Cha-Mei Tang

Dynamics of space-charge waves in the laser beat wave accelerator

The excitation of plasma waves by two laser beams, whose frequency difference is approximately the plasma frequency, is analyzed. This nonlinear analysis is fully relativistic in the axial and transverse directions and includes mismatching of the laser beat frequency to the plasma frequency, time-dependent laser amplitudes, and an applied transverse magnetic field (surfatron configuration). The analytical results for the large-amplitude plasma waves includes an axial constant of motion, accelerating electric field, and its phase velocity. The analytical results in the weak laser power limit are in good agreement with numerical results obtained from the complete equations. The imposed transverse magnetic field is found to increase the effective plasma frequency, but has little effect on the plasma dynamics.

Excitation of the plasma waves in the laser beat wave accelerator

The excitation of plasma waves by two laser beams, whose frequency difference is approximately the plasma frequency, is analyzed. Our nonlinear analysis is fully relativistic and includes mismatching of the laser beat frequency to the plasma frequency, time dependent laser amplitudes, and an applied transverse magnetic field (surfatron). For a given beat frequency, laser power, and plasma density, we find the peak accelerating electric field and its phase velocity. The transverse magnetic field is found to increase the effective plasma frequency, but has little effect on the plasma dynamics.

Planar lenses for field‐emitter arrays

Experimental data is analyzed for a method of collimating and focusing electron beams emitted from field‐emitter arrays (FEAs) where the focusing electrode is co‐planar or nearly co‐planar with the gate electrode. The focusing mechanism is provided by the fringe field formed along the edge of the gate electrode due to the potential difference between the grid and the substrate or the gate and another lens electrode. The concept is verified by EGUN2 simulations. Experiments performed to verify the theory and focusing of electron beams emitted from FEAs were observed. Emission data from 1×1, 1×100 and 2×100 arrays, located within long thin strips of gate electrodes are reported. Without focusing lenses, emission patterns on screen should have been approximately circular. However, elongated emission patterns were consistently observed, indicative of one‐dimensional focusing.

Field-Emission Arrays - A Potentially Bright Source

Abstract Field-emission arrays are potentially a compact and bright electron source. Unlike a laser photocathode, external laser systems are not required. It is possible to have spatial control of the cathode emission. Temporal modulation up to GHz may be possible. We estimate the emittance and brightness based on a reasonable FEA design. Since the FEAs are at an early stage of development for accelerator applications, many technical problems need to be addressed before they can become useful cathodes.

Emission measurements and simulation of silicon field‐emitter arrays with linear planar lenses

Results of beam collimation experiments on linear field‐emitter arrays with linear planar lenses are summarized. The electron beam is imaged on a phosphor screen. In general, as lens voltage is reduced relative to the gate voltage, the elliptically shaped screen images narrow, becoming fine lines with emission currents showing only modest reductions. This reduction of emission current can be overcome by increasing the gate voltage only a few volts without affecting beam collimation. As the lens voltage is reduced, screen current decreases relative to emission current while gate current increases, indicating that some emitted electrons in this linear lens geometry cannot propagate to the anode screen. Experimental data and qualitative modeling are in fair agreement.

Mini‐column silicon field‐emitter arrays

We report emission data of silicon field emitter arrays fabricated with the ultimate goal of their integration within a planar lens structure for beam focusing experiments. The field emitters have been fabricated as linear arrays (with 20 μm separation between tips), square arrays (with 12 μm tip‐to‐tip separation), and single tips. The emitters themselves are formed on columns approximately 0.7 μm in height and 0.4 μm in diameter, and have gate aperture diameters as small as 1.1 μm. Two types of field emitter tips have been fabricated: One has been formed via isotropic etching of silicon in an SF6 plasma, the second using an orientation‐dependent silicon wet etching procedure. Both have been sharpened with an oxidation process, and tip radii as small as 4 nm have been obtained. During testing, 14.4 μA of anode current was obtained from a single tip. From a 100 tip array, 3 mA of current was measured at a gate voltage of 85 V. From a 10 000 tip array, 14.5 mA was obtained.

Induced resonance electron cyclotron quasi-optical maser in an open resonator

In this letter we analyze an induced resonance electron cyclotron (IREC) quasi‐optical maser configuration, which has the unique features of being highly efficient and, with the proper choice for the index of refraction, relatively insensitive to the electron beam’s energy spread. The radiation mechanism can, however, be sensitive to the beam’s pitch angle spread. A system of nonlinear coupled orbit equations, describing the dynamics of electrons in a spatially varying magnetic field, and the electromagnetic field of a steady‐state open resonator configuration are derived and analyzed. Furthermore, we show that by appropriately tapering the magnetic field an induced resonance condition can be achieved which results in high interaction efficiencies. It is anticipated that an efficient, high power millimeter, submillimeter, and infrared radiation source can be realized with the IREC quasi‐optical maser configuration.

Induced Resonance Electron Cyclotron (IREC) quasi-optical maser

This paper analyzes an induced-resonance electron-cyclotron (IREC) quasi-optical maser configuration. This configuration has the unique features of being highly efficient and at the same time relatively insensitive to the detrimental effects of poor beam quality. A system of nonlinear coupled-orbit equations, describing the dynamics of electrons in a spatially varying external magnetic field and the electromagnetic field of a steady-state oscillator, are derived and analyzed. We show that with the proper choice for the index of refraction, the high-frequency cyclotron interaction can be made insensitive to a beam energy spread. The interaction can, however, be sensitive to the beams pitch-angle spread. The necessary conditions on beam quality for high-efficiency operation are derived and shown to be in good agreement with numerical solutions. Furthermore, it is shown that by appropriately tapering the magnetic field an induced resonance condition can be achieved that results in high interaction efficiencies. It is anticipated that an efficient, high power millimeter, submillimeter, and infrared radiation source can be realized with the IREC quasi-optical maser configuration.

Thomson Backscattered X-Rays from an Intense Laser Beam

Abstract We have formulated and obtained analytical expressions for Thomson backscattered X-ray radiation for an electron beam incident on a linearly polarized electromagnetic undulator at a small angle. The analytical expressions are valid for fundamental and harmonics with arbitrarily large laser intensities. The intensity distribution pattern is evaluated numerically.

Analysis of free-electron-laser performance utilizing the National Bureau of Standards' CW microtron

The National Bureau of Standards’ (NBS) cw racetrack microtron (RTM) will be utilized as a driver for a free electron laser (FEL) oscillator. The NBS RTM possesses many exceptional properties of value for the FEL: (i) cw operation; (ii) energy from 20–185 MeV; (iii) small energy spread and emittance; (iv) excellent energy stability; and (v) high average power. The 1D FEL gain formula predicts that the FEL would oscillate at the fundamental approximately from 0.25–10 μm when upgrading the peak current to ≥2 A. In this paper, we present 3D self‐consistent numerical results including several realistic effects, such as emittance, betatron oscillations, diffraction, and refraction. The results indicate that the design value of the transverse emittance is small enough that it does not degrade the FEL performance for intermediate to long wavelengths, and only slightly degrades the performance at the shortest wavelength under consideration. Due to the good emittance, the current density is high enough that focusi...