A New Configuration for Two-beam Linear Colliders

A promising approach to multi-TeV linear colliders is the Two-Beam approach being pursued at CERN[1,2]. In this technique the energy for acceleration is provided by a drive beam that is accelerated efficiently by a fully loaded, low frequency linac. To achieve the correct pulse structure, the beam is compressed in delay rings before delivery to the linac where it is decelerated. The resulting high-frequency RF power is transferred to the main linac accelerator structures to accelerate the primary beams. In this paper we discuss the application of this technique to lower frequency, X-band, and we discuss a new configuration for the generation of the drive beam. We show how the RF power pulse for the drive beam can be extended to be used to accelerate the main beam up to the injection energy of 8 GeV. We also show a recirculating configuration for the drive beam that can substantially reduce the number of RF sources necessary for the Linear Collider. 1 TWO-BEAM ACCELERATION The basic concept of two-beam acceleration is rather simple. In Figure 1 you see it illustrated with parameters relevant to this paper. A high-current beam, tightly bunched at the operating frequency or a sub harmonic, is decelerated by a low-impedance decelerator structure. The resulting output RF is transferred in a waveguide to a high-gradient accelerating structure where it is used to accelerate the low-current, high-energy beam. In the example shown the drive beam is decelerated by 1.5 MeV each meter while the main beam is accelerated by 93 MeV each meter. The combination of decelerating and accelerating structures acts like a transformer moving the energy stored in the drive beam to the main beam. Depending on the final desired energy, the drive beam must be replaced after it loses about 90% of its energy to maintain its stability during deceleration[2]. Accelerator Accelerator Accelerator Accelerator 760 MW 760 MW MAIN LINAC Drive Beam Deceleration (190 A, 1.3 GeV 1.5 MV/m) Main Beam Acceleration (0.8 A, 8 GeV + 93 MV/m) DRIVE LINAC Decelerator Decelerator Figure 1: The Basic Concept of Two-Beam Acceleration The creation of the drive beam is a key problem for two-beam schemes because it must be done efficiently with reliable technology. Recently, a new technique was introduced which uses a low frequency normal conducting linac to accelerate the drive beam. After acceleration the drive beam pulse structure is modified using rings that serve to compress the energy into pulses appropriate for RF production at high frequency. This technique is described in Refs. 1 and 2. Table 1 An 11.4 GHz Two-Beam Linear Collider Parameter Units 1⁄2 TeV 1 TeV Energy TeV 2 Eb 0.5 1.0 Luminosity 10cms L 1.4 1.4 Beamstrahlung % δb 3.2 8.3 Beamstrahlung Υ 0.11 0.32 Photons/electron nγ 0.86 1.12 Linac rate Hz fr 120 60 Particles/bunch 10e Ne 7 7 Bunches/pulse nb 248 248 Bunch spacing nsec ∆b 1.4 1.4 Emittances 10m-r γεx/y 400/6.4 400/6.5 Beta functions mm βx/y 12/.12 12/.12 RMS size nm σx/y 310/4.0 220/2.8 Bunch length μm σz 90 90 Enhancement HD 1.46 1.46 Beam power MW Pb 8 8 Main Linac: RF frequency GHz f 11.4 11.4 Loaded Grad. MV/m G 93 93 Two linac length km lT 6.5 13 RF power/Acc. Mwatts Ps 380 380 RF pulse length μsec ∆K 0.46 0.46 # Drive Beams ND 3 6 Frequency Mult. 1 8 8 Compression 16 16 Drive Beam Freq. MHz 1428 1428 # of klystrons NK 80 80 Klystron power Mwatts PK 50 50 Klystron Pulse μsec TK 185 360 AC to RF eff. % ηRF 40 40 RF to beam eff. % ηRF-B 28 28 AC to beam eff. % ηAC-B 10 10 Wall AC for RF MW PAC 160 160 In this paper we introduce two new aspects to the drive beam creation process and the concept of a two-beam linear collider. First, the drive beam is accelerated in multiple passes through the drive beam linac. XX International Linac Conference, Monterey, California