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The design, simulation, manufacture and measurement of a W-band five-fold (5F) helically corrugated waveguide (HCW) is reported. The 5F HCW is based on the coupling of the traveling TE31 and near cut-off TE22 modes to create an operating eigenwave. The fabricated test structure has circular waveguide ports and features elliptical polariser sections and broadband TE11 to TE31 mode converters on either side of the 5F HCW. The optimised mode converter design, based on a four-fold (4F) HCW, has a predicted power conversion efficiency greater than 90% from 89 to 102.5 GHz, and 96% peak efficiency at 94 GHz. The optimization of the 5F HCW geometry produced an eigenwave suitable for gyro-devices, but the optimization could equally well have been directed to applications such as pulse compression and microwave undulators. Analysis of simulated electric field profiles showed that the propagating power in the 5F HCW was increased by a factor of 6 over that in the 3F HCW at equivalent peak electric field strength. This is due to the larger diameter of the waveguide. Test structures were manufactured through a combination of precision machining of a sacrificial mandrel, copper growth by electroforming followed by removal of the aluminium mandrel by chemically etching. Measurements of the 5F HCW structure’s dispersion showed excellent agreement with the prediction over the design range of 90 to 98 GHz.

A manufacturing study of a 0.372 THz 8-fold (8F) helically corrugated interaction region (HCIR) and measurement of its wave dispersion characteristics are reported. This demonstrates the structure’s suitability as the electron beam / electromagnetic wave interaction region in a high power frequency tunable Gyroresonant THz source. The 8F HCIR has an eigenmode, which is ideal for broadband tuning, created by the coupling of the TE61 and TE23 modes. Maximum power handling of 14 times larger than a 3-fold HCIR at the same frequency is calculated. A TE11 to TE61 7-fold (7F) helically corrugated waveguide (HCW) mode converter, needed to measure the wave dispersion of the 8F HCIR, was designed and constructed. Negative aluminium mandrels of the 7F HCW mode converter and 8F HCIR were manufactured. Copper structures were constructed through electroforming. The measured wave dispersion of the 0.372 THz 8F HCIR agreed well with simulated dispersion characteristics.

The azimuthally rippled cavity for a large-orbit, coharmonic gyro-multiplier, designed to operate at the second and fourth harmonics, at frequencies of 37.5 and 75 GHz, respectively, has been numerically and experimentally confirmed to be insensitive to the polarization of quadrupole, ${\text{TE}}_{2,n}$ -like modes, including the second-harmonic operating mode of the multiplier, a cylindrical TE2,2-like waveguide mode. To test the cavity with this mode required the design, construction, and measurement of ripple wall mode converters, converting the cylindrical TE2,1 mode into the TE2,2 mode. These were designed to operate at a central frequency of ~37.9 GHz, with predicted mode purity of better than 85%, and 3-dB bandwidth of 161 MHz. The constructed converter had a central operating frequency of 37.7 GHz, with $S$ -parameter measurements used to infer suitable mode purity and an operational 3-dB bandwidth of 50 MHz. This has allowed far-field phase measurements of the corrugated cavity to be conducted, where the orientation of the geometry to the polarization of both the TE2,1 and TE2,2 modes was shown to have no effect on the dispersion.

A broadband, kW-level gyrotron traveling wave amplifier (gyro-TWA) in W-band was developed based on a helically corrugated interaction region and a cusp electron gun. Experiments were carried out to demonstrate the versatility of the gyro-TWA and to investigate its broad instantaneous amplification bandwidth. Following the successful prototyping, research is being carried out to improve the stability of the amplifier and widen its capability for continuous wave operation for long-range high-data-rate communication applications.

In this article, a transmission line system for the propagation of millimeter-wave radiation is presented. The full system includes a TE11-to-TE01 mode converter, waveguide tapers, miter bends, and many straight sections. The design of each of these components is described, and the optimized simulation results are given. The mode converter shows a greater than 96% mode conversion efficiency that can be achieved over a 2% bandwidth at the ${W}$ -band. The miter bends demonstrate a transmission loss of 0.04 dB each over the same bandwidth when they are configured to introduce a mixture of higher order waveguide modes before the reflecting surface. An example transmission line system with a propagation length of 20 m, inclusive of four 90° bends with an oxygen-free high conductivity (OFHC) copper waveguide material, was studied over a 90–96-GHz frequency range and showed a 0.84-dB transmission loss at 93 GHz.

