A CONCEPT OF PLASMA WAKE FIELD ACCELERATION LINEAR COLLIDER (PWFA-LC)* A PWFA-LC CONCEPT

Abstract Plasma Wake-Field Acceleration (PWFA) has demonstrated acceleration gradients above 50 GeV/m. Simulations have shown drive/witness bunch configurations that yield small energy spreads in the accelerated witness bunch and high energy transfer efficiency from the drive bunch to the witness bunch, ranging from 30% for a Gaussian drive bunch to 95% for a shaped longitudinal profile. These results open the opportunity for a linear collider that could be compact, efficient and more cost effective that the present microwave technologies. A concept of a PWFA-based Linear Collider (PWFA-LC) has been developed and is described in this paper. The drive beam generation and distribution, requirements on the plasma cells, and optimization of the interaction region parameters are described in detail. The R&D steps needed for further development of the concept are also outlined. A PWFA-LC CONCEPT The requirements for an electron-positron linear collider in the TeV energy range are well understood from 20 years of conceptual design work based on conventional rf cavity acceleration systems. The high gradients possible with plasma wakefield acceleration may provide a path to a new lower cost approach to achieving these energies. However, a practical collider must also meet the luminosity requirements imposed by the physics goals, which is the order of 10 34 cm -2 s -1 . To achieve this high luminosity requires beams of about ten MW average power, which are low emittance and can be focused to nanometre size for collisions. A practical collider technology must also have high power transfer efficiency into the beam. Several ideas for plasma wakefield-based linear colliders (PWFA-LC) have been suggested in the past. The "afterburner" [1] is an approach that uses short plasma sections to double the energy of a conventional rf linear collider just before the collision point. Each beam is split into pairs of microbunches with the first driving a plasma wake that accelerates the second. Luminosity of the energy-doubled collider is maintained by employing plasma lenses to reduce the spot size before collision. A multiple-stage PWFA-LC concept has been suggested at the 2006 Advanced Accelerator Workshop [2], which is essentially a multi-stage afterburner employing a highcharge beam with multiple bunches and multiple plasma cells to reach high energy. One implementation would use a 100 GeV drive beam and five (four if the incoming witness bunch also has 100 GeV) plasma stages to accelerate the main beam to 500 GeV. The design presented here is an attempt to optimize the advantages of PWFA and conventional linear collider concepts, based on a reasonable set of R&D milestones that could be realized over the next ten years. This approach benefits from the extensive R&D for conventional linear colliders and has relatively relaxed requirements on the plasma acceleration systems while still potentially lowering the cost. These considerations led to a larger number of PWFA stages and imposed specific requirements on the parameters for the main and drive beams. This PWFA-LC concept addresses these requirements, and, in contrast to the approaches discussed above, uses an electron drive beam for both electron and positron main beams. This design will evolve with better understanding of plasma wakefield physics based on future experimental results and simulation studies. Therefore, it is crucial to maintain flexibility in the parameter space for a PWFA linear collider. The design for a PWFA-based Linear Collider is shown schematically in [3] which describes a 10 TeV linear collider design. The proposed plasma wakefield research program at FACET is designed to demonstrate the viability of this concept. This PWFA-LC design uses a conventional 25 GeV electron drive beam accelerator, to produce trains of drive bunches distributed in counter-propagating directions to 20 PWFA cells for both the electron and the positron arms of the collider to reach energy of 500 GeV for each beam. Each cell provides 25 GeV of energy to the main beam in about a meter of plasma. The layout and parameters were chosen to optimize PWFA performance while also providing feasible parameters at the interaction point and a practical design for the main beam injector and the drive beam acceleration and distribution system. The drive beam system is very similar to the CLIC drive beam concept which is being tested at the CTF3 test facility The main beam bunch train consists of 125 bunches, each separated by 4 ns. The drive beam train consists of 20 mini-trains each with 250 bunches separated by 2 ns (as described in details i