Franz-Josef Decker

Energy doubling of 42 GeV electrons in a metre-scale plasma wakefield accelerator

The energy frontier of particle physics is several trillion electron volts, but colliders capable of reaching this regime (such as the Large Hadron Collider and the International Linear Collider) are costly and time-consuming to build; it is therefore important to explore new methods of accelerating particles to high energies. Plasma-based accelerators are particularly attractive because they are capable of producing accelerating fields that are orders of magnitude larger than those used in conventional colliders. In these accelerators, a drive beam (either laser or particle) produces a plasma wave (wakefield) that accelerates charged particles. The ultimate utility of plasma accelerators will depend on sustaining ultrahigh accelerating fields over a substantial length to achieve a significant energy gain. Here we show that an energy gain of more than 42u2009GeV is achieved in a plasma wakefield accelerator of 85u2009cm length, driven by a 42u2009GeV electron beam at the Stanford Linear Accelerator Center (SLAC). The results are in excellent agreement with the predictions of three-dimensional particle-in-cell simulations. Most of the beam electrons lose energy to the plasma wave, but some electrons in the back of the same beam pulse are accelerated with a field of ∼52u2009GVu2009m-1. This effectively doubles their energy, producing the energy gain of the 3-km-long SLAC accelerator in less than a metre for a small fraction of the electrons in the injected bunch. This is an important step towards demonstrating the viability of plasma accelerators for high-energy physics applications.

E-157: A 1.4-m-long plasma wake field acceleration experiment using a 30 GeV electron beam from the Stanford Linear Accelerator Center Linac

In the E-157 experiment now being conducted at the Stanford Linear Accelerator Center, a 30 GeV electron beam of 2×1010 electrons in a 0.65-mm-long bunch is propagated through a 1.4-m-long lithium plasma of density up to 2×1014u200ae−/cm3. The initial beam density is greater than the plasma density, and the head of the bunch expels the plasma electrons leaving behind a uniform ion channel with transverse focusing fields of up to several thousand tesla per meter. The initial transverse beam size with σ=50–100u200aμm is larger than the matched size of 5 μm resulting in up to three beam envelope oscillations within the plasma. Time integrated optical transition radiation is used to study the transverse beam profile immediately before and after the plasma and to characterize the transverse beam dynamics as a function of plasma density. The head of the bunch deposits energy into plasma wakes, resulting in longitudinal accelerating fields which are witnessed by the tail of the same bunch. A time-resolved Cherenkov imag...

Beam Matching to a Plasma Wake Field Accelerator using a Ramped Density Profile at the Plasma Boundary

An important aspect of plasma wake field accelerators (PWFA) is stable propagation of the drive beam. In the under dense plasma regime, the drive beam creates an ion channel which acts on the beam as a strong thick focusing lens. The ion channel causes the beam to undergo multiple betatron oscillations along the length of the plasma. There are several advantages if the beam size can be matched to a constant radius. First, simulations have shown that instabilities such as hosing are reduced when the beam is matched [1]. Second, synchrotron radiation losses are minimized when the beam is matched. Third, an initially matched beam will propagate with no significant change in beam size in spite of large energy loss or gain. Coupling to the plasma with a matched radius can be difficult in some cases. This paper shows how an appropriate density ramp at the plasma entrance can be useful for achieving a matched beam. Additionally, the density ramp is helpful in bringing a misaligned trailing beam onto the drive beam axis. A plasma source with boundary profiles useful for matching has been created for the E-164X PWFA experiments at SLAC.

