Radio Frequency Deflector Research Articles

Sub-femtosecond time-resolved measurements of electron bunches with a C-band radio-frequency deflector in x-ray free-electron lasers

Time-resolved diagnostics are fundamental for x-ray free-electron lasers (FELs). Radio-frequency (RF) transverse deflector structures (TDSs) are typically employed to characterize the temporal properties of the electron beams driving FELs. In this article, we present time-resolved measurements with a resolution below one femtosecond using a C-band RF TDS at SwissFEL, the x-ray FEL facility at the Paul Scherrer Institute in Switzerland. The sub-femtosecond resolution is partially achieved due to an optimized optics setup and fits the expected values, showing a good understanding of our models. Measurements with a sub-femtosecond resolution are of crucial importance for ultra-fast x-ray FEL applications.

Performance of the OEDO beamline

The OEDO (Optimized Energy Degrading Optics for RI beam) beamline has been developed to obtain low-energy (10 – 50 MeV/u) RI beams by slowing down the secondary beams at RIBF, RIKEN. Such beams are useful probes to investigate nuclear structure by means of nucleon transfer and Coulomb excitation reactions. The beamline is designed to produce a well-focused beam of small energy spread with the help of the angle-tuneable wedge degrader and the radio-frequency deflector. The commissioning and first physics experiments using the OEDO beamline were carried out in 2017. OEDO successfully provided the energy-degraded beams of Se, Zr, and Pd. Its performance including the optics and transmission was evaluated using those experimental data and it functions properly as designed. We expect that our novel beamline shed a light on the experimental approaches involving nuclear reactions at several tens of MeV/u to expand our knowledge of nuclei far beyond the line of stability.

Design of a multipurpose RF deflector system applied in a compact linac

A thermionic gun usually suffers from the long beam tail problem, and this is also the case in the compact terahertz free-electron laser facility in the Huazhong University of Science and Technology. Here we propose to use a radio frequency (RF) deflector operated in the dipole mode as a beam chopper to cut off the unnecessary beam tail. The RF deflector, followed by a scraper and a scintillation screen, will be installed in the drift space between the gun and the linac. The RF deflector system can be operated in two configurations: (1) a beam chopper, with the scraper pushed in, to prevent the tail particles from entering the linac; (2) a bunch length diagnostic instrument, with the scintillation screen pushed in, to measure the longitudinal profile from the electron gun. The systematic design of the RF deflector is presented, and the performance on beam chopping and bunch length measurement is investigated.

A Measurement Method Based on RF Deflector for Particle Bunch Longitudinal Parameters in Linear Accelerators

In high-brightness electron linear accelerators (LINACs), the particle bunch length is measured by a radio frequency deflector (RFD). The electron bunch is deflected vertically toward a screen and its length can be obtained using vertical spot size measurements after a proper calibration, e.g., measuring the vertical bunch centroid while varying the deflecting voltage phase. The energy parameters of the bunch (the energy chirp and the energy spread) and the correlation between particle positions, divergences, and energies contribute to the bunch vertical dimension at the screen position after the RFD and so far were considered as a source of systematic errors in a bunch length measurement. The measurement theory and production model for bunch length, energy spread and chirp, as well as correlations are described. As usual in particle accelerators physics, the method is validated using numerical simulations of state-of-the-art LINACs with a reference simulation code showing a typical accuracy in the few percent levels.

Enhancing particle bunch-length measurements based on Radio Frequency Deflector by the use of focusing elements

A method to monitor the length of a particle bunch, based on the combination of a Radio Frequency Deflector (RFD) with magnetic focusing elements, is presented. With respect to state-of-the-art bunch length measurement, the additional focusing element allows to measure also the correlations between the longitudinal and transverse planes in terms of both position and divergence. Furthermore, the quadrupole-based focusing increases the input dynamic range of the measurement system (i.e. allows for a larger range of beam Twiss parameters at the entrance of the RFD). Thus, measurement resolution and precision are enhanced, by simultaneously preserving the accuracy. In this paper, the method is first introduced analytically, and then validated in simulation, by the reference tool ELEctron Generation ANd Tracking, ELEGANT. Finally, a preliminary experimental validation at CLEAR (CERN Linear Electron Accelerator for Research) is reported.

