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Positron Source Options for Linear Colliders Proceedings of EPAC 2004, Lucerne, Switzerland POSITRON SOURCE OPTIONS FOR LINEAR COLLIDERS K. Floettmann, DESY, 22607 Hamburg, Germany Abstract development is influenced by many other processes like A common feature of future linear collider designs is Compton scattering, Møller scattering, Bhabha scattering, the requirement of a huge number of particles which have photo-effect. to fit into the acceptance of the damping ring. The requirements are far beyond the current SLC source at rep. # of # of # of SLAC which is the positron source with the highest rate bunches positrons positrons intensity operated up to now. In addition the acceptance [Hz] per pulse per bunch per pulse of the damping rings is in some cases smaller as at the TESLA 5 2820 2·1010 5.6·1013 SLC. NLC 120 192 0.75·1010 1.4·1012 Linear colliders work in a burst mode with a large SLC 120 1 5·1010 5·1010 number of bunches in a burst, repeated at a low frequency Table 1: Parameters of planned and running positron between 5 Hz and 120 Hz. Hence a high number of sources. bunches has to be produced at the repetition frequency. 12 13 A fundamental intensity limit for positron sources is Linear collider require 10 -10 positrons per RF pulse given by thermal stress which is built up in the conversion (Table 1); thermal stress in the target has to be kept under target due to the energy deposition of the particles. control in order to avoid target damage. Besides improvements of conventional positron sources, The heating of the target is dominated by the ionization i.e. sources where an electron beam creates electron losses of electrons and positrons given by Edep ~2 MeV 2 position pairs in an electromagnetic cascade, new cm /g per charged particle. Thus the temperature rise of concepts based on the direct conversion of gamma the target ∆T can be estimated as: radiation offer possibilities for increased particle E ∆= η dep intensities. In these sources the hard gamma radiation has TN2 cA⋅ to be produced either in an undulator or by backscattering of laser light off an electron beam. An additional c = heat capacity advantage of gamma radiation based sources is the A = source area possibility to produce polarized positrons. In order to get N positrons, 2·N·η particles (electrons BASIC CONSIDERATIONS and positrons) have to emerge from the target. The factor η describes the overproduction which is necessary to Positrons have to be produced from photons of some compensate for the limited efficiency of the capture optics MeV energy by means of pair production in the field of a behind the target. The length of the RF pulse of a linear nucleus in a production target. The straight forward collider is too short to allow for significant thermal method to produce the required photons is based on diffusion within a burst; hence N is, without counter- bremsstrahlung from electrons passing through the same measures, the integrated number of positrons required in a target. Other methods utilize undulator radiation from an pulse. The heat load of the target is determined by three electron beam of ≥ 150 GeV or Compton scattering of more or less free parameters: laser photons off an electron beam. In case of - The number of positrons penetrating through the bremsstrahlung a full electromagnetic cascade develops in source area N/A. a rather thick target, while in case of the direct conversion - The efficiency of the capture optics 1/η. of photons a rather thin target is required. - And the heat capacity of the target material c. Bremsstrahlung and pair production are essentially An increased source area counteracts a high capture inverse processes; hence they can be characterized by a efficiency, because the phase-space density of the common parameter, the radiation length X0. It is emerging positrons is reduced, thus a small source area is convenient to measure the target thickness in terms of the favorable. In case of a rather long RF pulse (~ms), as it is radiation length, because for important electromagnetic typical for a superconducting approach (TESLA), the processes (bremsstrahlung, pair production but also ratio N/A can be improved by a factor of >10 by means of multiple scattering) some or all of the dependence upon a rotating target [1]. The short pulses of normal the medium is contained in the radiation length. Roughly conducting approaches (~300 ns) can be distributed onto speaking materials with a high nuclear charge Z have several targets by means of RF deflectors as proposed for short radiation lengths, while low Z materials have a long the conventional NLC positron source (Figure 1). radiation length. Conventional positron