Measurement of Magnetic Axis in Accelerator Magnets: Critical Comparison of Methods and Instruments

We review 19 measurement systems for the magnetic axis of accelerator magnets, used to align machine components. First, we provide some background information and we describe briefly the instruments and methods used for the magnetic and the geometric measurements. For all systems we give then a performance summary in terms of magnet parameters and measurement uncertainties. The dataset is analyzed statistically to identify the parameter with the most influence on the total uncertainty, which is magnet length. Finally we derive scaling laws relating uncertainties to magnet's parameters, and we discuss the relative performance of the various methods

Experience with short-period, small gap undulators at the SwissFEL aramis beamline

The SwissFEL Aramis beamline provides hard X-ray FEL radiation down to 1 Angström with 5.8 GeV and short period, 15mm, in-vacuum undulators (U15). To reach the maximum designed K-value of 1.8 the U15s have to be operated with vacuum gaps down to 3.0 mm. The thirteen-undulator modules are 4m long and each of them is equipped with a pair of permanent magnet quadrupoles at the two ends, aligned magnetically to the undulator axis. Optical systems and dedicated photon diagnostics are used to check the alignment and improve the K-value calibration. In this talk the main steps of the undulator commissioning will be recalled and a systematic comparison between the magnetic results and the electron and photon based measurements will be reported to highlight achievements and open issues.peer-reviewe

Two-pass two-way acceleration in a superconducting continuous wave linac to drive low jitter x-ray free electron lasers

We present a design study of an innovative scheme to generate high rep rate (MHz-class) GeV electron beams by adopting a two-pass two-way acceleration in a Superconducting (SC) linac operated in Continuous Wave (CW) mode. The electron beam is accelerated twice by being re-injected in opposite direction of propagation into the linac after the first passage. Acceleration in opposite directions is accomplished thanks to standing waves supported in RF cavities. The task of recirculating the electron beam when it leaves the linac after first pass is performed by a Bubble-shaped Arc Compressor composed by a sequence of Double Bend Achromat. In this paper we address the main issues inherent to the two-pass acceleration process and the preservation of the electron beam quality parameters (emittance, energy spread, peak current) required to operate X-ray Free Electron Lasers with low jitters in the amplitude, spectral and temporal domain, as achieved by operating in seeding and/or oscillator mode a CW FEL up to 1 MHz rep rate. Detailed start-to-end simulations are shown to assess the capability of this new scheme to double the electron beam energy as well as to compress the electron bunch length from picoseconds down to tens of femtoseconds. The advantage of such a scheme is to halve the requested linac length for the same final electron beam energy, which is typically in the few GeV range, as needed to drive an X-ray FEL. The AC power to supply the cryogenic plant is also significantly reduced with respect to a conventional single-pass SC linac for the same final energy. We are reporting also X-ray FEL simulations for typical values of wavelengths of interest (in the 200 eV \u2013 8 keV photon energy range) to better illustrate the potentiality of this new scheme

Conceptual design of a laser-plasma accelerator driven free-electron laser demonstration experiment

Up to now, short-wavelength free-electron lasers (FEL) have been systems on the scale of hundreds of meters up to multiple kilometers. Due to the advancements in laser-plasma acceleration in the recent years, these accelerators have become a promising candidate for driving a fifth-generation synchrotron light source – a lab-scale free-electron laser. So far, demonstration experiments have been hindered by the broad energy spread typical for this type of accelerator. This thesis addresses the most important challenges of the conceptual design for a first lab-scale FEL demonstration experiment using analytical considerations as well as simulations. The broad energy spread reduces the FEL performance directly by weakening the microbunching and indirectly via chromatic emittance growth, caused by the focusing system. Both issues can be mitigated by decompressing the electron bunch in a magnetic chicane, resulting in a sorting by energies. This reduces the local energy spread as well as the local chromatic emittance growth and also lowers performance degradations caused by the short bunch length. Moreover, the energy dependent focus position leads to a focus motion within the bunch, which can be synchronized with the radiation pulse, maximizing the current density in the interaction region. This concept is termed chromatic focus matching. A comparison shows the advantages of the longitudinal decompression concept compared to the alternative approach of transverse dispersion. When using typical laser-plasma based electron bunches, coherent synchrotron radiation and space-charge contribute in equal measure to the emittance growth during decompression. It is shown that a chicane for this purpose must not be as weak and long as affordable to reduce coherent synchrotron radiation, but that an intermediate length is required. Furthermore, the interplay of the individual concepts and components is assessed in a start-to-end simulation, confirming the feasibility of the envisioned experiment. Moreover, the setup tolerances for a first demonstration experiment are determined, confirming the general practicability. The revealed challenges, besides the energy spread, especially concern the source stability and the precision of the beam optics setup

A simple method for the determination of the structure of ultrashort relativistic electron bunches

In this paper we propose a new method for measurements of the longitudinal profile of 100 femtosecond electron bunches for X-ray Free Electron Lasers (XFELs). The method is simply the combination of two well-known techniques, which where not previously combined to our knowledge. We use seed 10-ps 1047 nm quantum laser to produce exact optical replica of ultrafast electron bunches. The replica is generated in apparatus which consists of an input undulator (energy modulator), and the short output undulator (radiator) separated by a dispersion section. The radiation in the output undulator is excited by the electron bunch modulated at the optical wavelength and rapidly reaches 100 MW-level peak power. We then use the now-standard method of ultrashort laser pulse-shape measurement, a tandem combination of autocorrelator and spectrum (FROG -- frequency resolved optical gating). The FROG trace of the optical replica of electron bunch gives accurate and rapid electron bunch shape measurements in a way similar to a femtosecond oscilloscope. Real-time single-shot measurements of the electron bunch structure could provide significant information about physical mechanisms responsible for generation ultrashort electron bunches in bunch compressors. The big advantage of proposed technique is that it can be used to determine the slice energy spread and emittance in multishot measurements. It is possible to measure bunch structure completely, that is to measure peak current, energy spread and transverse emittance as a function of time. We illustrate with numerical examples the potential of the proposed method for electron beam diagnostics at the European X-ray FEL.Comment: 41 pages, 18 figure

Periodic Poisson Solver for Particle Tracking

A method is described to solve the Poisson problem for a three dimensional source distribution that is periodic into one direction. Perpendicular to the direction of periodicity a free space (or open) boundary is realized. In beam physics, this approach allows to calculate the space charge field of a continualized charged particle distribution with periodic pattern. The method is based on a particle mesh approach with equidistant grid and fast convolution with a Green's function. The periodic approach uses only one period of the source distribution, but a periodic extension of the Green's function. The approach is numerically efficient and allows the investigation of periodic- and pseudo-periodic structures with period lengths that are small compared to the source dimensions, for instance of laser modulated beams or of the evolution of micro bunch structures. Applications for laser modulated beams are given.Comment: 33 pages, 22 figure