This paper presents the dispersion curve study of the helically corrugated waveguide using the 2D Finite Element Method (FEM) method based on the helicoidal coordinate transform. Calculation of the dispersion curve is 60 times faster compared with 3D FEM simulation while maintaining similar accuracy. The 2D method was used to investigate the relative error between it and a 1D analytical method based on perturbation theory. Over a narrow parametric range, the 1D method has a similar dispersion (within a relative error of 3%) in comparison to the 2D/3D methods but the error greatly increased when the parameter space was expanded. Correction factors were determined to greatly enhance the 1D method accuracy over the wider parametric range. Assisted by the improved 1D method, it allows identifying the desired operating mode automatically within the multiple eigenvalues from the 2D simulation results. Therefore it can be integrated into an optimization routine to explore the applications of the helically corrugated waveguides at wider parameter ranges.

High-power microwave sources are typically relativistic in nature, employing multi-kilo-ampere electron beams that require significant magnetic confinement for efficient operation. As the desired output power increases so does the complexity, and overall energy requirements, of the source. It can therefore be advantageous to consider the use of several, moderate-power, sources operating as a phased array; for an array of N sources the far-field peak intensity scales as N2, and the peak-of-field may be steered electronically by varying the relative phases of the different output signals. In this paper we present the numerical analysis of a short-pulse (∼1ns) X-band backward-wave oscillator, driven by a 210keV, 1.4kA electron beam, suitable for use as the radiative element in such an array. Investigation of the required magnetic confinement showed two peaks in performance, with the highest efficiency, of 43%, predicted at the low magnetic confinement peak at 0.3T, corresponding to 125MW peak output power. The magnitude, and timing, of the peak in the output pulse were functions of the rise-time of the electron beam energy, with longer rise-times resulting in delayed peak-of-field and lower peak output power. When operating in an array, to maintain effective output in the region of N2, it was determined that the beam rise-times, across all sources, should be ≤150ps with the adjustment of the relative timing between output’s being ±30ps.

Many important plasma processes, such as heating and current drive in tokamaks and laser-plasma interactions, involve the interaction of an externally generated electromagnetic wave with the plasma. The plasma can couple the incident wave to a range of other waves. Important examples include Raman scattering, where the incident wave couples to an electrostatic Langmuir wave and a scattered electromagnetic wave, and Brillouin scattering, where the Langmuir wave is replaced by an ion sound wave 1 . Both are observed in laser plasma experiments; however, these instabilities can also be generated by intense, short pulse microwave signals in a cool, tenuous plasma. A magnetized plasma supports additional beat wave interactions with natural cyclotron motions of charged particles and with magnetized plasma waves such as upper/lower hybrid waves as well as kinetic modes such as ion/ electron Bernstein waves. Applications include heating plasma that is inaccessible at low harmonics of the cyclotron frequency in high density fusion plasmas. Enabling these experiments is a large helicon apparatus, 1 m diameter, 3m length. The stainless-steel vacuum vessel is surrounded by several electromagnets providing a B 0 up to 0.0875 T. The plasma will be ionized by m = 0 helicon waves with 3 < f < 30 MHz launched by a flat spiral antenna 2 . Plasmas of 10 15 < n e < 10 18 m -3 and T e < 10 eV will be produced in a low pressure (

The numerical study of a low confining-field (~0.3T) X-band Backward Wave Oscillator (BWO), suitable for implementation as the microwave source(s) in a phase-synchronous array, is presented. Analytical theory and CST Particle Studio simulations have been used to design a ~9.6GHz BWO, generating 120MW, 1ns, pulses when driven by a 4ns, 210kV, 1.5kA Radan 303B modulator.

This paper presents a numerical investigation of alternate gyrotron Traveling Wave Amplifier (gyro-TWA) configurations for increased gain. Previous experimental results showed a maximum stable gain of 37 dB over the frequency range 90-96 GHz. These results were limited in bandwidth by the input power from the solid-state pre-amplifier. The simplest way to mitigate the limited input power available is to increase amplifier gain. Doing so without inducing instability requires the introduction of losses to the RF circuit. We present PIC simulations contrasting a range of new designs combining two gain stages and lossy elements with results for the single section design presented in our earlier experimental program.

A cavity-type microwave undulator (MU) based on a corrugated cavity operating at 36 GHz was designed. A short-corrugated section prototype was manufactured and measured to validate the resonance frequency and quality factor. The electron beam trajectory in the MU with a full length of 1-meter was simulated to exam the electron drift distance in the radial direction. The radiation spectrum from the MU shows soft X-ray radiation can be produced with 250 MeV electron bunches.

A broadband, high-power gyrotron traveling wave amplifier (gyro-TWA) was demonstrated to show single-shot millimeter wave output over a 3 dB bandwidth of 91-96.5 GHz, with max. power of 3.4 kW and gain of 36-38 dB. Following on from this, the system will be upgraded for high, Pulse Repetition Frequency (PRF) output, suitable to be employed in radar applications. Amongst the changes required is the design and construction of a modulated anode electron gun and water cooled collector system to be installed with a cryogenic liquid free, up to 5.5 T, superconducting magnet to replace a conventional solenoid.

