The test facility ELISE represents an important step in the European R&D roadmap towards the neutral beam injection (NBI) systems at ITER. ELISE provides early experience with operation of large radio frequency (RF) driven negative hydrogen ion sources. Starting with first plasma pulses in March 2013, ELISE has meanwhile demonstrated stable 1 h plasma discharges with repetitive 10 s beam extraction pulses every 3 min in hydrogen and deuterium at the ITER required pressure of 0.3 Pa. Stable ion currents of 9.3 A and 5.8 A have been extracted using only one quarter of the available RF power and reducing the extraction voltage in order to control the co-extracted electrons. The best hydrogen pulse for the required 1000 s for hydrogen gave an extracted current of 21.4 A and resulted in an accelerated current of 17.9 A, using only 53 kW per driver. Linear scaling towards full RF power (90 kW/driver) predicts that the target value of the negative ion current (H‾: 33 A extracted, 23 A accelerated; D‾: 28 A extracted and 20 A accelerated) can be achieved or even exceeded. Issues in long pulse operation are the caesium dynamics and the stability of the co-extracted electron current, for which the caesium management and the magnetic field configuration are promising tools for optimisation. Operation at high RF power for long pulses has highest priority for the next experimental campaign. In parallel or in a later stage, ELISE could serve as a test bed for studies on a DEMO NBI system. Examples are concepts concerning RF efficiency, operation with largely reduced caesium consumption or with caesium alternatives, and neutralization of the accelerated ion beam by a laser neutralizer in order to improve efficiency and reliability of NBI systems. Lab scale experiments on these topics are carried out presently in parallel with ELISE operation.
Caesium (Cs) is applied in high power negative hydrogen ion sources to reduce a converter surface's work function and thus enabling an efficient negative ion surface formation. Inherent drawbacks with the usage of this reactive alkali metal motivate the search for Cs-free alternative materials for neutral beam injection systems in fusion research. In view of a future DEMOnstration power plant, a suitable material should provide a high negative ion formation efficiency and comply with the RAMI issues of the system: reliability, availability, maintainability, inspectability. Promising candidates, like low work function materials (molybdenum doped with lanthanum (MoLa) and LaB 6 ), as well as different non-doped and boron-doped diamond (BDD) samples were investigated in this context at identical and ion source relevant parameters at the laboratory experiment HOMER. Negative ion densities were measured above the samples by means of laser photodetachment and compared with two reference cases: pure negative ion volume formation with negative ion densities of about 1×10 15 m −3 and the effect of H − surface production using an in-situ caesiated stainless steel sample which yields 2.5 times higher densities. Compared to pure volume production, none of the diamond samples did exhibit a measurable increase in H − densities, while showing clear indications of plasma-induced erosion. In contrast, both MoLa and LaB 6 produced systematically higher densities (MoLa: ×1.60; LaB 6 : ×1.43). The difference to caesiation can be attributed to the higher work functions of MoLa and LaB 6 which are expected to be about 3 eV for both compared to 2.1 eV of a caesiated surface.
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