BNL, Upton, Long Island, New York
UCLA, Los Angeles, California
Trains of subpicosecond electron bunches are essential to reach high transformer ratio and high efficiency in compact, beam-driven, plasma-based accelerators. These trains with a correlated energy chirp can also be used in pump-probe experiments driven by FELs. We demonstrate experimentally for the first time that such trains with controllable bunch-to-bunch spacing, bunch length, and charge can be produced using a mask technique. With this simple mask technique, the stability of the bunch train in energy and time is guaranteed by the beam feedback system.
UCLA, Los Angeles, California
USC, Los Angeles, California
BNL, Upton, Long Island, New York
Funding: Work supported by US Department of Energy.
One of the challenges for plasma wakefield accelerators (PWFAs) is to accelerate a trailing bunch with a narrow energy spread. The real challenge is to produce a bunch train with a least one drive bunch and one trailing bunch. We have demonstrated experimentally at the BNL-ATF a mask technique that can produce trains of bunches with variable spacing in the sub-picosecond range*. This 60 MeV train with one to five drive bunches and a trailing bunch propagates in a 1 to 2 cm long plasma capillary discharge with a variable plasma density. When the plasma density is tuned such that the plasma wavelength is equal to the drive bunches spacing the plasma wakefield is resonantly excited. The distance between the last drive bunch and the trailing bunch is one and a half time that between the drive bunches, putting the trailing bunch in the accelerating phase of the wakefield. The resonance is characterized by a maximum energy loss by all the drive bunches and maximum energy gain by the trailing bunch. Experimental results will be presented.
*P. Muggli et al., Phys. Rev. Lett. {10}1, 054801, 2008
UCLA, Los Angeles, California
BNL, Upton, Long Island, New York
Funding: Work supported by US Department of Energy.
At the BNL-ATF we have recently demonstrated the generation of trains of electron with sub-picosecond spacing*. These trains of equidistant bunches can be used to resonantly excite large amplitude wakefields in plasmas. The resonance is reached when the plasma wavelength is equal to the drive bunch train spacing. However, in order accelerate an electron bunch with a narrow energy spread, a trailing witness bunch must be generated. The witness bunch must be separated from the last drive bunch by one and a half time the distance between drive bunches. We show that such a drive/witness bunch train can be generated. The mask can also be designed to produce witness bunches trailing the drive bunch train by 2.5,3. 5, times the drive bunch spacing in order to probe the coherence of the plasma wake in subsequent wave bucket. Resonantly driving plasma wakes with trains of bunches could lead to multiplication of the trailing bunch energy by up to the number of bunches in the drive train with high efficiency in a single stage. Experimental results will be presented.
* P. Muggli et al., Phys. Rev. Lett. {10}1, 054801, 2008
SLAC, Menlo Park, California
UCLA, Los Angeles, California
Duke University, Durham, North Carolina
USC, Los Angeles, California
Funding: Work supported in part by the U.S. Department of Energy under contract number DE-AC02-76SF00515.
Recent progress in generating gradients in the 10's of GV/m range with beam driven plasmas has renewed interest in developing a linear collider based on this technology. This talk will explore possible configurations of such a machine, discuss the key demonstrations and the facilities needed to advance this effort and highlight possible alternative uses of this technology.
SLAC, Menlo Park, California
UCLA, Los Angeles, California
Duke University, Durham, North Carolina
USC, Los Angeles, California
Funding: Work supported by the DOE under contract DE-AC02-76SF00515.
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 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 scheme of the drive beam generation and distribution, requirements on the plasma cells, and optimization of the interaction region parameters are described in detail. The research and development steps, necessary for further development of the concept, are also outlined.
SLAC, Menlo Park, California
UCLA, Los Angeles, California
Duke University, Durham, North Carolina
USC, Los Angeles, California
Funding: Work supported in part by the U.S. Department of Energy under contract number DE-AC02-76SF00515.
An electron beam driven Plasma Wake-Field Accelerator (PWFA) has recently sustained accelerating gradients above 50GeV/m for almost a meter. Future experiments will transition from using a single bunch to both drive and sample the wakefield, to a two bunch configuration that will accelerate a discrete bunch of particles with a narrow energy spread and preserved emittance. The plasma works as an energy transformer to transform high-current, low-energy bunches into relatively lower-current higher-energy bunches. This method is expected to provide high energy transfer efficiency (from 30% up to 95%) from the drive bunch to the accelerated witness bunch. The PWFA has a wide variety of applications and also has the potential to greatly lower the cost of future accelerators. We discuss various possible uses of this technique such as: linac based light sources, injector systems for ring based synchrotron light sources, and for generation of electron beams for high energy electron-hadron colliders.
UCLA, Los Angeles, California
SLAC, Menlo Park, California
Duke University, Durham, North Carolina
USC, Los Angeles, California
Funding: Work supported by DOE under contracts DE-FG03-92ER40727, DE-FG52-06NA26195, DE-FC02-07ER41500, DE-FG02-03ER54721.
