Harmonic operation of a free-electron laser

Harmonic operation of a free-electron-laser amplifier is studied. The key issue investigated here is suppression of the fundamental. For a tapered amplifier with the right choice of parameters, it is found that the presence of the harmonic mode greatly reduces the growth rate of the fundamental. A limit on the reflection coefficient of the fundamental mode that will ensure stable operation is derived. The relative merits of tripling the frequency by operating at the third harmonic versus decreasing the wiggler period by a factor of 3 are discussed

Experimental demonstration of a W-band gyroklystron amplifier

The experimental demonstration of a four cavity W-band (93 GHz) gyroklystron amplifier is reported. The gyroklystron has produced 67 kW peak output power and 28% efficiency in the TE011 mode using a 55 kV, 4.3 A electron beam. The full width at half maximum instantaneous bandwidth is greater than 460 MHz, a significant increase over the bandwidth demonstrated in previous W-band gyroklystron amplifier experiments. The amplifier is unconditionally stable at this operating point. Experimental results are in good agreement with theoretical predictions

Mode competition effects in free electron lasers and gyrotrons

In many cases in high frequency, high power coherent radiation generators (such as free electron laser and gyrotrons) the linear gain is positive for many modes and therefore these modes will grow and compete for the beam energy. The questions related to mode competition, coherency of the radiation and maximization of the interaction efficiency are of great importance. To address these issues simple multi-mode models have been formulated. This paper is a short review of the recent results from both simulation and analyses of these models. 3 figs

Microwave generation for magnetic fusion energy applications. Progress report, July 15, 1992--July 14, 1993

This work strives to develop high average power FELs at voltages below I MV allowing for smaller and less costly power supplies. To achieve operation of an FEL with 100 GHZ {approx_lt} f {approx_lt} 150 GHz and with relatively modest voltage, we have been investigating the use of small period ({lambda}{sub {omega}} {approximately} 1 cm) planar wiggler magnets together with sheet electron beams. The sheet beam geometry allows for an FEL interaction region in the form of a narrow slit with high wiggler field at the center plane where the electrons are concentrated. The total current and power may then be increased without making current density excessive by increasing the wide dimension of the sheet beam. Sheet beam FEL design parameters for both a Proof-of-Principle (PoP) FEL experiment, which is current in progress, and an ITER relevant FEL design are shown. A central issue in the sheet beam FEL concept is propagation of the beam through the interaction region without excessive interception by the walls. In section 2 below we describe a successful experimental demonstration of sheet beam propagation through a 56 period uniform wiggler. Cold testing and initial hot test operation of the (PoP) FEL amplifier are also described. Finally, we present a theoretical investigation of the bandwidth of an FEL amplifier with a tapered wiggler operating in saturation is described

An Advanced Electromagnetic Eigenmode Solver for Vacuum Electronics Devices -CTLSS ࣿ

Abstract The Cold-Test and Large-Signal Simulation code (CTLSS), a design tool for vacuum electronics devices, is presented. The prototype tool is a three-dimensional, frequencydomain cold-test code that operates on a rectangular structured grid. It uses a generalisation [1] of the JacobiDavidson algorithm [2] that has proven effective in solving for eigenmodes in problems having sharp-edged structures with materials having large dielectric constants and loss tangents as high as 100%. We present the CTLSS algorithm and code features that are useful for vacuum electronics design. Analysis of both closed cavities and periodic slow-wave structures are given. Tests indicate that the CTLSS algorithm can determine mode frequencies to well below 0.1% accuracy for all modes computed. A new formulation has been implemented to compute the complex axial wavenumber, k z , in a periodic waveguide, as the eigenvalue for a specified real frequency, and test results will be presented. This code is being extended to include an unstructured mesh for the conformal representation of structures using high order element

CIRCUIT ASPECTS OF THE NRL/INDUSTRIAL 94 GHz GYROKLYSTRON AMPLIFIER

Abstract A wide bandwidth, high average power W-band gyroklystron amplifier is currently under cooperative development by NRL, Litton Electron Devices, and Communication and Power Industries. The amplifier circuit is comprised of 4 stagger-tuned cavities operating in the fundamental TE 011 circular cavity mode. The input coupler is the first cavity of the circuit and must exhibit reasonable coupling strength between the TE 10 mode in rectangular waveguide and the desired TE 011 circular cavity mode over a better than 600 MHz bandwidth centered at 93.4 GHz, with high TE 01 mode purity. A single WR-8 rectangular waveguide drives a combined coaxial/cylindrical cavity system. The coaxial cavity resonating in the TE 411 mode is tightly coupled to the cylindrical cavity, excited to resonate in the TE 011 mode. The rf magnetic field couples the cavities through 4 azimuthally spaced apertures

Microwave generation for magnetic fusion energy applications. Task A, Free electron lasers with small period wigglers; Task B, Theory and modeling of high frequency, high power gyrotron operation: Progress report, May 1, 1993--May 1, 1994

This task involves the feasibility of high-power pulsed, high- efficiency, millimeter-wave free electron lasers (FEL) for ecr of thermonuclear plasmas. The research undertaken is to develop high average power FEL at voltage below IMV allowing for smaller and less costly power supplies. Linear amplification experiments employing a No. 56 period untapered wiggler have been conducted, and substantial small signal gain was observed at 95 GHz over a wide range of experimental conditions consistent with the prediction of one-dimensional numerical simulation. Progress is also reported on theoretical studies relating to the development of high-power gryotron and the ability to predict and improve the performance of various cavity designs