The “Laser-hybrid Accelerator for Radiobiological Applications,” LhARA, is conceived as a novel, flexible facility dedicated to the study of radiobiology. The technologies demonstrated in LhARA, which have wide application, will be developed to allow particle-beam therapy to be delivered in a new regimen, combining a variety of ion species in a single treatment fraction and exploiting ultra-high dose rates. LhARA will be a hybrid accelerator system in which laser interactions drive the creation of a large flux of protons or light ions that are captured using a plasma (Gabor) lens and formed into a beam. The laser-driven source allows protons and ions to be captured at energies significantly above those that pertain in conventional facilities, thus evading the current space-charge limit on the instantaneous dose rate that can be delivered. The laser-hybrid approach, therefore, will allow the radiobiology that determines the response of tissue to ionizing radiation to be studied with protons and light ions using a wide variety of time structures, spectral distributions, and spatial configurations at instantaneous dose rates up to and significantly beyond the ultra-high dose-rate “FLASH” regime. It is proposed that LhARA be developed in two stages. In the first stage, a programme of in vitro radiobiology will be served with proton beams with energies between 10 and 15 MeV. In stage two, the beam will be accelerated using a fixed-field alternating-gradient accelerator (FFA). This will allow experiments to be carried out in vitro and in vivo with proton beam energies of up to 127 MeV. In addition, ion beams with energies up to 33.4 MeV per nucleon will be available for in vitro and in vivo experiments. This paper presents the conceptual design for LhARA and the R&D programme by which the LhARA consortium seeks to establish the facility.

Linear accelerators operating at millimeter or sub-terahertz frequencies and short pulse duration have the advantages of lower power consumption and high repetition rate. In this paper planar metallic accelerating structures with different modes operating at 210 GHz were designed. A tolerance study was also carried out to determine the sensitivities of the geometric parameters to the wakefield acceleration performance. The generated Wakefield was simulated using the beam parameter of the Compact Linear Advanced Research Accelerator (CLARA) test facility at Daresbury Laboratory. For a 55 MeV single electron bunch with charge of 250 pC and a bunch length of 0.27 mm (0.9 ps), an equivalent acceleration gradient of 20 MV/m was achieved in the simulation. The relatively modest acceleration gradient was limited by the charge in a single bunch. The acceleration gradient could be further improved by using a bunch train which has larger total bunch charge. From the simulation, the acceleration gradient of 100 MV/m can be generated when it is driven by a 10-bunch beam train.

The development of high-frequency RF linear accelerators (linacs) requires the consideration of several technological challenges, such as electron bunch linearization. Presented in this paper is the design of the interaction circuit for a 48 GHz MW-level three-cavity gyroklystron amplifier, appropriate for application as a millimeter wave power source in a fourth harmonic linearizing system for an X-band linac. The output cavity is operated at the cylindrical TE0,2,1 mode, while the input and buncher cavities are operated at the TE0,1,1 mode. The interaction circuit has been designed using a combination of analytical calculations and particle-in-cell simulations. The optimized gyroklystron is shown, through simulation, to deliver an output power of up to 2.3 MW with a gain of 36 dB and an efficiency of 44% at 48 GHz, when driven by a 140 kV, 37 A electron beam. The support of the STFC UK (Cockcroft Institute Core GrantR160525-1) is gratefully acknowledged. This work is supported by European Union (EU) Horizon 2020 Project “CompactLight” 2017-2021. “CompactLight” grant code: 777431-XLS. Published data March 2020, under Self-archiving / 'green' OA.

An electron plasma lens is a cost-effective, compact, strong-focusing element that can ensure efficient capture of low-energy proton and ion beams from laser-driven sources. A Gabor lens prototype was built for high electron density operation at Imperial College London. The parameters of the stable operation regime of the lens and its performance during a beam test with 1.4 MeV protons are reported here. Narrow pencil beams were imaged on a scintillator screen 67 cm downstream of the lens. The lens converted the pencil beams into rings that show position-dependent shape and intensity modulation that are dependent on the settings of the lens. Characterisation of the focusing effect suggests that the plasma column exhibited an off-axis rotation similar to the $m=1$ diocotron instability. The association of the instability with the cause of the rings was investigated using particle tracking simulations., Comment: 22 pages, 17 figures, preprint submitted to Applied Sciences

