Space-charge Dominated Electron Beam Research Articles

Study for the alignment of focusing solenoid of ares rf gun and effect of misalignment of solenoid on emittance of space charge dominated electron beam

SINBAD (Short and INnovative Bunches and Accelerators at DESY) facility will host multiple experiments relating to ultra-short high brightness beams and novel experiments with ultra-high gradient. ARES (Accelerator Research Experiment at SINBAD) Linac is an S-band photo injector to produce such electron bunches at around 100 MeV. The Linac will be commissioned in stages with the first stage corresponding to gun commissioning. In this paper, we present studies about the scheme adopted for the alignment of focusing solenoid for the ARES gun. The method is bench marked using ASTRA simulations. Moreover the effect of misalignment of the solenoid on the emittance of space charge dominated scheme and its compensation is also discussed.

Generation and focusing of electron beams with initial transverse-longitudinal correlation

In charged particle beams, one of the roles played by space charge is to couple the transverse and longitudinal dynamics of the beam. This can lead to very complex phenomena which are generally studied using computer simulations. However, in some cases models based on phenomenological or analytic approximations can provide valuable insight into the system behavior. In this paper, we employ such approximations to investigate the conditions under which all the slices of a space charge dominated electron beam with slowly varying current could be focused to a waist with the same radius and at the same location, independent of slice current, and show that this can be accomplished approximately if the initial transverse-longitudinal correlation introduced onto the beam by the electron gun is chosen to compensate for the transverse-longitudinal correlation introduced onto the beam in the drift section. The validity of our approximations is assessed by use of progressively more realistic calculations. We also consider several design elements of electron guns that affect the initial correlations in the beams they generate.

An emittance measurement device for a space-charge dominated electron beam

Beam emittance is one of the most important parameters for electron sources. To investigate the beam emittance of the 3.5-cell DC-SC photocathode injector developed at Peking University, a multi-slit emittance measurement device has been designed and manufactured. The designed slit width, mask thickness and beamlet drift length are 100 μm, 3 mm and 430 mm respectively. It is suitable for the electron beam with energy of about 5 MeV and the average current less than 0.1 mA. The preliminary measurement result of the rms emittance of the electron beam produced by the DC-SC injector is about 5–7 mm·mrad.

Experimental verification of tomographic phase-space imaging for beams with space-charge using a pinhole-scan

A model is described for tomographic phase-space mapping and emittance measurement in beams with space-charge. The tomographic results of an experiment with a 10 keV space-charge dominated electron beam are presented. The data are compared against a direct experimental sampling of the phase-space using a pinhole scan. It is found that the accuracy of the tomographic method is within 10% for beams with space-charge intensity less than 90%. With the aid of numerical simulation the observed similarities and differences between the tomography method and pinhole scan are discussed.

Multislit-Based Emittance Measurement of Electron Beam from a Photocathode Radio-Frequency Gun

Measurement of emittance for a space-charge dominated electron beam from a photocathode rf gun is performed by employing the multislit-based method at Accelerator Laboratory of Tsinghua University. We present the design considerations on the multislit system and the experimental results, with special attention to the study of space charge induced emittance growth. The experimental results are in reasonable agreement with the PARMELA simulations.

Time-dependent imaging of space-charge dominated electron beams

When considered as non-neutral plasmas, space-charge dominated charged particle beams have significantly smaller Debye lengths than their beam sizes. Therefore, collective effects due to space-charge forces are very important to the beam dynamics. As an example, density perturbations generated in a space-charge dominated charge particle beam will stimulate longitudinally space-charge waves. To study the modification of transverse beam distributions by longitudinal beam dynamics, experimental studies of low-energy electron beams, with and without longitudinal density perturbations, have been undertaken at the University of Maryland’s electron ring and the Long Solenoid Experiment (LSE). We have taken time-resolved beam images on these machines using two different diagnostics: Optical transition radiation, produced from an intercepting aluminized silicon screen, and a fast (<3ns decay time) phosphor screen. Results from both techniques show that both the transverse size and transverse particle distribution of a space-charge dominated beam are affected by the longitudinal dynamics of the beam. In addition to the fast imaging measurements, longitudinal mean energy profiles of different beams have also been measured at different locations in the LSE system.

Radiation from a space charge dominated linear electron beam

It is commonly known that radiation loss in linear beam transport is largely unimportant. For a space charge dominated linear beam, however, radiation power loss can be an appreciable fraction of the injected beam power [Biswas, Kumar, and Puri, Phys. Plasmas 14, 094702 (2007)]. Exploring this further, the electromagnetic nature of radiation due to the passage of a space charge dominated electron beam in a “closed” drift tube is explicitly demonstrated by identifying the cavity modes where none existed prior to beam injection. It is further shown that even in an “open” drift tube from which radiation may leak, the modes that escape contribute to the time variation of the electric and magnetic fields in the transient phase. As the window opening increases, the oscillatory transient phase disappears altogether. However, the “bouncing ball” modes survive and can be observed between the injection and collection plates.

