ILC undulator based positron source, tests and simulations

Abstract

Abstract. An undulator based positron source allows generation of polarized positrons in quantities required by ILC. Here we describe the results of modeling and testing of elements for such a system. Classification 3: Linear Colliders, Lepton Accelerators and New Acceleration Techniques, A03. Work supported by NSF grant PHY-0202078 INTRODUCTIONThe scheme for polarized positron production was proposed a long time ago in a framework of VLEPP project [1]. The basis of the method is a two stage process, where at first stage the circularly polarized photons generated in helical electromagnetic field and then, at second stage, these photons converted into positrons and/or electrons in a thin (~half radiation length) target. Secondary particles carry longitudinal polarization transferred from the primary photon beam in accordance with theirs energy. In this first publication [1] the gammas considered to be generated by energetic particle in the following substances: in a field of electromagnetic wave, in static magnetic helical field of undulator and in crystals with helical dislocations (helical crystals). In [2] the laser radiation was considered as a specific example of an electromagnetic wave. With application of selection of energetic positrons only, the final polarization increased and defined by the length of undulator (as one needs to compensate partial collection of secondary positrons). For typical length of undulator~175 m, the degree of polarization reaches 60% and it could reach 85% with 300 m long undulator. One peculiarity associated with helical undulator scheme is that this system is able to generate polarized positrons with degree of polarization ~30% if no energy selection mechanism applied to the positrons at all. The undulator scheme of positron production has been chosen as a baseline for ILC [3] accommodated from TESLA design [4]. One peculiarity here is that the beam, like in original VLEPP scheme is going through the undulator on its way to IP. One positive moment of this is that the beam can be made having small transverse dimensions as the emittance is small. This allows small aperture in undulator and hence makes engineering problem less severe. From the other hand nonlinear field of undulator could disturb this tiny emittance and polarization of this primary beam while it is going to IP. Considerations show that this is not a problem here however as the beam trajectory remains line-type with accuracy ~1/ γ so the nonlinearities cancel each other. So the ILC scheme looks as it is represented in Fig.1

On Generation of Photons Carrying Orbital Angular Momentum in the Helical Undulator

“…Production of polarized positrons with higher undulator harmonics also needs revision. In the positron production scheme with undulator for ILC, the initial suggestion was to operate at low K-factor, K<0.4 [29], where the content of higher harmonics is minimal (even at K~0.7 only 50% of radiation is carried by higher harmonics). Meanwhile the baseline of ILC deals with undulator having K~0.92 [32] where content of higher harmonics is dominating.…”

Feasibility and conceptual design of a C.W. positron source at CEBAF

“…The future International Linear Collider (ILC) will require of order 10 14 e + /s to reach the desired luminosity requirements. For this current, a baseline design has been studied by using an undulator based positron source [43,44], which is illustrated in Fig. 24.…”

“…With B 0 = 1 T, harmonic number n = 1 and 100 m length undulator with a period of λ u = 0.01 m, the electron beam must be more than 130 (170) GeV to get this required ∼ 10 (20) MeV photon energy [43,44]. The photon beam creating these positrons will be incident on the rim of a titanium alloy composed of 90% titanium with a composition of Ti-6%Al-4%V target wheel, 0.4 radiation lengths thick (14 mm) with 2 m diameter.…”

High Power Polarized Positron Source

“…For ILC polarized positrons are created by gammas generated by the main beam in helical undulator K<1, L~150m, λ u =1cm, (a). We also mentioned the possibility to use (c) for ILC [14].…”