Magnetized Electron TRansport in Ion Sources – METRIS

Magnetized plasma transport plays a key role not only in hot fusion plasmas but also in low-temperature plasma sources operating at low pressure, in which magnetic fields are used to limit charged particle losses to the walls and/or obtain special kinds of energy coupling. Such plasma sources are widely used in applications like materials processing, space propulsion, and neutral beam injection, and are generally developed by a combination of experimental research and numerical modeling. However, in contrast to the progress made in fusion plasma research, the knowledge of magnetized transport in low-temperature plasma sources has hardly evolved since the 1950s-1960s and is insufficient to meet the modern needs for modeling. Due to the presence of chamber walls, magnetized low-temperature plasmas can show ill-understood complex behavior even in a non-turbulent regime, while most available experimental data is too application-oriented and not detailed enough for model validation. This problem has become particularly urgent in recent efforts to model the magnetic filter stage of the negative ion source of the neutral beam heating system for ITER.

In this context, the objective of the METRIS project is to improve the general understanding and modeling of transport in magnetized low-temperature plasma sources, in particular negative ion sources for fusion, but with strong benefits for low-temperature plasma modeling in general. In order to achieve this, we will study the magnetized transport as a problem of its own, as much as possible isolated from other aspects of the discharge operation, for different basic magnetic field configurations. We will combine numerical modeling, theoretical analysis, and basic experiments. These different approaches will be developed by a closely-interacting team, working at the same site of LAPLACE, which will naturally lead to efficient synergy between the approaches, while at the same time establishing an intensive collaboration between several young researchers with complementary backgrounds.

The modeling part of the METRIS program concerns the development of robust numerical model schemes for plasma transport in arbitrary magnetic field configuration, as well as simplified analytical models that show the interdependence of the relevant parameters. Our main focus is fluid modeling, which we will back up with more detailed particle-in-cell simulations. As various aspects of this model development are known also in fusion plasma research, we will explore the methods used in that field and profit as much as possible from existing knowledge.

Since it is impossible to clarify the magnetized transport problem by modeling alone, an essential part of the METRIS program consists of basic experiments. In order to avoid the practical constraints of application-oriented plasma source designs, we will build a new dedicated laboratory set-up with flexible magnetic field and simple but detailed diagnostics of the plasma transport. In addition to probe diagnostics and imaging, we will develop and perform innovative space- and time-resolved wall-current measurements, allowing detailed model validation. Our aim is to characterize different transport regimes governed by classical cross field mobility, magnetic drifts, and instabilities/turbulence, with the accent on non-turbulent transport and the onset of turbulence.

The METRIS project can be expected to yield numerous scientific publications in leading international journals and conferences, as well as numerical codes and methods directly exploitable for plasma-based technologies such as negative ion sources for fusion. The project will allow LAPLACE to maintain and reinforce its position among the international leaders in the field of low-temperature plasma simulation.