P. Krejcik

Simulation of a 50GeV PWFA Stage

The plasma afterburner has been proposed as a possible advanced acceleration scheme for a future linear collider. In this concept, a high energy electron(or positron) drive beam from an existing linac such as the SLC will propagate in a plasma section of density about one order of magnitude lower than the peak beam density. The particle beam generates a strong plasma wave wakefield which has a phase velocity equal to the velocity of the beam and this wakefield can be used to accelerate part of the drive beam or a trailing beam. Several issues such as the efficient transfer of energy and the stable propagation of the particle beam in the plasma are critical to the afterburner concept. We investigate the nonlinear beam‐plasma interactions in such scenario using a new 3D particle‐in‐cell code called QuickPIC. Preliminary simulation results for electron acceleration, beam‐loading and hosing instability will be presented.

Start-to-end simulation of self-amplified spontaneous emission free-electron lasers from the gun through the undulator.

Abstract It is widely appreciated that the performance of self-amplified spontaneous emission free-electron lasers (FELs) depends critically on the properties of the drive beam. In view of this, a multi-laboratory collaboration has explored methods and software tools for integrated simulation of the photoinjector, linear accelerator, bunch compressor, and FEL. Rather than create a single code to handle such a system, our goal has been a robust, generic solution wherein pre-existing simulation codes are used sequentially. We have standardized on the use of Argonne National Laboratorys Self-Describing Data Sets file protocol for transfer of data among codes. The simulation codes used are PARMELA, elegant , and GENESIS. We describe the software methodology and its advantages, then provide examples involving Argonnes Low-Energy Undulator Test Line and Stanford Linear Accelerator Centers Linac Coherent Light Source. We also indicate possible future direction of this work.

Start-to-end simulation for the LCLS X-ray FEL

X-ray FELs, such as the LCLS and TESLA FEL, require electron beams with large peak current and very small emittance. The X-ray peak power, temporal and spectral properties, depend significantly on details of the electron beam phase space distribution. The electron beam distribution is determined by many effects, as the emission process at the gun photo-cathode, bunch compression, acceleration and wakefields within the undulator. Although analytical results can give an estimate of the expected performance, the complexity of the electron beam generation, acceleration and compression can only be evaluated using a numerical simulation of all these processes, a start-to-end simulation. In this presentation we discuss the LCLS X-Ray FEL performance estimated by a start-to-end simulation, and we compare the results with those obtained using a simpler model.

Formation of secondary electron cascades in single-crystalline plasma-deposited diamond upon exposure to femtosecond x-ray pulses

Secondary electron cascades were measured in high purity single-crystalline chemical vapor deposition (CVD) diamond, following exposure to ultrashort hard x-ray pulses (140 fs full width at half ma ...

High-intensity single bunch instability behavior in the new SLC damping ring vacuum chamber

New low-impedance vacuum chambers were installed in the SLC damping rings for the 1994 run after finding a single bunch instability with the old chamber. Although the threshold is lower with the new vacuum chamber, the instability is less severe, and we are now routinely operating at intensities of 4.5/spl times/10/sup 10/ particles per bunch (ppb) compared to 3/spl times/10/sup 10/ ppb in 1993. The vacuum chamber upgrade is described, and measurements of the bunch length, energy spread, and frequency and time domain signatures of the instability are presented.

USING A FAST-GATED CAMERA FOR MEASUREMENTS OF TRANSVERSE BEAM DISTRIBUTIONS AND DAMPING TIMES*

With a fast‐gated camera, synchrotron light was used for studying the transverse beam distributions and damping times in the Stanford Linear Collider (SLC) damping rings. By digitizing the image in the camera signal, the turn‐by‐turn time evolution of the transverse beam distribution was monitored and analyzed. The projections of the digitized image were fit with Gaussian functions to determine the moments of the distribution. Practical applications include the determination of injection matching parameters and the transverse damping times. In this report we describe a synchrotron light monitor and present experimental data obtained in the SLC damping rings.

Commissioning of the SPPS linac bunch compressor

First results and beam measurements are presented for the recently installed linac bunch compressor chicane. The new bunch compressor produces ultra-short electron bunches for the Sub-Picosecond Photon Source (SPPS) and for test beams such as the E164 Plasma Wakefield experiment. This paper will give an overview of the first experiences with tuning and optimizing the compressor together with a description of the beam diagnostics and beam measurements. These measurements form the basis for further detailed study of emittance growth effects such as CSR and wakefields in a previously unmeasured regime of ultra-short bunch lengths.

Correction of the first order beam transport of the SLC Arcs

Correction of the first order transport of the SLC (Stanford Linear Collider) Arcs has been made possible by a technique which allows the full 4*4 transport matrix across any section of Arc to be experimentally determined. By the introduction of small closed bumps into each achromat it is possible to substantially correct first order optical errors, and notably the cross plane coupling at the exit of the Arcs. The careful analysis of the errors of the 4*4 matrix reconstruction allows correction of the Arc R matrix to an accuracy equivalent to 10 mu m random magnet misalignments.<<ETX>>

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.

Waveguide stub-line tuning of RF cavities with heavy beam loading

A method is proposed for using adjustable, waveguide stub-line tuners to match the load impedance of the cavity plus beam to the power source, without the need to detune the cavity. Adjustable stub-line tuners are shown to be able to completely match the resistive and reactive parts of the load impedance to a generator under all conditions of beam loading, ensuring optimum power transfer between generator and cavity. This technique may have advantages in high-current storage rings such as the SLC damping rings and the new PEP II storage rings. The coupled-bunch instability driven by the fundamental mode of the cavity is re-appraised in this coupling scheme in which the cavity is no longer detuned. The consequences of this matching scheme are also considered for the beam loading stability limit.