Application of Plasma Lenses As Optical Matching Device for Positron Sources at Linear Colliders

In the baseline design of the International Linear Collider (ILC) an undulator-based positron source is foreseen. The proposed luminosity of the recently chosen first energy stage with √s = 250 GeV requires an improvement by a factor of 2500 to the world’s first linear collider, the past SLC experiment. This ambitious luminosity goal can only be achieved, if all technological limits are being pushed. One such area is the captured positron number, which is primarily determined in the capture section within the positron source and specifically by its optical matching device. It is responsible for transforming the phase-space of the outgoing particles produced in the target for the succeeding accelerator sections. The plasma lens is a new candidate for this task, providing a specifically adequate method due its magnetic field being azimuthal. Optimizing an idealised tapered active plasma lens for the ILC led us to a design with improved captured positron yield, outperforming ILC’s currently proposed quarter wave transformer by approximately 50%. The captured yield also proved to be stable within ±1.2% for deviations in design parameters of ±10%. OPERATION CYCLE OF AN ACTIVE PLASMA LENS An active plasma lens (PL) consists of an open cylindrical cavity (capillary) with ring electrodes around both ends and gas inlets. The capillary’s symmetry axis coincides with the beam axis. In the first step, gas (e.g. H2) is introduced into the capillary. Followed by applying some kV pulsed voltage to the ring electrodes, producing a strong longitudinal electric field inside the capillary. In fact the field is of such magnitude that the neutral gas atoms are ionized and form a ion-electron mixture, the so called plasma. Whereas the ions’ momenta are virtually unchanged as a result of their high rest mass, now the electrons are accelerated in longitudinal direction by the electric field, forming a short-lived electric discharge current of some kA. The discharge current in longitudinal direction in turn generates an azimuthal magnetic field. Now inserting a charged particle bunch leads to a radial force, resulting in a focused bunch exiting the plasma lens. Depending on the discharge current’s life time, which is limited by the high voltage pulse, multiple bunches or even a whole particle pulse can be transformed. Afterwards the electrons and ions recombine and the gas disperses through the capillary openings into the vacuum. This marks the end of the duty cycle and a new one can be initiated. ISSUES OF CONVENTIONAL OPTICAL MATCHING DEVICES The optical matching device (OMD) resides right between the target and the pre-accelerator section and is responsible for matching the phase-space of the produced particles appropriately to the succeeding accelerator sections. This requires the transformation of the initial particles from a highly divergent beam with a small effective cross-section to a wide, parallel one. In the past this problem has been approached by different types of sophisticated coils like the quarter wave transformer (QWT) and flux concentrator (FC). Both have fundamentally a problem with strong dephasing. Also, the QWT and FC also suffer from chromaticity and eddy currents in rotating targets, respectively. The Plasma lens as a new alternative OMD option could have less issues in those three areas.