Following on from a series of laboratory plasma experiments1, 2 that simulated natural plasma instabilities3, a larger plasma apparatus is being constructed. This laboratory plasma will be used for studying nonlinear wave coupling between one or more electromagnetic (EM) waves interacting in plasma. Coupling of injected EM waves to Langmuir and ion acoustic waves is of interest for a number of laser plasma interactions and in ionospheric physics experiments. Long (and short) pulse signals with normalized intensities approaching those used in some recent laser plasma interactions can be generated using flexible microwave amplifiers. Multi -wave microwave interactions are also directly relevant to the delivery of heating and current drive in future magnetic confinement fusion reactors. Microwave sources and amplifiers will be used to precisely launch and electronically control multiple EM signals. The relatively long lived, benign and accessible plasma relevant to coupling of microwave frequency signals will enable the use of insertion diagnostics in addition to analyzing the EM signals.

Backward Wave Oscillators (BWO's) utilizing moderately relativistic (~550kV), high-current (~10 kA) electron beams are capable of producing hundreds of MWs of pulsed radiation in the centimeter wavelength range. Such relativistic BWOs (RBWOs) allow for broadband, smooth, frequency-tuning via adjustment of the accelerating potential; making them an attractive source for use in frequency-swept pulse compression. This paper presents results of a 2.86m long 5-fold helically corrugated, dispersive pulse compressor where a maximum power compression ratio of 25 was achieved by compressing an input microwave pulse of 80 ns duration swept from 9.65 GHz to 9.05 GHz into a 1.6ns Gaussian-envelope pulse. For an average input power of 5.8 kW generated by a conventional traveling wave tube amplifier, a peak pulse output power of 145 kW was measured corresponding to an energy efficiency of 66%. An X-band relativistic BWO, designed to drive a 0.97m long 5-fold compressor, was built and tested using the accelerating potential generated by a SINUS-6 highcurrent accelerator. The experimental RBWO operated close to the predicted power of 700MW with its oscillation frequency varied from 10 to 9.6GHz via the falling edge of the voltage pulse. It was demonstrated that the ~15ns duration frequency-swept part of the RBWO pulse was effectively compressed resulting in a 4.5-fold peak power increase with a maximum power of 3.2 GW generated. The potential for a 5-fold helical waveguide to compress longer duration pulses generated by a RBWO is discussed.

Muon accelerators offer an attractive option for a range of future particle physics experiments. They can enable high energy (TeV+) high energy lepton colliders whilst mitigating the difficulty of synchrotron losses, and can provide intense beams of neutrinos for fundamental physics experiments investigating the physics of flavor. The method of production of muon beams results in high beam emittance which must be reduced for efficient acceleration. Conventional emittance control schemes take too long, given the very short (2.2 microsecond) rest lifetime of the muon. Ionisation cooling offers a much faster approach to reducing particle emittance, and the international MICE collaboration aims to demonstrate this technique for the first time. This paper will present the MICE RF system and its role in the context of the overall experiment., Comment: 2 pp

The Muon Ionization Cooling Experiment (MICE) has been built at the STFC Rutherford Appleton Laboratory to demonstrate the principle of muon beam phase-space reduction via ionization cooling. Muon beam cooling will be required at a future proton-derived neutrino factory or muon collider. Ionization cooling is achieved by passing the beam through an energy-absorbing material, such as liquid hydrogen, and then re-accelerating the beam using RF cavities. This paper describes the hydrogen system constructed for MICE including: the liquid-hydrogen absorber, its associated cryogenic and gas systems, the control and monitoring system, and the necessary safety engineering. The performance of the system in cool-down, liquefaction, and stable operation is also presented., Comment: 25 pages, 21 figures, part of the MICE collaboration

A corrugated rectangular waveguide, commonly used in radio frequency antennas applications, was considered for use in a gyro-TWA experiment. This waveguide transducer produced broadband circularly polarised radiation in the Ka-band frequency range (26??40 GHz). Computer simulations, using computer simulation technology Microwave Studio??, were carried out and predicted return losses exceeding 30 dB and a 6° phase variation for 20% bandwidth centred at 31 GHz. Comparison between experimental measurements and simulations showed good agreement. The performance of this polariser was also compared to that of a conventional elliptical converter. Owing to its broadband performance the authors consider some of the technical aspects of applying this component to a Ka-band gyro-TWA experiment.

We report operation of a co-axial Free-Electron Maser (FEM) based on two-dimensional distributed feedback. The 37.3GHz FEM was driven by an annular electron beam (475kV, 500A, 250ns) and generated 15MW ±1.5MW of radiation with a 6% conversion efficiency. To investigate an increase in FEM output power and efficiency a new cavity configuration based on a copper 2D Bragg reflector will be studied. ©2005 IEEE.