Measurement of beam energy spread in a space-charge dominated electron beam

Characterization of beam energy spread in a space-charge dominated beam is very important to understanding the physics of intense beams. It is believed that coupling between the transverse and longitudinal directions via Coulomb collisions will cause an increase of the beam longitudinal energy spread. At the University of Maryland, experiments have been carried out to study the energy evolution in such intense beams with a high-resolution retarding field energy analyzer. The temporal beam energy profile along the beam pulse has been characterized at the distance of 25 cm from the anode of a gridded thermionic electron gun. The mean energy of the pulsed beams including the head and tail is reported here. The measured rms energy spread is in good agreement with the predictions of the intrabeam scattering theory. As an application of the beam energy measurement, the input impedance between the cathode and the grid due to beam loading can be calculated and the impedance number is found to be a constant in the operation region of the gun.

Nonlinear behavior of localized space-charge waves in space-charge dominated electron beams

We present experimental observations of the abnormal growth of localized nonlinear space-charge waves in space-charge dominated electron beams passing through a resistive channel. The energy width of the space-charge waves is measured on both ends of the channel. Previous experiments had shown that, for small initial perturbations, the energy width of the slow waves increases, while the energy width of the fast waves decreases, in agreement with linear theory. We report that in the nonlinear regime (large initial perturbations), the energy width of the fast wave increases, which is unexpected, and, to the best of our knowledge, no theory exists that would predict this phenomenon.

Observations and Simulations of Transverse Density Waves in a Collimated Space-Charge Dominated Electron Beam

Experiments and particle-in-cell simulations demonstrate the appearance of wavelike transverse density variations in a space-charge dominated electron beam. Simulations show how an aperture located near the source gives rise to a nonequilibrium phase-space distribution with strong force imbalance confined to a sheath near the beam edge. Tracking of particles in this sheath, starting near the aperture's edge, reproduces well the onset of the perturbation. The subsequent evolution of the perturbation over about one meter suggests the appearance of a transverse wave. For the parameters investigated, simulations further indicate that the perturbation damps out over a few plasma periods without causing any rms emittance growth.

Transport of a space-charge dominated electron beam in a short-quadrupole channel

Results are presented for electron beam transport experiments in a 1-m-long straight section consisting of a solenoid and five short printed-circuit quadrupoles. A linear computer code for rms envelope matching, SPOT, is used for channel design, while final simulations with more realistic elements are obtained with a $2\frac{1}{2}\mathrm{D}$ version of WARP, a particle-in-cell code. Reasonable agreement is found between calculations and the effective beam envelope obtained from pictures of the beam on a movable phosphor screen. The results validate, within experimental errors, the use of short magnetic quadrupoles for the transport of space-charge dominated beams. The straight section constitutes the prototype matching section for an electron recirculator to be built at the University of Maryland.

Longitudinal space-charge waves and instabilities in intense beams

Space-charge effects and collective behavior play an important role in the dynamics of intense beams for accelerator applications. One of the research issues is the longitudinal space-charge waves and instabilities in high current accelerators. At the University of Maryland, an experimental program has been conducted to systematically study space-charge waves and longitudinal instabilities in space-charge dominated electron beams. This includes the generation of localized space-charge waves in an active device; the propagation and end reflection of space-charge waves in a conducting transport channel; and the longitudinal instabilities in a resistive-wall channel. This paper reports and discuss our experimental results.

Excimer laser induced electron beams on an Al target: Plasma effect in a ‘‘nonplasma’’ regime

Experiments on electron-beam generation from an Al target illuminated by XeCl and KrCl excimer lasers were carried out under plasma ignition threshold intensities. Direct experimental evidence of such a plasma formation under ignition threshold is given for the first time. By Langmuir-like probe measurements, positive potentials as large as 0.36 and 0.75 V at distances down to about 2 μm from the grounded target over the laser spot area were reproducibly observed during the XeCl and KrCl laser action, respectively. From this result and in absence of cathode voltage, an accelerating electric field on the target of about 375 kV/m was estimated. The highest positive charge density near the cathode that resulted was about 3.3 mC/m3. The strong electric field caused by the plasma formation modified the acceleration conditions for a space-charge dominated electron beam and it should be the main cause of the discrepancies between the experimental current values and the calculated ones.