New energy-degrading beamline for in-flight RI beams, OEDO

The Optimized Energy Degrading Optics (OEDO) beamline was constructed at RIBF to produce slowed-down, well-focused radioactive-isotope beams at 10–50 MeV/u, and began operation in June 2017. We examined the performance of the OEDO system by decelerating high-energy medium-mass radioactive ions. The main components of the OEDO beamline are two superconducting quadrupole triplet magnets and one radio-frequency deflector. This paper introduces the unique design of the ion optics of the beamline and describes the achieved performance of the beamline in the commissioning and following physics experiments. We will discuss the properties of energy-degraded beams produced by the OEDO beamline by comparison of simulations and the achieved performance.

Longitudinal Bunch Size Measurements with an RF Deflector at J-PARC LINAC

We have measured the longitudinal bunch size of beams from the radio-frequency quadrupole linac (RFQ) at the Japan Proton Accelerator Research Complex (J-PARC). The radio-frequency deflector (RFD) is installed in the medium-energy beam transport section (MEBT1) between the RFQ and the drift-tube linac (DTL). The RFD, which is usually used to produce a macro pulse configuration for the 3-GeV rapid-cycling synchrotron, is utilized to produce a different deflecting angle depending on the bunch arrival phase. Transverse beam profiles were measured after the RFD with and without deflection to extract the longitudinal bunch size. The Courant-Snyder parameters in the longitudinal direction were estimated by measurements for different strength of the upstream buncher. These measurements are one of the important steps to understand the longitudinal beam properties at a high beam current.

Study of a tuner for a high-accuracy bunch shape monitor

In the J-PARC linac, a higher precision bunch shape monitor (BSM) was developed to measure the longitudinal-beam distribution. To transform a longitudinal-beam profile into a transverse one with a radio frequency (RF) field, we need a field with an acceleration synchronizing frequency. The RF deflector of a BSM comprises a λ/2 cylindrical cavity and two electrodes for deflection. In general, the resonance frequency can be tuned by adjusting the electrode length. We designed a new tuner using CST studio, wherein control over the resonance frequency was achieved by adjusting not only the electrode length but also the cavity volume. We found the optimum electrode lengths and volume for tuning.

Observation of the Self-Modulation Instability via Time-Resolved Measurements.

Self-modulation of an electron beam in a plasma has been observed. The propagation of a long (several plasma wavelengths) electron bunch in an overdense plasma resulted in the production of multiple bunches via the self-modulation instability. Using a combination of a radio-frequency deflector and a dipole spectrometer, the time and energy structure of the self-modulated beam was measured. The longitudinal phase space measurement showed the modulation of a long electron bunch into three bunches with an approximately 200 keV/c amplitude momentum modulation. Demonstrating this effect is a breakthrough for proton-driven plasma accelerator schemes aiming to utilize the same physical effect.

Effects of correlations between particle longitudinal positions and transverse plane on bunch length measurement: a case study on GBS electron LINAC at ELI-NP

