Bunch Length Measurement Research Articles

Experimental characterization of CANDELA photo-injector

CANDELA photo-injector is made of a 2-cell S-band RF gun, using a dispenser cathode illuminated by a Ti : sapphire laser. This electron source provides a single bunch (at 12.5 Hz), with a charge of 1 nC and an energy of 2 MeV. After recalling the experimental set-up, this paper presents some results concerning mainly energy and bunch length measurements, and also comparisons with simulations done with the PARMELA code. Measured pulse durations of less than 10 ps show for the first time that dispenser photocathodes are “fast response” cathodes.

Sub-nanosecond pulse modulation of a triode electron gun at 25 MHz

Although the electron injection system of the infrared free electron laser FELIX permits a high electron bunch repetition frequency of 1 GHz, user experiments often require a lower frequency. At the same time, the pulses from the gun have to be sufficiently short to fit within the longitudinal acceptance of a 1-GHz prebuncher. The generation of 300 ps electron bunches with a 25 MHz repetition rate, produced by pulse modulation of a triode gun, is addressed in this paper. The modulating waveform was generated by superposition of a 1 GHz sinusoidal wave, a 25 MHz train of pulses with a duration of 2 ns and a DC bias voltage. The hardware for the adopted modulation method, which consists of two RF cavities and an adjustable delay line, is described and analysed. Measurements of the electron bunch length are presented.

Electron bunch length measurements at the Vanderbilt FEL

Pulse compression of submillimeter long electron bunches at the Vanderbilt FEL has been observed. The bunch length is determined by analyzing the coherent transition radiation spectrum produced by the charge distribution, and we show that bunch length changes as small as 15 μm (corresponding to 50 fs) can be detected.

Device for electron bunch length measurement in the picosecond region

We designed, created, and tested a device for measuring electron bunch length in the picosecond region. The basic idea behind the method is a circular scanning of the electron bunch in the rotating magnetic field of a TM110 rf cavity. As a result, the longitudinal position of the electrons is transformed into an angular position in a plane orthogonal to the axis. The resolution limit of the instrument is 4 ps (rms).

Generation and measurement of 50-fs(rms) electron pulses.

An electron source has been developed at the Stanford SUNSHINE facility which can produce electron bunches as short as 50 fs (rms) with $(2\ensuremath{-}4.6)\ifmmode\times\else\texttimes\fi{}{10}^{8}{e}^{\ensuremath{-}}$ per microbunch. This source consists of a 2.6 MeV rf gun with a thermionic cathode and an alpha magnet for bunch compression. Coherent transition radiation emitted at wavelengths equal to the bunch length and longer is used in a Michelson interferometer to determine the bunch length by optical autocorrelation. The experimental setup and results of bunch length measurements are described.

Bunch length measurements on CLIO

The lengths of both electron and laser micropulses have been measured under various conditions. The width of the electron bunches varies between 8 and 16 ps FWHM for standard operation of the linac in our usual 32–50 MeV range. The laser micropulse length varies between 1.5 and 6 ps at 8 μm, depending on the detuning of the optical cavity length. Shorter pulses of subpicosecond duration have been produced by running our linear accelerator in a configuration where the electron bunches acquire a quasi-linear time-energy relationship while travelling on the side, rather than on the crest, of the accelerating high-frequency wave.

Comments on advanced, time-resolved imaging techniques for free-electron laser experiments

An extensive set of time-resolved imaging experiments has been performed on rf-linac driven free-electron lasers (FELs) over the past few years. These experiments have addressed both micropulse and macropulse timescales on both the charged-particle beam and the wiggler/undulator outputs(spontaneous emission and lasing). A brief review of first measurements on photoinjector micropulse elongation, submacropulse phase slew in drive lasers, submacropulse wavelength shifts in lasers, etc. is presented. This is followed by discussion of new measurements of 35-MeV electron beam micropulse bunch length (≤ 10 ps) using optical transition radiation, some of the first single bend synchrotron radiation beam profile measurements at γ < 80, and comments on the low-jitter synchroscan streak camera tuner. These techniques will be further developed on the 200–650 MeV linac test stand at the Advanced Photon Source (APS) in the next few years. Such techniques should be adaptable to many of the present FEL designs and to some aspects of the next generation of light sources.

Time-resolved electron-beam characterizations with optical transition radiation

Time-resolved characterizations of electron beams using optical transition radiation (OTR) as a prompt conversion mechanism have recently been extended on the Los Alamos free-electron Laser (FEL) facility 40-MeV linac. Two key timescales for rf-linac driven FELs are the mupulse (10 ps) and the macropulse (5 μs to 1 ms). In the past we have used gated, intensified cameras to select a single or few mupulses (25–400 ns gate width) out of the pulse train to evaluate submacropulse effects. Recently, we have obtained some of the first measurements of mupulse bunch length (7–10 ps) and submacropulse spatial position and profile using OTR and a Hamamatsu streak camera. Additionally, mupulse elongation effects and head-to-tail transverse kicks are reported as a function of charge.

