Cyclotron Autoresonance Maser Research Articles

Radio-Frequency Undulators, Cyclotron Auto Resonance Maser and Free Electron Lasers

We discuss a hybrid Free Electron Laser (FEL) architecture operating with a RF undulator provided by a powerful Cyclotron Auto-Resonance Maser (CARM). We outline the design elements to operate a compact X-ray device. We review the essential aspects of wave undulator FEL theory and of CARM devices.

State-of-the-Art of High-Power Gyro-Devices and Free Electron Masers

This paper presents a review of the experimental achievements related to the development of high-power gyrotron oscillators for long-pulse or CW operation and pulsed gyrotrons for many applications. In addition, this work gives a short overview on the present development status of frequency step-tunable and multi-frequency gyrotrons, coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fast- and slow-wave cyclotron autoresonance masers (CARMs), gyroklystrons, gyro-TWT amplifiers, gyrotwystron amplifiers, gyro-BWOs, gyro-harmonic converters, gyro-peniotrons, magnicons, free electron masers (FEMs), and dielectric vacuum windows for such high-power mm-wave sources. Gyrotron oscillators (gyromonotrons) are mainly used as high-power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control, and diagnostics of magnetically confined plasmas for clean generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 140 GHz, megawatt-class gyrotrons employing synthetic diamond output windows is 30 min (CPI and European KIT-SPC-THALES collaboration). The world record parameters of the European tube are as follows: 0.92 MW output power at 30-min pulse duration, 97.5% Gaussian mode purity, and 44% efficiency, employing a single-stage depressed collector (SDC) for energy recovery. A maximum output power of 1.5 MW in 4.0-s pulses at 45% efficiency was generated with the QST-TOSHIBA (now CANON) 110-GHz gyrotron. The Japan 170-GHz ITER gyrotron achieved 1 MW, 800 s at 55% efficiency and holds the energy world record of 2.88 GJ (0.8 MW, 60 min) and the efficiency record of 57% for tubes with an output power of more than 0.5 MW. The Russian 170-GHz ITER gyrotron obtained 0.99 (1.2) MW with a pulse duration of 1000 (100) s and 53% efficiency. The prototype tube of the European 2-MW, 170-GHz coaxial-cavity gyrotron achieved in short pulses the record power of 2.2 MW at 48% efficiency and 96% Gaussian mode purity. Gyrotrons with pulsed magnet for various short-pulse applications deliver Pout = 210 kW with τ = 20 μs at frequencies up to 670 GHz (η ≅ 20%), Pout = 5.3 kW at 1 THz (η = 6.1%), and Pout = 0.5 kW at 1.3 THz (η = 0.6%). Gyrotron oscillators have also been successfully used in materials processing. Such technological applications require tubes with the following parameters: f > 24 GHz, Pout = 4–50 kW, CW, η > 30%. The CW powers produced by gyroklystrons and FEMs are 10 kW (94 GHz) and 36 W (15 GHz), respectively. The IR FEL at the Thomas Jefferson National Accelerator Facility in the USA obtained a record average power of 14.2 kW at a wavelength of 1.6 μm. The THz FEL (NOVEL) at the Budker Institute of Nuclear Physics in Russia achieved a maximum average power of 0.5 kW at wavelengths 50–240 μm (6.00–1.25 THz).

The Cyclotron Auto-Resonance Maser: Analytical and numerical study

We study the small signal regime of Cyclotron Auto-Resonance Maser (CARM), using different computational tools. The analysis is motivated by the necessity of an accurate benchmarking of the design parameter for the CARM device under construction at ENEA Frascati center. The main goal is that of comparing the results from three different procedures, the analytical formulation and two different numerical items: the home-made code GRAAL (Gyrotron Radiation Auto-Resonance Amplification Laser) and the, commercially available, software package CST. We prove the substantial consistence between the different methods and comment on the reasons leading to the discrepancies. The extension to the non-linear regime is finally touched on.

Optimization of TE11/TE04 mode converters for the cold test of a 250 GHz CARM source

Electron Cyclotron Resonance Heating (ECRH) systems in future fusion devices, like the DEMO-nstration reactor, foresee an operational frequency in the range 230–280 GHz to match the plasma characteristics. Cyclotron Auto Resonance Masers (CARMs) could represent an alternative to gyrotrons as effective source of mm and sub-mm waves and the project of a 250 GHz, 0.5 MW CARM has been undertaken at ENEA. Mode converters from the TE10 in WR3 rectangular waveguide to the operational mode (TE53) are required to perform the RF cold test of Bragg reflectors and cavities. In this paper an improved and feasible structure for the TE11 to TE04 conversion, in oversized circular waveguides, using only two transitions with profile and radius optimized in order to obtain high efficiencies and sufficient bandwidth, has been designed and the simulation results are presented. The first transition is a TE11/TE01 serpentine mode converter in circular waveguide (efficiency of 95%) with average radius of 1.48 mm and an appropriate number of geometrical periods. The second one, a TE01/TE04 rippled wall mode converter with efficiency of 97%, consists of a single conversion section with an average radius of 3 mm and sixteen sinusoidal periods instead of three conversion sections (TE01/TE02, TE02/TE03, TE03/TE04) in cascade, thus reducing the complexity and the length of the full transition chain.