In high-brightness LINear ACcelerators (LINACs), electron bunch length can be measured indirectly by a radio frequency deflector (RFD). In this paper, the accuracy loss arising from non-negligible correlations between particle longitudinal positions and the transverse plane (in particular the vertical one) at RFD entrance is analytically assessed. Theoretical predictions are compared with simulation results, obtained by means of ELEctron Generation ANd Tracking (ELEGANT) code, in the case study of the gamma beam system (GBS) at the extreme light infrastructure—nuclear physics (ELI-NP). In particular, the relative error affecting the bunch length measurement, for bunches characterized by both energy chirp and fixed correlation coefficients between longitudinal particle positions and the vertical plane, is reported. Moreover, the relative error versus the correlation coefficients is shown for fixed RFD phase 0 rad and rad. The relationship between relative error and correlations factors can help the decision of using the bunch length measurement technique with one or two vertical spot size measurements in order to cancel the correlations contribution. In the case of the GBS electron LINAC, the misalignment of one of the quadrupoles before the RFD between −2 mm and 2 mm leads to a relative error less than 5%. The misalignment of the first C-band accelerating section between −2 mm and 2 mm could lead to a relative error up to 10%.

Mapping few-femtosecond slices of ultra-relativistic electron bunches

Free-electron lasers are unique sources of intense and ultra-short x-ray pulses that led to major scientific breakthroughs across disciplines from matter to materials and life sciences. The essential element of these devices are micrometer-sized electron bunches with high peak currents, low energy spread, and low emittance. Advanced FEL concepts such as seeded amplifiers rely on the capability of analyzing and controlling the electron beam properties with few-femtosecond time resolution. One major challenge is to extract tomographic slice parameters instead of projected electron beam properties. Here, we demonstrate that a radio-frequency deflector in combination with a dipole spectrometer not only allows for single-shot extraction of a seeded FEL pulse profile, but also provides information on the electron slice emittance and energy spread. The seeded FEL power profile can be directly related to the derived slice emittance as a function of intra-bunch coordinate with a resolution down to a few femtoseconds.

Misalignment measurement of femtosecond electron bunches with THz repetition rate

Generation of a well-aligned bunch train is of vital importance for preserving the beam quality in a plasma wake-field accelerator. The simultaneous measurements of the relative misalignments in both planes, of the individual femtosecond electron bunches with THz repetition rate in a train, are reported and analyzed with simulations. The new method proposed in this paper demonstrates that the measurements can be done quickly by applying a single quadrupole scan together with a radio-frequency deflector (RFD), while the RFD strength and the RFD phase for each bunch do not need to be known

Aberration corrected emittance exchange

Full exploitation of emittance exchange (EEX) requires aberration-free performance of a complex imaging system including active radio-frequency (RF) elements which can add temporal distortions. We investigate the performance of an EEX line where the exchange occurs between two dimensions with normalized emittances which differ by multiple orders of magnitude. The transverse emittance is exchanged into the longitudinal dimension using a double dog-leg emittance exchange setup with a five cell RF deflector cavity. Aberration correction is performed on the four most dominant aberrations. These include temporal aberrations that are corrected with higher order magnetic optical elements located where longitudinal and transverse emittance are coupled. We demonstrate aberration-free performance of an EEX line with emittances differing by four orders of magnitude, \textit{i.e.} an initial transverse emittance of 1~pm-rad is exchanged with a longitudinal emittance of 10~nm-rad.

Single-shot 35 fs temporal resolution electron shadowgraphy

We obtain single-shot time-resolved shadowgraph images of the electromagnetic fields resulting from the interaction of a high intensity ultrashort laser pulse with a metal surface. Using a high brightness relativistic electron beam and a high streaking speed radiofrequency deflector, we report <35 fs temporal resolution enabling a direct visualization of the retarded-time dominated field evolution which follows the laser-induced charge emission. A model including the finite signal propagation speed well reproduces the data and yields measurements of fundamental parameters in short pulse laser-matter interaction such as the amount of emitted charge and the emission time scale.

Femtosecond x-ray pulse temporal characterization in free-electron lasers using a transverse deflector

We propose a novel method to characterize the temporal duration and shape of femtosecond x-ray pulses in a free-electron laser (FEL) by measuring the time-resolved electron-beam energy loss and energy spread induced by the FEL process, with a transverse radio-frequency deflector located after the undulator. Its merits are simplicity, high resolution, wide diagnostic range, and noninvasive to user operation. When the system is applied to the Linac Coherent Light Source, the world's most powerful x-ray FEL, it can provide single-shot measurements of the electron-beam and x-ray pulses with a resolution on the order of 1--2 femtoseconds rms.

