Yuzheng Lin

Phase motion of accelerated electrons in vacuum laser acceleration

The phase stability in the capture and acceleration scenario (CAS) is studied and compared with that of conventional linear electron accelerators (CLEAs). For the CAS case, it has been found that a slow phase slippage occurs due to the difference between the electron velocity and the phase velocity of the longitudinal accelerating electric field. Thus, CAS electrons cannot remain in a fixed small phase region of the accelerating field to obtain a quasimonoenergy gain in contrast to the stability of phase oscillation in CLEAs. Also, the energy spread of the output electron beam for the CAS case cannot be kept as small as the CLEA because there is no good phase bunching phenomenon generated by phase oscillation.

First principle measurements of thermal emittance for copper and magnesium

There are growing interests in generation, preservation and applications of high brightness electron beam. With the rapid development in the techniques for emittance compensation, laser shaping and generation of ellipsoidal beam, we are approaching the limit--the uncorrelated thermal emittance. In this paper, we describe the design of the angle-resolved photoemission spectroscopy for measurements of thermal emittance for Cu and Mg. The measurement is conducted in a field-free region. The energy spectrum and angular distribution of the electrons will be measured immediately after its emission and further used to reconstruct the initial phase space and the corresponding thermal emittance. We also show how cathode surface roughness and laser incidence angle as well as its polarization state affect the quantum efficiency and thermal emittance.

An X-band disk-and-washer accelerating structure for electron accelerators

X-band electron linacs are an excellent choice for portable accelerators. In this pager we present an X-band disk-and-washer standing wave structure is studied in this paper for a low energy accelerating tube. The DAW cavities are optimized to obtain the largest shunt impedance and quality for the TM020 mode. A set of differential equations is used to describe the optimizing process. The optimized results of a DAW structure working at 9300 MHz are given using SUPERFISH and MAFIA400. Four radial stem supports are adopted. The modes near to the working frequency are given with and without supports. The electromagnetic field building process was analyzed with T3 of MAFIA400. OFC model DAW cavities are built and the first test results are described.

High accuracy analysis of arbitrary modes in tapered disk-loaded structures

In this paper, a high-precision eigenmode-computation analysis of arbitrary azimuthal mode numbers in periodic disc-loaded structure based on variational method will be discussed. It allows for rounding the edge of a disk hole without any approximation in shape treatment and calculates the modes exactly synchronous to the beam. It converges much faster than the mesh-based computer code. Good agreement was observed between the results of variational method and those of other methods.

Acceleration of electron bunches by intense laser pulse in vacuum

Abstract This paper addresses the output characteristics of real electron bunches accelerated with ultra-intense laser pulse in vacuum by the capture & acceleration scenario (CAS) scheme (see, e.g., Phys. Rev. E66 (2002) 066501). Normally, the size of an electron bunch is much larger than that of a tightly focused and compressed laser pulse. We examine in detail the features of the intersection region, the distribution of electrons which can experience an intense laser field and be accelerated to high energy. Furthermore, the output properties of the accelerated CAS electrons, such as the energy spectra, the angular distributions, the energy–angle correlations, the acceleration gradient, the energy which can be reached with this scheme, the emittances of the outgoing electron bunches, and the dependence of the output properties on the incident electron beam qualities such as the emittance, focusing status, etc. were studied and explained. We found that with intense laser systems and electron beam technology currently available nowadays, the number of CAS electrons can reach 10 4 –10 5 , when the total number of incident electrons in the practical bunch reaches ∼10 8 . These results demonstrate that CAS is promising to become a novel mechanism of vacuum laser accelerators.

Multiple parameter characterizations for electron beam with diffraction radiation

There are growing interests in developing non-intercepting method for real-time monitoring electron beam parameters for international linear collider (ILC) and X-ray free electron lasers (XFEL). In this paper we briefly review the theories on using optical and coherent diffraction radiation (ODR and CDR) to measure electron beam profile, divergence, emittance and bunch length. We focus on using ODR to direct image electron beam profile. A new method for bunch length measurement with diffraction radiation deflector is introduced. We also report the preliminary study on radiation spectrum distortion that generally occurs when using CDR to measure bunch length with Martin-Puplett or Michelson interferometers.

Measurements of laser temporal profile and polarization-dependent quantum efficiency

The ultrashort ultraviolet (UV) laser system and the optical transport line for driving the photocathode RF gun at Accelerator Laboratory of Tsinghua University are introduced in the article. Temporal profile of the UV pulse was measured by non-colinear difference frequency generation (DFG) between the UV pulse itself and the jitter-free residual IR laser pulse after third harmonic generation (THG) process. Experiments to measure the dependence of quantum efficiency (QE) on laser polarization state are also performed. Results show that in our case the ratio of QE between p- and s- polarization is more than 2.6.

Design of a source to supply ultra-fast electron and X-ray pulses

In this paper we report the preliminary design and considerations on a multi-discipline ultra-fast source, which is capable of providing the user community with femtosecond electron bunch and light pulses with the wavelength ranging from IR to X-ray. The facility is based on photocathode RF gun driven by a Ti:Sapphire laser system. The low emittance subpicosecond electron bunch at the gun exit can be used in femtosecond electron diffraction setup to visualize the ultrafast structural dynamics. After acceleration and compression, the electron beam with the energy of 50 MeV is further used to provide high peak brightness X-ray by inverse Compton scattering with TW laser. We also consider the possibility and reliability of storing the electron beam in a compact storage ring and the laser pulse in a super-cavity Operating in this scheme may increase the average flux of the X-ray photons by orders of magnitude.

Status of the photocathode RF gun at tsinghua university

A photocathode RF gun system was built to develop the electron source for the Thomson scattering X-ray source at Accelerator laboratory of Tsinghua University. The system consists of a BNL/ATF type 1.6 cell S-band RF cavity, a solenoid for emittance compensation, a laser system and some simple equipments for beam diagnosis. The status of the system is introduced in this paper, and the first beam measurements of the photocathode RF gun, including the dark current, transverse beam profile, charge and quantum efficiency is also reported.

Simulation of Laser Wake Field Acceleration using a 2.5D PIC Code

A 2.5D PIC simulation code is developed to study the LWFA( Laser WakeField Acceleration ). The electron self‐injection and the generation of mono‐energetic electron beam in LWFA is briefly discussed through the simulation. And the experiment of this year at SILEX‐I laser facility is also introduced.