A Passive Wireless Sensor Network for Temperature Mapping Inside a Shielded Coaxial Enclosure

This contribution addresses the electromagnetic feasibility of the wireless temperature monitoring inside a coaxial cavity resembling a portion of a high-power high-frequency cyclotron auto-resonance maser for plasma heating in the new generation of DEMO TOKAMAK machines. The scenario is investigated as a potential communication channel for a ultrahigh-frequency radiofrequency identification (RFID) sensor network where cavity probes are used to both excite the coaxial cavity and to collect the temperature data scattered back by sensor antennas. By using a theoretical near-field analysis of a simplified model of the cavity and of the reader/sensor devices it is demonstrated that a two-probes architecture is suitable to interact with more than $N =$ 16 equally spaced RFID temperature sensors (having power sensitivity of −8.3 dB mW) over the surface of a 0.5 m tube by using less than 20 dB mW power emitted by the reader. The theoretical results are corroborated by experimental data with a mock-up of the cavity and realistic prototypes of miniaturized RFID radio-sensors and excitation probes.

Beam–Wave Interaction From FEL to CARM and Associated Scaling Laws

The development of a microwave tube providing high output power (~1 MW) at a high frequency (~250 GHz) with high efficiency of the beam–wave power conversion is a challenging task. A great deal of theoretical and experimental efforts is directed toward such a goal. A promising powerful source of microwave radiation is the cyclotron autoresonance maser (CARM) oscillator. In this paper, we revisit the well-known physical models in a way, which is suitable for their implementation in the numerical tools for computer-aided design and optimization of a CARM operating at high frequency. The analysis developed by us is an attempt directed toward the realization of an adequate design tool for the development of CARM devices.

Application of Modified Bragg Structures in High-Power Submillimeter Cyclotron Autoresonance Masers

We perform theoretical analysis and numerical simulation of cyclotron autoresonance masers, in which we propose using electrodynamic systems in the form of hybrid two-mirror cavities based on modified and conventional Bragg reflectors. It is shown that a stable regime of narrow-band generation with a multimegawatt power level can be achieved at a frequency of about 300 GHz in the described scheme of a generator based on a near-axis, moderately relativistic electron beam with an accelerating voltage of 500 kV and a current of 10 A, where the transverse size (diameter) of the interaction space is about 5 wavelengths and the guiding magnetic field is about 5 T.

Traditional vs. advanced Bragg reflectors for oversized circular waveguide

This paper compares two types of distributed Bragg reflector, based on the periodic wall perturbation of an oversized circular waveguide. The first type is a traditional mirror, where wall ripples with a period of half a guided wavelength for the working mode couple forward and backward waves. The other type is an advanced reflector with a ripple period of about a guided wavelength, exploiting an intermediate conversion into a quasi-cutoff mode. The design of both reflectors has been optimized with a mode matching code to deliver a reflectivity >96% for the TE5,3 mode at 250GHz and a power to gun <0.5% in copper waveguides with a diameter of 15mm. Such specifications are relevant to the upstream mirror of a cyclotron auto-resonance maser under development at ENEA Frascati. The two types of reflector are compared in terms of mechanical dimensions, reflectivity, bandwidth and losses.

Cyclotron auto resonance maser and free electron laser devices: a unified point of view

We take advantage of previous research in the field of cyclotron auto resonance maser (CARM) and undulator-based free electron laser (U-FEL) sources to establish a common formalism for the relevant description of the underlying physical mechanisms. This strategy is aimed at stressing the deep analogies between the two devices and at providing a practical tool for their study based on the use of well-tested scaling formulae developed independently for the two systems.