Radio frequency picosecond phototube

We propose a photon detector for recording low-level and ultra-fast optical signals, based on radio frequency (RF) analysis of low-energy photoelectrons (PEs). By using currently developed 500 MHz RF deflector, it is possible to scan circularly and detect single PEs, amplified in multi-channel plates (MCPs). The operation of the tube is investigated by means of thermionic electron source. It is demonstrated that the signals generated in the MCP can be processed event by event; by using available nanosecond electronics and that time resolution better than 20 ps can be achieved. Timing characteristics of the Cherenkov detector with RF phototube in a ‘head-on’ geometry is investigated by means of Monte Carlo simulation.

The development and beam test of an RF chopper system for J-PARC

The linac of the Japan Proton Accelerator Research Complex (J-PARC) is now under construction. It is a high-intensity linac with a peak H − current of 50 mA and a 1.25% duty factor. As a key component of the medium-energy beam-transport line of the linac, a chopper is necessary to obtain the required beam structure for the following 3-GeV rapid-cycling synchrotron, and also plays an important role in beam-loss control in such a high-intensity accelerator. Clean chopping requires a short rise time and a high deflecting field. An radio frequency deflector (RFD) was proposed as a chopper for the J-PARC linac, because of its merits of high deflecting field, compact structure, ease to manufacture and both reliability and flexibility of the total chopper operation. An RF chopper, consisting of two RFDs, has been developed. In a low-level RF test, the fundamental RF properties show good agreement with those of a simulation in the design. In a high-power test, the chopper works well without any discharge under 36 kW peak driving power. In a test of the coupled RFD system, the longer rise time of the second RFD and the delay of the second RFD excitation were observed. A beam test of the RF chopper was also successfully performed up to a peak current of 25 mA. The chopper system worked very stable during the beam test, and the sufficient separation between chopped and normal beam was obtained, with a short rise/fall time (10 ns) of the chopped beam.

Design study on a medium-energy beam-transport line for the JHF proton linac

The JHF/JAERI linac is an injector of a 3-GeV ring. It consists of an H − ion source, a 3-MeV radio-frequency quadrupole linac, a 50-MeV drift-tube linac, a 200-MeV separated-type drift-tube linac and a 400-MeV ACS accelerator in its normal conducting section. As an intense beam linac with an average current of 0.2 mA (first stage) and higher in the second stage, the issue of decreasing beam losses is of prime importance. The MEBT between the RFQ and the DTL has been designed with a modified TRACE3-D code, aimed at perfect matching, clear chopping, a small emittance growth and sufficient space for beam measurements. The line consists of eight quadrupoles, two bunchers and two choppers with a total length of about 3 m. Multiparticle codes PARMILA and LINSAC were used to check the beam dynamics of the design. Mismatch due to errors in the elements of the MEBT or the input beam was also studied. A radio-frequency deflector (RFD) is adopted in the chopper. It is characterized by a high deflecting field and compactness. The RF-deflector cavity was designed using the MAFIA code and the electromagnetic field distribution, including fringe field, is applied in TRACE3-D simulation. It illustrates the effect of the beam tube in the cavity on large deflection. A deflection initiated by the RF-deflector is amplified more than twice by the following quadrupole. Owing to this merit, the required RF field of 1.6 MV/m is rather low to generate an adequate separation of 4.3 mm between chopped and unchopped beams at the beam stopper. By means of rapid changes in the RF amplitude and phase during the rise and fall time, the chopper has an equivalent rise and fall time of less than 3.1 ns. Beam losses during the transient time of the chopper are less than 0.08% at the exit of the 50-MeV DTL. An analysis of unstable particles in the coupled RF-deflectors shows no obvious difference from the case of an independent cavity.