameters increase the effective emission while the others act the opposite. The electric field effect is equivalent to a reduction of the magnetic field tilt. A very good agreement is shown between the analytical and numerical results for a wide range of parameters. The analytical solution is a convenient tool for the theoretical study and design of magnetically assisted applications, providing realistic input for subsequent simulations.
“…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.…”
Providing an efficacious plasma facing surface between the extreme plasma heat exhaust and the structural materials of nuclear fusion devices is a major challenge on the road to electricity production by fusion power plants. The performance of solid plasma facing surfaces may become critically reduced over time due to progressing damage accumulation. Liquid metals, however, are now gaining interest in solving the challenge of extreme heat flux hitting the reactor walls. A key advantage of liquid metals is the use of vapour shielding to reduce the plasma exhaust. Here we demonstrate that this phenomenon is oscillatory by nature. The dynamics of a Sn vapour cloud are investigated by exposing liquid Sn targets to H and He plasmas at heat fluxes greater than 5 MW m−2. The observations indicate the presence of a dynamic equilibrium between the plasma and liquid target ruled by recombinatory processes in the plasma, leading to an approximately stable surface temperature.
“…Langmuir probes are widely used to measure various plasma properties [25], and have also previously been employed in Magnum-PSI [26,27].…”
Section: Flush-mounted Langmuir Probementioning
confidence: 99%
“…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.…”
Section: Flush-mounted Langmuir Probementioning
confidence: 99%
“…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 .…”
In the direct vicinity of plasma-facing surfaces, the incident plasma particles interact with surface-recombined neutrals. Remarkably high near-surface plasma pressure losses were observed in the high-flux linear plasma generator Magnum-PSI. Combining the incoherent and coherent Thomson scattering diagnostics, we directly measured particle, momentum and energy fluxes down to 3 mm from the plasma target surface. At the surface, the particle and total heat flux were also measured, using respectively an in-target Langmuir probe and thermographic methods. The near-surface momentum and energy losses scale with density, and amount to at least 50 % and 20 %, respectively, at ne = 8•10 20 m −3 . These losses are attributed to the efficient exchange of charge, momentum and energy between incident plasma and surface-recombined neutrals. In low-temperature plasmas with sufficient density, incident particles go through several cycles of interaction and surface deposition before leaving the plasma, thereby providing an effective alternative dissipation channel to the incident plasma. Parallel plasma parameter profiles exhibit a transition with increasing plasma density. In lowdensity conditions, the plasma temperature is constant and near-surface ion acceleration is observed, attributed to the ambipolar electric field. Conversely, deceleration and plasma cooling are observed in dense conditions. These results are explained by the combined effect of ionneutral friction and electron-ion thermal equilibration in the so-called thermalized collisional pre-sheath. The energy available for ambipolar acceleration is thus reduced, as well as the upstream flow velocity. In the ITER divertor, enhanced near-surface p-n interaction is expected as well, given the overlap in plasma conditions. Including these effects in finite-element scrape-off layer models requires a near-surface resolution smaller than the neutral mean free path. This amounts to 1 mm in Magnum-PSI, and possibly an order of magnitude smaller in ITER.
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