Nonthermal Microwave Emission Features under the Plasma Ohmic Heating and Low-hybrid Current Drive in the FT - 2 Tokamak

Study of behavior of accelerated electrons (AE) in the ohmically heated (OH) plasma and low-hybrid current drive (LHCD) in tokamak has the practical interest especially for estimations of AE radiation energy losses and solving of the non-inductive CD problem for the ITER. Studies based on measurements of non-thermal microwave radiation (MR) intensity and hard X-ray (HXR) spectra are conducted in the FT-2 tokamak having large local magnetic ripples under LHCD in the omically heated (OH) plasma with the fan instability excited. The paper presents experimental data first obtained in such conditions witnessing about some AE behavior features. Nature of MR abnormal intensity and short giant flashes together with the fast additional electron heating in the plasma core are discussed. The first high additional fast electron heating was registered together with the synchrotron radiation (SR) intensity increase. It was accompanied by short MR spikes observed in the narrow frequency range (53 ÷ 78) GHz. They arise owing to the AE transverse energy and pitch angle increase under each crossing the cyclotron auto-resonance region and SR maser gain. It is proposed the heating mechanism owing to absorption of SR and Bernstein waves arising at linear transformation the extraordinary component of intensive MR spikes in the black plasma layers. It was found that the non-thermal MR of abnormal intensity arising during OH in the frequency range (10 ÷ 40) GHz is due to the fan instability development and the substantial local magnetic ripples. MR is accompanied by short giant flashes having a narrow frequency spectrum. Together with the SR growth the less intensive MR flashes appear in the range (57 ÷ 75) GHz. In our case it becomes possible the maser amplification of both SR and collective radiation. Appearance of the giant flashes may be initiated under transition of the maser - amplifier into the self - excitation regime, when low-frequency quasi- coherent MR flashes are generated.

Conceptual design of pulsed high voltage and high precision power supply for a cyclotron auto-resonance maser (CARM) for plasma heating

Due to the high electron temperature during the plasma burning, both a higher power (>1MW) and a higher frequency (up to 300GHz) are required for plasma heating in future fusion experiments like DEMO. For this task, ENEA started a project to develop a cyclotron auto-resonance maser (CARM) able to produce an electron radiation in synchronism with the electromagnetic field and to transfer the electron beam kinetic energy to the plasma.This facility requires an advanced pulsed high voltage power supply (HVPS) with the following technical characteristics: variable output voltage up to 700kV; variable pulse length in the range 5–50μs; overshoot<2%; rise time<1μs; voltage accuracy (including drop, ripple and stability) <0.1%. This paper describes the conceptual design and the technical solutions adopted to achieve the performance requested for the CARM HVPS.

The High Gain Integral Equation for CARM-FEL Devices

The theory of cyclotron autoresonance maser (CARM) is formulated in a free electron laser (FEL) like fashion. We get a one-to-one correspondence between the equations describing the two devices and show that the wealth of scaling relations developed for the FEL evolution can be exploited to describe the CARM dynamics too.

THz回旋自谐振脉塞多模束波互作用非线性模拟

A nonlinear description via the self-consistent coupling equations of waves with relativistic electron beam, is provided for the nonlinear interaction of multiple modes with the relativistic electron beam in cyclotron autoresonance maser(CARM), to analyze the mode competition in CARM. Specific analysis of a sub-terahertz (0.35 THz) CARM with higher-order mode TM51 and a large-orbit electron beam shows that, contrasted with, both larger output power and higher gain can be achieved for a CARM with TM operating mode. By optimizing parameters, the operating mode can play an absolutely dominant role while the competing modes are efficiently suppressed. However, a multi-mode nonlinear simulation must be performed for a higher-order terahertz CARM, because the output power is quite sensitive to the operating parameters.

Nonlinear Simulation of a Cyclotron Autoresonance Maser (CARM) Operating in a Transverse Magnetic Mode

In the gyrotron operation, the transverse-magnetic (TM) mode is excluded because the TM mode instability vanishes when the vacuum waveguide mode and the beam mode are at grazing incidence. However, situation changes in a cyclotron autoresonance maser (CARM) interaction. In this paper nonlinear formulation of a TM-mode CARM is derived, and detailed simulations are presented for the TM1,1-mode CARM. Simulation results show that a TM1,1-mode CARM can reach high power of megawatts and ultrahigh gain of more than 70 dB, as a TE1,1-mode gyrotron traveling wave tube (gyro-TWT) and TE1,1-mode CARM did in the reported experiments.

Comparison between Single-mode Treatment and Multi-mode Treatment of a Coaxial Bragg Resonator for a Terahertz Cyclotron Autoresonance Maser Oscillator

As a coaxial Bragg structure has more complex geometry configuration than a cylindrical or a planar Bragg structure, for simplicity of calculation a single-mode treatment is often adopted. However, heterogeneity of a coaxial Bragg structure always gives rise to the multi-mode conversion, which can change the performance of the resonator. Based on the scattering matrix method, detailed comparison between the single-mode treatment and the multi-mode treatment is presented to a coaxial Bragg resonator for a cyclotron autoresonance maser (CARM) oscillator operating in the higher-order mode TE61 at frequency of 0.35 THz. Results indicate that there is a little difference between the results obtained by these two treatments to the upstream reflector due to its shallow ripple amplitude, whereas to the downstream reflector, due to its relatively deep ripple amplitude, an apparent difference occurs at the operating frequency and consequently it gives notable difference to the frequency response and Q-factor of the whole resonator. Optimization shows possible application of a coaxial Bragg resonator with a single higher-order mode operation in constructing a terahertz CARM oscillator.

