27 October 2005
Last week I discussed the recommendation from Snowmass for the baseline electron source to be used for the linear collider reference design, and today I will briefly discuss the more challenging positron source. One of the consequences of having made the choice of superconducting RF technology for the main linac of the linear collider is that the machine pulse structure is such that more electrons and positrons are required per bunch. In fact, the resulting requirement is for 2 1010particles for 2820 bunches per pulse or half that for 5600 bunches per pulse. This is a quite challenging requirement for positron production, although it does appear possible.
Three candidate schemes for positron production have been proposed: a helical undulator- based source; a conventional source; and a laser Compton based source. All three were presented, discussed and compared at Snowmass. The pluses and minuses of the three schemes make the choice a difficult one; however, after considerable discussion a set of recommendations were made by the end of the workshop. Those recommendations are now under discussion and consideration, and we expect that a final decision for the baseline positron source will be made at the Frascati GDE meeting in December.
At Snowmass, Working Group 3a (chaired by by M. Kuriki, J. Clarke, J. Sheppard and P. Piot) concluded that both the schemes using an undulator and the ones using a conventional sources are mature enough to be considered as candidates for the baseline. In the end, the undulator was recommended primarily because it had more margin of safety to reach the required positron intensity. This analysis resulted in a recommendation that the baseline be a helical undulator-based system, and the conventional source should be made a backup with no further R&D on it proposed. The third scheme, the Compton based source was considered promising and it was recommended that it should become an alternative to the undulator, to be considered after further R&D is performed.
Although there are many possible configurations for an undulator based system, the general arrangement proposed at Snowmass uses a 65 m long helical undulator placed at the end of the ILC electron linac. The accelerated electron beam passes through this undulator at 250 GeV and generates circularly polarized photons that in turn produce electron positron pairs in a thin rotating Ti target. The positrons are collected, accelerated up to 5 GeV and transported to the damping ring. The proposed design yields ~1.5 positrons into the damping ring per electron that pass through the undulator giving some safety margin.
An advantage of the helical undulator over the conventional source is that ~60% polarization can be achieved without a major upgrade, while the path to develop polarized positrons for the conventional source is uncertain. Finally, in order to maintain sufficient availability for the accelerator complex using an undulator source, it is proposed to incorporate a 'keep-alive' beam using a conventional source to allow tuning of the rest of the machine while the undulator is down. I will discuss the alternative Compton based source at another time.