High power millimeter and sub-millimeter wave sources operating in the high GHz - THz (35GHz-1THz) frequency range are important for a number of applications. This work will focus on the overview of recent theoretical and experimental research conducted at the University of Strathclyde to design, build and study masers operating at 37.3GHz, 91.2GHz and 0.74THz. The results obtained will be presented and discussed. ©IEEE.

Summary form only given. There has been a drive in recent years to produce ultra-high power short pulses for a range of applications. These high power pulses can be produced by microwave pulse compression. Sweep-frequency based microwave pulse compression using smooth bore hollow waveguides is one technique of pulse compression, however at very high powers this method has some limitation due to its operation close to cut-off. In optimum cases, the frequency at the beginning of an input pulse should be only 0.5-1% above the cutoff frequency. If a Cherenkov type TWT is used to drive the compressor then it is inevitable that the low-frequency part of the amplification band is below the cutoff which then reflected from the compressor back to the amplifier resulting in its possible parasitic self-oscillation. If a relativistic BWO is used as a source of frequency-modulated pulses for a smooth-waveguide compressor, then the necessary frequency sweep that would be required can only be produced by using a difficult-to-realize voltage modulation on the BWO. The two combined problems of wave reflection from a compressor and optimum frequency modulation can be solved using a waveguide with a special helical corrugation of its inner surface. A special helical corrugation of a circular waveguide ensures an eigenwave having a strongly frequency-dependent group velocity far from cut-off, which makes the helically corrugated waveguide attractive for using as a compressor of pulses from very high power amplifiers and oscillators. The results of proof-of-principle experiments and calculations of the wave dispersion using a PIC code are presented. In the experiments a 70 ns 1 kW pulse from a conventional TWT was compressed in a 2 metre long helical waveguide. The compressed pulse had a peak power of 10.9 kW and duration of 3 ns. In order to find the optimum pulse compression ratio the waveguide's dispersion characteristics must be well known. The dispersion of the helix was calculated using the PIC code MAGIC and verified using an experimental technique. For a helical compressor the optimum negative frequency sweep can quite naturally be realized at the falling edge of a relativistic BWO pulse. For example, if a BWO generates hundreds of MW during 50 ns over a voltage drop from 700 kV to 400 kV, this frequency modulated radiation can be compressed up to 10-20 times in power with efficiency higher than 50%. Future work detailing plans to produce short, ultra-high power (multi-GW) pulses discussed.

The authors report the study of a high-current accelerator, capable of producing thin (2 mm) annular, mildly relativistic (450 keV) electron beams (up to 1.5 kA) of diameter of 70 mm. Propagation of an electron beam through a 2 m long, coaxial beam channel, with inner and outer conductor radii of 30 and 40 mm, respectively, was demonstrated with the electron beam current at the beam-channel output measured using a Rogowski coil. The results obtained are compared with those predicted via numerical simulation. The electron beam obtained was used for proof-of-concept work demonstrating the operation of a high-power maser operating at millimeter wavelengths, using two-dimensional distributed feedback. © 2006 American Institute of Physics.

The study of a coaxial free-electron maser (FEM) based on two-dimensional (2D) and one-dimensional (1D) distributed feedback, driven by a 70 mm diameter, annular electron beam is presented. A new cavity formed with 2D and 1D periodic lattices, positioned at the input and output of the interaction region, respectively, was used. It has been demonstrated that 2D distributed feedback in the input mirror allowed 8 mm radiation emitted from different parts of the electron beam to be synchronized. The FEM operating in the 35.9-38.9 GHz frequency region generated 65 MW, 150 ns duration millimeter wave pulses which contained ∼10 J of energy in the pulse. © 2008 American Institute of Physics.

Authors respond to comments on their paper: 'High current oversized electron beam formation for high-power microwave research' [appl. phys. lett. 89, 171503 (2006)].

We report on experiments which have recently been conducted at the University of Strathclyde investigating rapid amplitude modulations occurring in the microwave output radiation of an electron cyclotron maser (ECM). The experiment used an electron beam injected from a co-axial diode with knife-edged graphite cathode in the fringing field of an adjustable magnet system producing a beam of up to 175kV and 140A. The time evolution of the electron beam was measured as the cathode plasma expanded using a Faraday cup in conjunction with upstream beam interceptors as a function of the magnetic compression ratio. The ECM cavity was configured so that its length and the length of the interaction space could be readily adjusted. The microwave output signal was studied using special fast rectifying diode detectors, a high performance deep memory oscilloscope and cut-off filters. Steady-state output was observed at high magnetic compression ratios (16:1) at a frequency of 16GHz corresponding to cyclotron resonant maser (CRM) coupling between the beam and the radiation in the expected TE1,2 mode. At lower compression ratios modulation was observed after an initial steady-state period and shown by antenna pattern measurements to be associated with transverse mode competition in the microwave cavity. © 2004 Elsevier B.V. All rights reserved.