Recent Plasma Wake-Field Acceleration (PWFA) experiments at Stanford Linear Accelerator Center has demonstrated electron acceleration from 42GeV to 84GeV in less than one meter long plasma section. The accelerating gradient is above 50GeV/m, which is three orders of magnitude higher than those in current state-of-art RF linac. Further experiments are also planned with the goal of achieving acceleration of a witness bunch with high efficiency and good quality. Such PWFA sections with 25 GeV energy gain will be the building blocks for a staged TeV electron-positron linear collider concept based on PWFA (PWFA-LC). We conduct Particle-In-Cell simulations of these PWFA sections at both the initial and final witness beam energies. Different design options, such as Gaussian and shaped bunch profiles, self-ionized and pre-ionized plasmas, optimal bunch separation and plasma density are explored. Theoretical analysis of the beam-loading* in the blow-out regime of PWFA and simulation results show that highly efficient PWFA stages are possible. The simulation needs, code developments and preliminary simulation results for future collider parameters will be discussed.
*M. Tzoufras et al, Phys. Rev. Lett. {10}1, 145002 (2008).
USC, Los Angeles, California
Funding: Work supported by US Department of Energy
Preservation of the incoming beam emittance is a key characteristic needed for any accelerating system, including the beam-driven, plasma-based accelerator or plasma wakefield accelerator (PWFA). Electron beams with a density larger than the plasma density propagate in a pure and uniform plasma ion column that acts as a focusing element free of geometric aberrations, and the beam emittance is preserved. On the contrary, positron beams attract plasma electrons that flow through the beam and create a non-uniform charge density inside the beam that can exceed the beam density. The resulting plasma focusing force is non-uniform and non-linear. Experimentally, we observe the formation of a beam halo on a screen placed downstream from the plasma. Analysis of the beam images as a function of the plasma density show that the transverse beam size at the screen is strongly reduced in the high emittance plane, and that in the low emittance plane charge is transferred from the beam core to the halo. Numerical simulations of the experiments show the same behavior and indicate that there is emittance growth is both planes. Experimental and simulations will be presented.
USC, Los Angeles, California
UCLA, Los Angeles, California
Duke University, Durham, North Carolina
Funding: Work supported by US Department of Energy.
Plasma Wakefield Accelerator has been proven to be a promising technique to lower the cost of the future high energy colliders by offering orders of magnitude higher gradients than the conventional accelerators. However, it has been shown that ion motion is an important issue to account for in the extreme regime of ultra high intensities and ultra low emittances, characteristics of future high energy colliders. In this regime, the transverse electric field of the beam is so high that the plasma ions cannot be considered immobile at the time scale of electron plasma oscillations, thereby leading to a nonlinear focusing force. Therefore, the transverse emittance of a beam matched to the initial linear focusing will not be preserved under these circumstances. However, Vlasov equation predicts a matching profile even in the nonlinear regime. Furthermore, we extend the idea and introduce a plasma section that can match the entire beam to the mobile-ion regime of plasma. We also find the analytic solution for the optimal matching section. Simulation results will be presented.
USC, Los Angeles, California
UCLA, Los Angeles, California
Funding: Work supported by the US Department of Energy
Studies on propagation of electron beams in plasma have shown that in the blowout regime of the plasma wakefield accelerator (PWFA), the emittance of the incoming beam is preserved because of the linear focusing force exerted by a uniform ion column [1]. However, for positron beams the focusing force is nonlinear and they suffer emittance growth. We simulated the propagation of a positron beam in the uniform plasmas with different densities. We calculated the beam emittance from the simulation results and observed the beam size and emittance grow with increasing plasma density. Simulation results agree well with that of previous work.
USC, Los Angeles, California
UCLA, Los Angeles, California
Duke University, Durham, North Carolina
BNL, Upton, Long Island, New York
Funding: Work supported by US Department of Energy.
Current Filamentation Instability, CFI, (or Weibel instability) is of central importance for relativistic beams in plasmas for the laboratory, ex. fast-igniter concept for inertial confinement fusion, and astrophysics, ex. cosmic jets. Simulations, with the particle-in-cell code QuickPic, with a beam produced by an RF accelerator show the appearance and effects of CFI. The instability is investigated as a function of electron beam parameters (including charge, transverse size and length) and plasma parameters (density and length) by evaluating the filament currents and magnetic fields. We present simulation results, discuss further simulation refinements, suggest criteria and threshold parameters for observing the presence of CFI and outline a potential future experiment.
SLAC, Menlo Park, California
UCLA, Los Angeles, California
USC, Los Angeles, California
High gradient acceleration of electrons has recently been achieved in meter scale plasmas at SLAC. Results from these experiments show that the wakefield is sensitive to parameters in the electron beam which drives it. In the experiment the bunch lengths were varied systematically at constant charge. Here we investigate the correlation of peak beam current to the wake amplitude. The effect of beam head erosion will be discussed and an experimental limit on the transformer ratio set. The results are compared to simulation.