M. Woodley

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.

Models and simulations

On-line mathematical models have been used successfully for computer controlled operation of SPEAR and PEP. The same model control concept is being implemented for the operation of the LINAC and for the Damping Ring, which will be part of the Stanford Linear Collider (SLC). The purpose of this paper is to describe the general relationships between models, simulations and the control system for any machine at SLAC. The work we have done on the development of the empirical model for the Damping Ring will be presented as an example.

Present status and first results of the final focus beam line at the KEK Accelerator Test Facility

ATF2 is a final-focus test beam line which aims to focus the low emittance beam from the ATF damping ring to a vertical size of about 37 nm and to demonstrate nanometer level beam stability. Several advanced beam diagnostics and feedback tools are used. In December 2008, construction and installation were completed and beam commissioning started, supported by an international team of Asian, European, and U. S. scientists. The present status and first results are described.

Vibration studies of the Stanford Linear Accelerator

Vibration measurements of the linear accelerator structures in the SLC linac show a 1 micron RMS vertical motion. This motion reduces to 0.2 micron RMS motion when the cooling water to the accelerator structures is turned off. The quadrupoles have 250 nanometer RMS vertical motion with the accelerator structure cooling water on and 60 nanometer motion with it off. These results together with measurements of the correlations as a function of frequency between the motions of various components are presented.

Proposal to Modify the Polarimeter Chicane in the ILC 14 mrad Extraction Line

A proposal is presented in this paper to modify the extraction line polarimeter chicane to allow the Compton backscattered electrons to be deflected further from the beam line, and to provide optics for the downstream GAMCAL detector.

Third-order corrections to the SLC final focus

The minimum /spl beta/ achievable at the interaction point (/spl beta/*) with the current design of the SLC final focus is limited to /spl sim/5 mm by third order optical aberrations, most notably the U/sub 1266/ and U/sub 3466/ terms (using the notation of K. Brown). A new lattice is presented which effectively zeros these terms. The remaining third order terms which accrue from the interleaved sextupole pairs in the chromatic correction section (CCS) can be cancelled by the inclusion of five octupoles (two in the CCS, and three in the final telescope). The resulting final focus system is corrected to third order for any usable range of /spl beta/* (given the constraints on the beam divergence at the interaction point). The potential luminosity obtainable from such a system is also presented.<<ETX>>

Analysis of higher order optical aberrations in the SLC final focus using Lie algebra techniques

The SLC final focus system is designed to have an overall demagnification of 30:1, with a /spl beta/ at the interaction point (/spl beta/*) of 5 mm, and an energy band pass of /spl sim/0.4%. Strong sextupole pairs are used to cancel the large chromaticity which accrues primarily from the final triplet. Third-order aberrations limit the performance of the system, the dominating terms being U/sub 1266/ and U/sub 3466/ terms (in the notation of K. Brown). Using Lie algebra techniques, it is possible to analytically calculate the size of these terms, in addition to understanding their origin. Analytical calculations (using Lie algebra packages developed in the Mathematica language) are presented of the bandwidth and minimum spot size as a function of divergence at the interaction point. Comparisons of the analytical results from the Lie algebra maps and the results from particle tracking (TURTLE) are also presented.<<ETX>>

The design for the LCLS RF photoinjector

Abstract We report on the design of the RF photoinjector of the Linac Coherent Light Source. The RF photoinjector is required to produce a single 150 MeV bunch of ∼1 nC and ∼100 A peak current at a repetition rate of 120 Hz with a normalized rms transverse emittance of ∼1 π mm-mrad. The design employs a 1.6-cell S-band RF gun with an optical spot size at the cathode of a radius of ∼1 mm and a pulse duration with an rms sigma of ∼3 ps. The peak RF field at the cathode is 150 MV/m with extraction 57° ahead of the RF peak. A solenoidal field near the cathode allows the compensation of the initial emittance growth by the end of the injection linac. Spatial and temporal shaping of the laser pulse striking the cathode will reduce the compensated emittance even further. Also, to minimize the contribution of the thermal emittance from the cathode surface, while at the same time optimizing the quantum efficiency, the laser wavelength for a Cu cathode should be tunable around 260 nm. Following the injection linac the geometric emittance simply damps linearly with energy growth. PARMELA simulations show that this design will produce the desired normalized emittance, which is about a factor of two lower than has been achieved to date in other systems. In addition to low emittance, we also aim for laser amplitude stability of 1% in the UV and a timing jitter in the electron beam of 0.5 ps rms, which will lead to less than 10% beam intensity fluctuation after the electron bunch is compressed in the main linac.

Beam-based monitoring of the SLC linac optics with a diagnostic pulse

The beam optics in a linear accelerator may be changed significantly by variations in the energy and energy spread profile along the linac. In particular, diurnal temperature swings in the SLC klystron gallery perturb the phase and amplitude of the accelerating RF fields. If such changes are not correctly characterized, the resulting errors will cause phase advance differences in the beam optics. In addition RF phase errors also affect the amplitude growth of betatron oscillations. We present an automated, simple procedure to monitor the beam optics in the SLC linac routinely and non-invasively. The measured phase advance and oscillation amplitude is shown as a function of time and is compared to the nominal optics.

Use of simulation programs for the modelling of the Next Linear Collider

The Next Linear Collider is an electron-positron accelerator unprecedented in its size, energy, and tight tolerances. We describe the suite of simulation tools which are widely used in designing and modelling the performance of the NLC. In order to achieve a uniform beamline description and permit simulation of all facets of the collider, an extended version of the Standard Input Format (xSIF) has been developed and implemented in MAD and DIMAD. We discuss several enhancements to the MAD and DIMAD calculation engines necessary to properly simulate the most challenging regions of the facility. We also describe enhancements to LIAR which allow it to be used as the tracking engine for a tuning/feedback simulation written in MATLAB. Finally, we discuss the additional software needed to model the beam stabilization and tuning processes.