A computational study has been made on the design of a 150-kV triodelike switchable electron gun. A smooth magnetic cusp arrangement was used to produce an axis-encircling beam. Numerical optimization of this arrangement led to a gun design that produced beam parameters of $I_{b} \sim \hbox{6}\ \hbox{A}$ , $\alpha = \hbox{1.2}$ , and $d\alpha \sim \hbox{15}\%$ . It has also been shown that this triode design can produce beam suppression at $\sim$ 2 kV while not interfering with the beam parameters under operation conditions.

Helically corrugated waveguide has been used in various applications such as gyro-backward wave oscillators, gyro-traveling wave amplifier and microwave pulse compressor. A fast prediction of the dispersion characteristic of the operating eigenwave is very important when designing a helically corrugated waveguide. In this paper, multi-mode coupling wave equations were developed based on the perturbation method. This method was then used to analyze a five-fold helically corrugated waveguide used for X-band microwave compression. The calculated result from this analysis was found to be in excellent agreement with the results from numerical simulation using CST Microwave Studio and vector network analyzer measurements.

High-power gigawatt-level radiation can be generated by the interaction of an electromagnetic wave and an annular electron beam with a transverse dimension much larger than the operating wavelength. The use of such a large-circumference annular beam allows the generation of high beam currents while also maintaining low space charge and RF power densities inside the interaction region. This circumvents the problems associated with potential depression in the beam channel and RF breakdown inside the oscillator. In this paper, we present the studies of high-current magnetically confined annular electron beams and discuss their production and transportation through a coaxial beam channel which formed the interaction region of a freeelectron maser (FEM). The results from numerical simulations, using the software packages KARAT and MAGIC, are compared with the experimental measurements. The operation of a FEM, driven by a high-current annular electron beam, is presented, and the tunability of the maser, inside a frequency range defined by an input 2-D Bragg mirror, is demonstrated. © 2009 IEEE.

A three-stage depressed collector was previously designed and simulated to recover the kinetic energy of the spent electron beam in an X-band gyrotron backward wave oscillator (gyro-BWO) by using the 3-D particle-in-cell code MAGIC. The geometry of the depressed collector was optimized using a genetic algorithm to achieve the optimum overall recovery efficiency for specific parameters of the spent beam. In this paper, secondary electron emissions were simulated, and a few emission models were compared to investigate the effects of the secondary electrons on the overall recovery efficiency and the backstreaming of the electrons from the collector region. The optimization of the shape and dimensions of each stage of the collector using a genetic algorithm achieved an overall recovery efficiency of more than 80% over the entire operating regime of the Gyro-BWO, with a minimized backstreaming of 1.4%. The heat distribution on the collector was calculated, and the maximum heat density on the electrodes was approximately 195 W/cm2, hence avoiding the generation of ?hot spots?.

A novel thermionic cusp electron gun operating in the temperature-limited regime that produces a large-orbit electron beam through a nonadiabatic magnetic-field reversal was designed, analyzed, and optimized to give an electron-beam ideal for driving gyro-devices, particularly in the millimeter-to-submillimeter-wavelength range due to its small cross-sectional size. The annular-shaped axis-encircling electron beam had a beam current of 1.5 A at an acceleration potential of 40 kV, a tunable velocity ratio alpha (= vperp/vz) between one and three, an optimized axial velocity spread A.vx/vz of ~8%, and a relative alpha spread Deltakappa/alpha of ~10% at an alpha value of 1.65.

Two-dimensional distributed feedback is an effective method of producing ultrahigh-power spatially coherent radiation from an active medium, that is spatially extended along two coordinates, including relativistic electron beams with sheet and annular geometry. This paper describes the progress in the investigations of planar and coaxial free-electron masers (FEMs) based on a novel feedback mechanism. The theoretical analysis of these FEM schemes was conducted in the frame of the coupled-wave approach and 3-D simulations and agrees well with the experimental data obtained in ldquocoldrdquo and ldquohotrdquo tests. As a result, the effective transverse (azimuthal) mode selection has been demonstrated under a transverse size of about 20-25 wavelengths, and narrow-frequency multimegawatt microwave pulses have been generated in the Ka- and W-bands.

