Transferring knowledge gained for pulsed extraction at the ELISE test facility to ITER-relevant CW extraction

Beam extraction at the large ELISE test facility is currently possible only pulsed, with short extraction phases, so-called extraction blips, of up to 10 s each ≈150 s. Over the past years, a good insight into the physics of this operational mode has been gained for both hydrogen and deuterium operation. The uniformity of the co-extracted electrons was identified as a key issue and it was possible to achieve 1000 s plasma pulses in hydrogen with repetitive extraction blips and an extracted current density of over 90 % of the ITER target value by improving the co-extracted electron symmetry. In deuterium roughly 67 % of the ITER target for the extracted current density has been achieved for long pulses. During such pulses an overall increase of the co-extracted electron current is typically observed between one blip and the next one, even though the electron current is observed to actually decrease during each blip. These opposing effects are explained by different caesium dynamics during the source plasma phase when compared to the beam phase and they were one motivation behind the currently ongoing upgrade of ELISE to a CW extraction system. This update consists of two main hardware changes: i) installation of a new CW high voltage power supply and ii) installation of a CW beam calorimeter. Being able to achieve beam pulses of up to 1 hour will allow knowledge to be gained on the physics of caesium redistribution and conditioning over the long timescales needed for ITER operation.