Instability Studies and Double RF-System Operation at BESSY

Longitudinal multi-bunch oscillations are observed at the BESSY storage ring even with beam currents of 1-2 mA, although stored currents up to 970 mA can be achieved. With a double rf-system the threshold currents can be increased when operating the system in the tune splitting or in the `Landau'-cavity mode. A longitudinal impedance of ?Z/n??12 ? is obtained from bunchlength measurements. No transverse multi-bunch instabilities have been observed. This may be explained by Landau damping induced by the non-linear fields of the ions trapped in the beam.

Real Time Bunch Length Measurements in the SLC Linac

The longitudinal charge distribution of bunches accelerated in the Stanford Linear Collider (SLC) linac will strongly affect the performance of the Collider. Bunch lengths are chosen in a balance between the deleterious effects of longitudinal and transverse wakefields. The former impacts on the beam energy spread whereas the latter is important to the transverse emittance. Two bunch length measurement ports have been installed in the SLC linac: one in the injector region and one after the emittance damping ring to linac reinjection point. These ports utilize a fused quartz Cerenkov radiator in conjunction with an electrooptic streak camera to permit real time monitoring of single s-band buckets with a resolution of several picoseconds. The design of the radiators and light collection optics is discussed with an emphasis on those issues important to high resolution. Experimental results are presented.

Measurement of Bunch Length with an Image Dissector Tube

The length of the electron bunches in the SRS storage ring has been measured by focusing the synchrotron light onto an image dissector tube. The technique is described, which is applicable both to the single bunch mode and to the multibunch mode. Bunch lengths of approximately 0.2 ns are measured with a resolution of 0.05 ns.

Anomalous Laser Induced Bunch Lengthening on-The ACO Storage Ring Free Electron Laser

Laser induced bunch lengthening has been measured on the ACO Storage Ring Free Electron Laser (SRFEL) in the anomalous bunch lengthening regime. The experimental results show correlations between the appearance of coherent modes in the electron bunch and anomalous behavior in the presence of the laser. Simultaneous time resolved laser induced bunch lengthening, mode strength, bunch length, and synchrotron sideband measurements were made as anomalous thresholds were traversed several times by changing the RF accelerating voltage. Bunch shortening, multiple time constants, and oscillatory behavior are among the phenomena which have been observed.

High-speed x-ray sensitive photoconducting detector

A novel photoconducting detector for time-resolved measurements in the x-ray region of the spectrum has been constructed. Time resolution of better than 40 ps at energies between 3 and 10 keV has been demonstrated in bunch length measurements on the Cornell Electron Storage Ring. At high beam currents, dramatic instabilities and a breakup of the electron bunch were directly observed with this detector.

Bunch length measurements in CESR using an X-ray sensitive photoconducting detector

A novel photoconducting detector has been used for bunch length measurements in CESR. The detector and measurements are described in detail. At 5.17 GeV beam energy no bunch lengthening was observed for beam currents up to 26.0 mA (4.2sx10 11 electrons) in a single bunch. These results agree well with bunch lengthening calculations.

Measurement of Bunch-Length by a Mode-Locked Laser with a Time Resolution of 10 Picoseconds

In accelerators working at accelerating frequencies of several 100 MHz the bunch length is in the order of several hundred picoseconds. For obtaining good measurements the time resolution of the measuring apparatus should be 10 ps. With convenient electronic components such a high time resolution cannot be obtained. In this paper a method is described for measuring the bunch length optically by measuring the time structure of the light emitted by relativistic particles (synchrotron or Cerenkov radiation). The heart of the system is a wideband Nd3+-Glass Laser emitting infrared light at 1.06 ?m. With a special technique, the so-called mode-locking, the laser is forced to emit high-intensity optical pulses of several GW peak power and a pulse-length of about 5 ps. These pulses enter a cell filled with a Kerr medium, e.g. CS2, and induce birefringence. Since the relaxation time of CS2 is small, the bire-fringent zone travels with c/n through the Kerr medium. Combinated with two polarizers the system works like a camera with a slit-shutter traveling at the velocity of light. By photographing a total time resolution of 10 ps is obtained.

Use of ion-exchange materials for preparing low-activity radiation sources

Although the electron injection system of the infrared free electron laser FELIX permits a high electron bunch repetition frequency of 1 GHz, user experiments often require a lower frequency. At the same time, the pulses from the gun have to be sufficiently short to fit within the longitudinal acceptance of a 1-GHz prebuncher. The generation of 300 ps electron bunches with a 25 MHz repetition rate, produced by pulse modulation of a triode gun, is addressed in this paper. The modulating waveform was generated by superposition of a 1 GHz sinusoidal wave, a 25 MHz train of pulses with a duration of 2 ns and a DC bias voltage. The hardware for the adopted modulation method, which consists of two RF cavities and an adjustable delay line, is described and analysed. Measurements of the electron bunch length are presented.