To realize the full potential of a gyrotron backward-wave oscillator (gyro-BWO), an energy recovery system was designed using Particle-In-Cell (PIC) simulations and optimized using both a genetic algorithm and PIC simulations. Simulations were carried out to optimize a periodic structure for separation of the spent electron beam and the output radiation produced by a gyro-BWO in the 8.0-9.5 GHz frequency range. The spent electron beam can be collected using a multistage depressed collector. The number and electric potentials of the electrodes were optimized to achieve the best overall recovery efficiency for specific parameters of the spent beam. The 3-D PIC code MAGIC was used to simulate the electrons' trajectories and a genetic algorithm was used to refine the electrode shapes for optimum efficiency.

Metal waveguide can be used as a dispersive medium to convert long duration, lower power pulses into short, higher peak power pulses. This provides an advanced method to generate radiation with gigawatts power in the millimeter and sub-millimeter wavelength range by compressing a megawatt level long duration pulse. In this paper, a five-fold helically corrugated waveguide operating in X-band was designed and constructed. The experiments conducted show that a 5.75 kW average power microwave pulse with a 6% bandwidth and duration of 80 ns can be compressed into a 144.8 kW, 1.6 ns pulse with a power compression factor of 25.2.

Numerical finite difference time domain and Particle-In-Cell simulations have demonstrated an electron wave interaction in a Cherenkov maser utilizing a cylindrical two-dimensional (2D) Periodic Surface Lattice (PSL) as a mode selective cavity [1-8]. Optimization of this structure's physical properties resulted in the design of a cavity with 16 longitudinal periods of 1.6 mm length, 7 azimuthal variations and an unperturbed inner radius of 4 mm. In numerical simulations this design produces an output power of 300 kW with 10 % efficiency at a frequency of 103.6 GHz. A proof of principle experiment demonstrating electron beam interaction with a wave formed when the volume field and surface field are coupled in an oversized 2D PSL cavity will be presented. The application of PSLs has the potential to deliver high average power, efficient, compact electromagnetic wave sources in the challenging THz frequency range.

A helically corrugated waveguide was used for a gyrotron backward-wave oscillator (gyro-BWO) experiment. A thermionic cathode was used to produce an electron beam of 90-215 keV in energy, 2-3 A in current, and pitch alpha of up to 1.6. The oscillator achieved high-efficiency frequency-tunable operation. At a fixed beam voltage of 185 kV and a current of 2 A, the output frequency was tuned by adjusting the magnetic field in the interaction cavity. A maximum power of 62 kW and a 3-dB frequency-tuning band of 8.0-9.5 GHz (17% relative tuning range) with a maximum electronic efficiency of 16.5% were measured. In addition, the interaction frequency could be tuned by varying the electron beam energy. At a fixed cavity magnetic field of 0.195 T, the output frequency and power from the gyro-BWO were measured as a function of tuning electron beam energy while the beam current was maintained at 2.5 A. A 3-dB relative frequency tuning range of 8% was measured when the electron beam voltage was changed from 215 to 110 kV.

Studies are presented of a series of computer simulations, using CST (Microwave Studio), of novel waveguide components for a mm-wave gyro-TWA. Each component has a range of operation that can be optimised through observations of the S-parameters and the phase data, while altering geometric parameters in the simulations. This approach allows optimum performance of mm-wave gyro-TWAs.

Cyclotron coupling between electrons and waves is thought to occur in both polar and equatorial regions of the Earth's magnetosphere. The cyclotron emissions from an electron beam moving into a waveguide with an increasing magnetic field have been investigated in an experiment1, scaled to microwave frequencies, supported by numerical simulation2,3 and theoretical analysis, to reproduce aspects of the magnetospheric environment. Magnetic compression produces an electron distribution with a high, quantified, degree of velocity spread4, as may be expected in the natural environment. The measurements1,5 and simulations2 indicate wave production efficiencies which are consistent with the magnetospheric observations and with theoretical predictions (typically ∼ few %), preferentially polarised and propagating nearly perpendicularly to the magnetic field, at frequencies close to the cyclotron frequency (in the experiments between 2 and 11GHz)5. The effectiveness of the wave generation mechanism is shown to be mitigated by the addition of a quasi-neutral background plasma through the use of a Penning discharge (plasma frequency ∼100–300MHz, reflecting the auroral density cavity ratio ω ce /ω pe ∼10 or more6) in two distinct ways: The efficiency is reduced, whilst the emission acquires an increasingly variable nature, where nominally identical experiments can either generate strong cyclotron signals, or very weak signals indeed, suggesting alternative competing near-threshold mechanisms. Simulations have shown a similar effect on the wave production efficiency, but so far have not provided an explanation for the statistical variability where there are conditions where no radiation is generated at all. The project has also considered the generation of signals in radiation modes where the wavevector has a significant component parallel to the bias magnetic field. Strong wave emission continued to be observed close to (but slightly downshifted from) the cyclotron frequency, with the E-field polarised perpendicular to the bias B-field. These measurements may have relevance to the radiation belts. Simulations, benchmarked against the experiments, have been used to study cyclotron emissions from beams in unbound environments, particularly relevant to the polar magnetosphere78, and these results will also be presented.

