Formation of large negative deuterium ion beams at ELISE

Abstract

Negative ion sources for neutral beam injection in fusion experiments are based on the surface production of Hor Don caesiated low work function surfaces. In the last years it was demonstrated at the large RF driven ion source of the ELISE test facility that the requirements for the ITER NBI systems can be fulfilled in hydrogen. This is a big step towards the first operational period of ITER, planned for up to 2035. However, for the following operational period neutral beam systems working in deuterium are needed. Operation of negative hydrogen ion sources in deuterium is significantly more demanding than in hydrogen: the amount of co-extracted electrons is much higher and their increase during pulses is much more pronounced, limiting the achievable performance. This paper presents results of investigations aimed to improve the insight in the physics related to this isotope effect. Due to the higher atomic mass of deuterium, caesium is removed much more effectively from reservoirs at the walls, resulting in a depletion of these reservoirs and a strongly increased caesium density in the plasma. Additionally, a correlation between the fluxes of charged particles towards the inner ion source surfaces and the co-extracted electrons is identified.

Negative Hydrogen Ion Sources for Fusion: From Plasma Generation to Beam Properties

“…The asymmetry in the co-extracted electron current is acceptable, i.e., the power load onto both segments of the EG is still below its limit. However, this asymmetry drastically increases with the RF power or when changing the isotope from hydrogen to deuterium [42], and it can be a limiting factor for the source performance.…”

“…The operation of negative hydrogen ion sources in deuterium is significantly more demanding than in hydrogen because for comparable source parameters (i.e., RF power, extraction voltage, filling pressure, strength and topology of the magnetic filter, ...) the co-extracted electron current is typically much higher, shows a higher asymmetry, and its increase during pulses is much more pronounced [42]. An isotope effect is also observed in the extracted negative ion current, but less pronounced: in deuterium, the extracted negative ions are reduced only by a few percent compared to hydrogen, and they show a slightly more pronounced decrease with time.…”

“…An isotope effect is also observed in the extracted negative ion current, but less pronounced: in deuterium, the extracted negative ions are reduced only by a few percent compared to hydrogen, and they show a slightly more pronounced decrease with time. Due to the stronger asymmetries in the coextracted electrons in deuterium, the demonstration of the ITER requirements is not possible up to now in deuterium: about 66% of the ITER target for the extracted negative ion current density in deuterium has been achieved at ELISE over 1 h [42].…”

“…Thus, compared to hydrogen, the operational space in deuterium is significantly reduced and defined by the power limit of the extraction grid. Hence, typically, a significantly stronger filter field strength is used in deuterium [42]: up to 4.6 mT close to the PG compared to up to 3 mT in hydrogen.…”

E × B electron drift current across the aperture of an ion source surrounded by a cusped magnetic field profile

“…The question of electron extraction mechanisms from negative ion sources (and in particular of those used for fusion applications [1][2][3][4][5][6]) and in micro-ECR neutralizers for space propulsion [7,8] is currently an open problem. In fusion type plasma sources, a magnetic cusp field (also called suppression field) is used to deflect extracted electrons onto the first grid of an electrostatic accelerator in order to prevent the acceleration to high energies of these particles.…”