Flat beams in the SLC

The Stanford Linear Collider was designed to operate with round beams; horizontal and vertical emittance made equal in the damping rings. The main motivation was to facilitate the optical matching through beam lines with strong coupling elements like the solenoid spin rotator magnets and the SLC arcs. Tests in 1992 showed that flat beams with a vertical to horizontal emittance ratio of around 1/10 can be successfully delivered to the end of the linac. Techniques developed to measure and control the coupling of the SLC arcs allow these beams to be transported to the Interaction Point (IP). Before flat beams could be used for collisions with polarized electrons, a new method of rotating the electron spin orientation with vertical arc orbit bumps [4] had to be developed Early in the 1993 run, the SLC was switched to flat beam operation. Within a short time the peak luminosity of the previous running cycle was reached and then surpassed. The average daily luminosity is now a factor of about two higher than the best achieved last year. In the following we present an overview of the problems encountered and their solutions for different parts of the SLC.<<ETX>>

Boundary effects: Refraction of a particle beam

The refraction of light at an interface is familiar as a rainbow or the bending of a pencil in a glass of water. Here we show that particles can also be refracted and even totally internally reflected, as evidenced by an electron beam of 28.5 × 109 electron volts being deflected by more than a milli-radian upon exiting a passive boundary between a plasma and a gas — the electron beam is bent away from the normal to the interface, just like light leaving a medium of higher refractive index. This phenomenon could lead to the replacement of magnetic kickers by fast optical kickers in particle accelerators, for example, or to compact magnet-less storage rings in which beams are guided by plasma fibre optics.

Energy Measurements of Trapped Electrons from a Plasma Wakefield Accelerator

Recent electron beam driven plasma wakefield accelerator experiments carried out at SLAC indicate trapping of plasma electrons. More charge came out of than went into the plasma. Most of this extra charge had energies at or below the 10 MeV level. In addition, there were trapped electron streaks that extended from a few GeV to tens of GeV, and there were mono-energetic trapped electron bunches with tens of GeV in energy.

Proposal for a one GeV plasma wakefield acceleration experiment at SLAC

Abstract A plasma-based wakefield acceleration experiment E-157 has been approved at SLAC to study acceleration of parts of an SLC bunch by up to 1xa0GeV/m over a length of 1xa0m. A single SLC bunch is used to both induce wakefields in the 1xa0m long plasma and to witness the resulting beam acceleration. The experiment will explore and further develop the techniques that are needed to apply high-gradient plasma wakefield acceleration to large-scale accelerators. The 1xa0m length of the experiment is about two orders of magnitude larger than for other high gradient plasma wakefield acceleration experiments and the 1xa0GeV/m accelerating gradient is roughly ten times larger than that achieved with conventional metallic structures. Using existing SLAC facilities, the experiment will study high gradient acceleration at the forefront of advanced accelerator research.

Test of the electron hose instability in the E157 experiment

The E157 experiment is designed to demonstrate high gradient plasma wake field acceleration over a significant length. It has been suggested that the electron hose instability of the drive beam will degrade the performance of this experiment because the hosing tail electrons will not fully sample the highest acceleration field. In this paper a parasitic experiment designed to test the extent of the hosing instability is described. In particular, we discuss how the initial beam conditions are determined so that the extent to which any transverse perturbations grow due to hosing can be determined.

High order mode heating observations in the PEP-II interaction region

High order mode (HOM) heating is observed in the PEP-II interaction region vacuum system by monitoring chamber temperatures in several locations. The region where the electron and positron beams enter a common chamber results in a RF cavity where the beams generate HOM heating. By determining the temperature dependence on the beam current, beam bunch length, and the phase of the two beams, information about the modes is extracted. The time response of the temperature and the geometry also aid in determining the source of the heat flow.

Progress toward E-157: a 1 GeV plasma wakefield accelerator

A plasma based wakefield acceleration (PWFA) experiment, scheduled to run this summer, will accelerate parts of a 28.5 GeV bunch from the SLAC linac by up to 1 GeV over a length of 1 meter. A single 28.5 GeV bunch will both induce the wakefields in the one meter long plasma and witness the resulting acceleration fields. The experiment will explore and further develop the techniques that are needed to apply high-gradient PWFA to large scale accelerators. This paper summarizes the goals of the first round of experiments as well as the status of the individual components: construction and diagnosis of the homogeneous lithium oven plasma source and associated ionization laser, commissioning of the electron beam, simulated performance of the electron beam energy measurement, and first PIC simulations of the full meter long experiment.