Paving the road towards ITER relevant long deuterium pulses at ELISE by investigating improved operational scenarios

Hydrogen plasma pulses of 1000 s with an extracted current density of over 90 % of the ITER target value (while also fulfilling the requirements for the electron-ion ratio and the global top/bottom beam homogeneity) were demonstrated at the ELISE test facility with its half ITER-size negative hydrogen ion source in pulsed extraction mode, i.e. short beam extraction phases of approximate to 10 s each approximate to 150 s, and using the so-called potential rods. In deuterium roughly 67 % of the ITER target for the extracted current density has been achieved for pulses longer than 45 minutes, limited by a pronounced vertical asymmetry and increase with time in the co-extracted electron current.In order to stabilize the co-extracted electron current during long pulses, particularly in deuterium, improved operational scenarios have to be developed. At ELISE, it was investigated the possibility to remove the potential rods and instead to set the bias plate to a positive voltage with respect to the source vessel. In this operational scenario short pulses in deuterium can be achieved with an extracted negative ion current density of up to 90 % of the ITER target value.In order to improve the physics insight, a 2D fluid code is applied. The first general result is that the modified potentials of surfaces close to the extraction system strongly affect the fluxes of charged particles towards different surfaces and consequently can result in a reduction of the co-extracted electron current.The temporal behaviour during long pulses with pulsed extraction, in particular of the co extracted electrons, demonstrates a significant impact of the beam extraction on the caesium dynamics. Thus, CW extraction is mandatory for developing fully ITER relevant operational scenarios. After an extensive upgrade, ELISE is now capable of performing CW pulses, i.e. long-pulse beam extraction.