sources require The capture efficiency of the optics behind the target is targets of 4-6 radiation length, while positron sources determined by the emittance of the positron beam and the based on direct photon conversion require targets of only acceptance of the optics. The bottleneck in the acceptance 0.4-0.5 radiation length. is however not the optics behind the target but the Besides bremsstrahlung and pair production the shower damping ring. Since linear collider require very small 69 Proceedings of EPAC 2004, Lucerne, Switzerland emittances to reach their luminosity goals the damping (bremsstrahlung and pair production) scales with the rings have in general a relatively small dynamic aperture. radiation length, while the dominant energy loss In order to increase the acceptance for the positron mechanism (ionization loss) scales with the density of the production a predamping ring, with a large dynamic material. The escape depth SEscape from which a positron acceptance and intermediate equilibrium emittance is of a given energy Epos can reach the target surface is proposed in conjunction with conventional positrons estimated as (ρ = density of the material): sources (NLC, GLC). S E Escape = pos X0 ⋅⋅ρ Edep X0 4 Targets & Capture Optics 2 RF Separator RF Combiner MeVcm E = 2 dep g - 6 GeV e 250 MeV e+ Table 2: Escape depth for 10 MeV positrons for various materials. NLC Positron Target System Layout Material Z SEscape/Xo W 74 0.74 Cu 29 0.39 Figure 1: The NLC positron source with RF separators and combiners. 3 targets are used during normal Ti 22 0.31 operation; the fourth is a spare target. Al 13 0.21 The emittance of the positron beam emerging from the Table 2 collects the escape depth in units of the target is determined by the source size and the beam radiation length for 10 MeV positrons for various divergence. The large divergence of the positrons is materials. When the required target thickness is of similar generated by multiple scattering in the target, while the order as the escape depth for typical particle energies the source size is influenced by both, multiple scattering and positron production efficiency is less depended on the the size of the incoming electron or photon beam. material properties. Multiple scattering plays however a much stronger role in 0.7 the thick target of a conventional positron source than in W the thin target of a source based on the direct conversion 0.6 Cu of photons. Ti 0.5 Al 0.16 0.14 0.4 0.12 0.3 0.1 positron yield 0.2 0.08 0.1 0.06 0 0.04 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.02 target thickness [X 0] number of positrons [arb. units] 0 Figure 3: Positron yield for different materials as function 0 5 10 of the target thickness. The incoming photon beam was in transverse momentum [MeV/c] this example generated by a 250 GeV electron passing Figure 2: Comparison of transverse momenta of a through a 1 m long wiggler of 1.7 T field strength. conventional positron source (6 X0, W, blue) and a thin target driven by undulator photons (0.4 X , Ti, green). Figure 3 shows the positron yield as function of the 0 target thickness for the case of a source based on the As a result of the narrower transverse momentum conversion of undulator photons. At the optimum target distribution (Figure 2) the phase-space density is higher in thickness of 0.4 X0 the yield for a low Z material as Ti is the case of a thin target source and the capture efficiency only about 16% below the yield of an equivalent W is increased by a factor of ~5 compared to a conventional target. (The optimum target thickness is somewhat before source with the same acceptance of the capture optics. the maximum yield, since the capture efficiency decreases Low Z materials have in general a higher heat capacity with target thickness.) Hence low Z materials with high than high Z materials (Dulong Petit Rule). Concerning the heat capacity can be employed in sources based on the heat load low Z materials are hence preferable as target direct conversion of photons, while conventional positron material. The positron production efficiency is however sources require high Z materials. The gain in the heat higher in high Z materials. This is due to the fact that the capacity is about a factor of 4. creation of particles in electromagnetic cascades 70 Proceedings of EPAC 2004, Lucerne, Switzerland Target material The condition for an adiabatic field variation is then given as: A high heat capacity of the target material reduces the temperature rise for a given energy deposition. A locally gP⋅ << 1 increased temperature is accompanied by mechanical eB⋅ stress which may lead to cracks and a destruction of the i target.
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