We present results from the X-band reversed guide magnetic field Raman Free Electron Maser (FEM) amplifier experiment at Strathclyde University. The FEM has been designed for zero slippage to achieve maximum instantaneous bandwidth. The tuneablity is further extended by the adjustment of beam voltage and undulator field strength. A 1 kW TWT has been used as the input source for broadband measurements and two 25 kW magnetrons for saturated output power measurements at discrete frequencies. Results show an instantaneous −3 dB bandwidth of 30% for a fixed cathode voltage and magnetic field. The device can be tuned over a range of 65% of the centre frequency by adjusting the cathode voltage. A saturated power of over 1 MW has been measured for 50 A input current. Broadband unsaturated gains of over 35 dB have been measured. An experiment using a 50 A beam from a thermionic cathode electron gun designed at Strathclyde is currently in progress. This will theoretically allow CW operation of the FEM using a DC undulator magnet and DC power source.

The use of two-dimensional (2D) Bragg resonators of coaxial geometry, realising two-dimensional distributed feedback, is considered as a method of providing additional mode selection and producing spatially coherent radiation from oversized annular electron beams. The main goals of the project are to prove operability of a FEM using 2D Bragg structures and compare the performance of the FEM using the novel 2D Bragg resonators with conventional 1D Bragg resonators.

A new microwave system in the form of a cylindrical waveguide with a helical corrugation of the inner surface is proposed for a gyrotron traveling wave tube (gyro-TWT). The corrugation radically changes the wave dispersion in the region of small axial wave numbers. This allows significant reduction in the sensitivity of the amplifier to the electron velocity spread and an increase in its frequency bandwidth. An X-band gyro-TWT operating at the second cyclotron harmonic with a 200-keV, 25-A electron beam produced an output power of 1 MW, corresponding to a gain of 23 dB and an efficiency of 20%.

The pseudospark discharge offers the possibility of producing electron beams which are very attractive for use in high-power microwave generation. A pseudospark-based Cherenkov maser amplifier is currently under development at Strathclyde University. The electron beam source for this maser is a multi-gap pseudospark discharge. Preliminary results from recent Cherenkov maser experiments and a comparison with a numerical simulation are presented. A microwave pulse of 100 ns duration and approximately 10 kW peak power was generated by a 80 kV, 20 A beam passed through an alumina-lined waveguide when the interaction was allowed to start up from noise, which appeared to originate from the pseudospark discharge. Simulations agree well with the experimental results when a beam energy spread of 1.5% is assumed.

A narrow-band Free-Electron Maser (FEM) oscillator with a Bragg resonator was operated in the reversed guide field regime. A high-current accelerator generated the 300 keV, 30 A, 100 ns electron beam. Radiation at 32 GHz frequency and 500 kW power, corresponding to ∼5% efficiency, was measured. The measured FEM parameters agreed with predictions of a numerical simulation. The observed radiation frequencies compared well with “cold” microwave calibration of the cavity.

Auroral Kilometric Radiation (AKR) emissions occur at frequencies similar to 300kHz polarised in the X-mode with efficiencies similar to 1-2% [1,2] in the auroral density cavity in the polar regions of the Earth's magnetosphere, a region of low density plasma similar to 3200km above the Earth's surface, where electrons are accelerated down towards the Earth whilst undergoing magnetic compression. As a result of this magnetic compression the electrons acquire a horseshoe distribution function in velocity space. Previous theoretical studies have predicted that this distribution is capable of driving the cyclotron maser instability. To test this theory a scaled laboratory experiment was constructed to replicate this phenomenon in a controlled environment, [3-5] whilst 2D and 3D simulations are also being conducted to predict the experimental radiation power and mode, [6-9]. The experiment operates in the microwave frequency regime and incorporates a region of increasing magnetic field as found at the Earth's pole using magnet solenoids to encase the cylindrical interaction waveguide through which an initially rectilinear electron beam (12A) was accelerated by a 75keV pulse. Experimental results showed evidence of the formation of the horseshoe distribution function. The radiation was produced in the near cut-off TE01 mode, comparable with X-mode characteristics, at 4.42GHz. Peak microwave output power was measured similar to 35kW and peak efficiency of emission similar to 2%, [3]. A Penning trap was constructed and inserted into the interaction waveguide to enable generation of a background plasma which would lead to closer comparisons with the magnetospheric conditions. Initial design and measurements are presented showing the principle features of the new geometry. Publisher PDF