Power Enhancement of a Small-Orbit Coaxial Cyclotron Autoresonance Maser (CARM) by Employing an External Radial Electrostatic Field

A proposal is presented to enhance the saturation power of a small-orbit coaxial cyclotron autoresonance maser (CARM) by employing an external electrostatic voltage. Nonlinear simulations show possible increase of 22% of the saturation power with high gain of 67 dB for a transverse electric mode at frequency of 140 GHz. Influences of initial parameters on the saturation power are simulated. It is found that due to the use of an external electrostatic field, saturation power of the wave is sensitive to the ratio of the transverse velocity to the axial velocity, as well as to the spread of the transverse velocity. The physical mechanism of the power enhancement may be that the radial force produced by the radial electrostatic field superposes an extra motion on the electron beam and thus causes an extra energy coupling between the electron beam and the radial electric component of the wave.

Terahertz transverse-magnetic-wave cyclotron autoresonance maser with a large-orbit relativistic electron beam

The cyclotron autoresonance maser (CARM) operating in a transverse-magnetic (TM) wave by employing a large-orbit relativistic electron beam shows a peculiarity that the growth rate range of coherent amplification of the TM wave is split into two branches as the electrons' cyclotron frequency increases: the upper branch is quite favorable to terahertz operation, whereas the lower branch can lead to a backward-wave CARM mechanism which is suitable to sub-terahertz operation. Compared to a terahertz large-orbit transverse-electric-wave gyrotrons and gyro-amplifiers which are experimentally realized up to date, a terahertz large-orbit TM-wave CARM requires much lower operating magnetic field and allows the cavity to have larger transverse dimension.

Linear Analysis of a Cyclotron Autoresonance Maser (CARM) Operating in a Transverse Magnetic Mode

In the fast-wave devices like gyrotron, gyro-peniotron and cyclotron autoresonance maser (CARM) that generate millimeter and sub-millimeter waves, the transverse dimensions of the resonator and the output cylindrical waveguide become small. In order to prevent loss of electrons and thermal loading of the rf structure, the electron beam must be kept relatively far from the walls. The latter requirement demands smaller transverse dimensions of the helical electron beam as well. In this paper linear formulation of a CARM operating in a general transverse-magnetic (TM) mode is derived, and a detailed analysis of the influences of the parameters is presented for the TM1,1 mode CARM. It is found that, compared to the TE1,1 mode which is often employed in gyrotron traveling wave tube (gyro-TWT) and CARM experiments, the TM1,1 mode has a greater eigen value and consequently leads to a greater waveguide radius for a given cutoff wave number, and also, allows the electron beam to be settled close to the waveguide axis to have a small transverse dimension. Results show that a TM-mode CARM can reach high power and ultrahigh gain, just as a TE-mode CARM or a TE-mode gyro-TWT does.

Mode competition in a large-orbit coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier

Physics issues of mode competition in a large-orbit coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier are investigated. It is shown that the use of a coaxial waveguide brings about two peculiarities: possible enhancement of the transverse dimensional size and the rarefied spectrum and that the applications of a large-orbit electron beam and CARM mechanism can efficiently exclude the parasitic effect of the cyclotron harmonics and spurious oscillations of undesired modes. The dispersion diagram and numerical simulation demonstrate that mode competition and spurious gyrotron-like or CARM-like oscillations can be suppressed when a large-orbit coaxial-waveguide CARM amplifier operates in a higher-order mode.

Nonlinear analysis of a large-orbit coaxial-waveguide cyclotron autoresonance maser amplifier

Nonlinear simulations are presented to analyze the influences of the electron beam and the magnetic field parameters on the output power of a large-orbit coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier. It is found that the guiding-center spread of the relativistic electrons has negligible impact on the output power due to the small field change felt by the large-orbit electrons. The electron-beam velocity spread and energy spread substantially decrease the output power, because these spreads directly affect the beam-wave interaction through the Doppler term and the relativistic cyclotron frequency term in the cyclotron resonance condition. However, this adverse effect may be offset by properly tapering the operating magnetic field. The output power is sensitive to both the slope and the amplitude of the tapered magnetic field. Nonlinear simulation demonstrates the feasibility that a large-orbit coaxial-waveguide CARM amplifier can be expected to provide output power with several megawatts, ultrahigh gain, and good bandwidth in the millimeter and submillimeter wavelength ranges.