A Theory of the Orotron with an Inclined Electron Beam

A linear and non-linear theory of the orotron with an electron beam inclined with respect to the surface of a periodic structure is presented. The beam inclination provides the possibility of the effective interaction of all particles of thick electron beam with slow evanescent harmonic of the cavity mode. On the basis of obtained analytical expression for the orotron starting current, the possibility to increase the device frequency up to 600 GHz is discussed. According to numerical simulations, the inclination of the beam allows increasing significantly both the electron efficiency and the output power of the device. The project of low-voltage orotron with the operating frequency of 100 GHz and output power of 10 W is proposed.     

Experimental study of Compton type free electron laser

In this paper, we report the initial results of the experimental study for the first Compton free electron laser in China. The relativistic electron beam of 22 MeV was injected into SmCo₅ permanent magnetic undulator with 3 cm period and total period number 98 by an RF LINAC. We obtain free electron radiation of which the average power over 1 μs macroscopic electron beam pulse train is 1·4 W at 10 μm     

Development of 28?GHz and 77?GHz, Mega-Watt Gyrotrons for Fusion Devices

A 28 GHz 1 MW with TE_8,3 cavity for GAMMA10 tandem mirror and a 77 GHz 1.5 MW gyrotron with TE_18,6 for Large Helical Device (LHD) have been developed to upgrade their Electron Cyclotron Heating (ECH) systems. In the 28 GHz gyrotron, the maximum power of 1.05 MW was obtained, which is in agreement with its design target value. And the high efficiency of 40% without collector potential depression (CPD) was obtained with 0.8 MW. In the first and second 77 GHz gyrotrons, 0.8 MW 3.6 sec., 0.3 MW 60 sec operations have been achieved, but several issues due to the stray RF and velocity dispersion have been found, too. In consideration of these, the design improvement was performed to aim at 1.5 MW in the third tube. In the short pulse test, the maximum output power of 1.6 MW and the maximum total efficiency of 49.4% with CPD were obtained. In the long pulse test, the pulse length extended to 5 sec with 1 MW, 1800 sec with 0.1 MW, 1.6 sec with 1.5 MW. Total injection power of 3.1 MW to LHD plasma has been achieved. It is shown that the electron beam pitch factor ?? decreases with increasing of beam current, comparing with the experimental and calculation results.     

Comparison of Electron-Beam-Misalignment Effect on Starting Current and Output Power in Coaxial-Cavity and Cylindrical-Cavity Gyrotron Oscillators

Comparative study is presented to the effect of the electron-beam misalignment on the starting current and output power of the coaxial-cavity and cylindrical-cavity gyrotron oscillators operating in the millimeter wave ranges. The numerical analysis is based on the gyrokinetic formulas for a TE_28,16,1 mode at a frequency of 140 GHz. Results show that the coaxial-cavity gyrotron oscillator has lower starting current and less power loss than the cylindrical-cavity gyrotron oscillator when the electron-beam axis has a misalignment to the cavity axis.     

Electron Gun for Powerful Short Pulse Gyrotron with Operating Magnetic Field 8 T

High power short pulse gyrotron with operating frequency 395 GHz operating on the second cyclotron harmonic is now under developing at FIR FU. The gyrotron is planned to use in future experiments for plasma diagnostics. For this purpose the output power about 100 kW and pulse duration 100 ns at least are needed. Preliminary estimations of parameters of some versions of the electron guns with accelerating potential U_0?=?70-100 kV were performed. Possibilities of non-adiabatic as well as adiabatic guns were considered. It was shown that non-adiabatic system is not reliable for such rather low value of U₀, the adiabatic magnetron injection gun (MIG) is more preferable for the gyrotron design. Analytical estimations of the suitable MIG dimensions and operating regime to form good quality electron beam were fulfilled. Numerical optimization of the gun shape and position was performed. It was shown that in spite of the extremely big ratio of the operating current (I_0?=?18 A) to the Langmuir current of the gun, close to 0.4-0.5, the suggested MIG can form the helical electron beam (HEB) which is suitable for gyrotron operation properties.     

110 GHz gyrotron with a built-in high-efficiency converter

A 110 GHz 1 MW pulse gyrotron has been elaborated. A built-in electrodynamic duct consisting of a new type quasi-optical converter and three matching mirrors transforms operating mode TE_15,4 into a gaussian beam going through the output window. The efficiency of the duct (ratio of the gaussian beam power outside the gyrotron to the total microwave power at the output of the cavity) is about 95%. A system consisting of the gyrotron and an outer mirror transmission line is proved to have efficiency (ratio of the microwave power measured by calorimeter at the end of line to the power of the electron beam) more than 40%.     

Hybrid Transmission Line for ECRH in the Helically Symmetric Experiment

The HSX oversized, mode-converting ECRH transmission line has been upgraded to a hybrid system to increase launched microwave power and reduce electrical arcing. Filtering of high-order, spurious modes ensures efficient coupling to a Gaussian beam for optimal electron heating. A Vlasov mode converter and two phase-correcting ellipsoidal mirrors convert the TE₀₂ gyrotron output mode to a symmetric, linearly polarized, microwave beam. A swappable twist reflector plate rotates beam polarization for 2nd-harmonic X-mode or fundamental O-mode ECRH. Long distances are traversed by coupling the beam to a dual-mode (TE₁₁?+?TM₁₁), smooth, circular cross-section waveguide. This system has been successfully tested without arcing for 50 ms pulses and over 100 kW of launched power. Analysis of the microwave beam for 50 kW, 2 ms microwave pulses reveals agreement with predicted beam shapes at two beam locations. The new system has also demonstrated increased plasma stored energy for ECRH plasmas with equal launched power.     

Generation of High-Power Sub-THz Waves in Magnetized Turbulent Electron Beam Plasmas

Sub-THz radiation can be generated by conversion of plasma waves into electromagnetic (EM) radiation in a plasma with strong Langmuir (LT) turbulence produced via a two-stream instability of a high current relativistic electron beam (REB). Nonlinear plasmon-plasmon merging results in the generation of photons nearby the 2^nd harmonic of the plasma frequency 2ω _p (“2ω _p -process”). For plasma densities of 10¹⁴???10¹⁵?cm^?3, these frequencies are in the range of sub-THz waves at 370–570 GHz. The specific power density of sub-THz-wave emission from plasmas in the multi-mirror magnetic trap GOL-3 (at BINP) during injection of a 10-μs-REB with a current density of about 1 kA/cm² at plasma densities n _e ?≈?5?10¹⁴?cm^?3, electron temperatures T _e ?≈?1.5 keV and magnetic induction B?≈?4 T was measured to be approx. 1 kW/cm³ in the frequency band around 300 GHz. In the case of a weakly relativistic 100-μs-electron beam (90 keV) with 250 A/cm² the corresponding results are 700 W/cm³ around 90 GHz with an efficiency of 1–2 % at n _e ?≈?3?10¹³?cm^?3 (total power?≈?30 kW). Theoretical investigations show that at a density of n _e ?≈?3?10¹⁵?cm^?3 and a turbulence level of 5 % the generated sub-THz power can reach?≈?1 MW/cm³.     

Performance Test of CW 300 GHz Gyrotron FU CW I

A 300 GHz CW gyrotron FU CW I has been developed and installed in the Research Center for Development of Far-Infrared Region, University of Fukui as a power source of a high frequency material processing system. Its performance was tested and the maximum power of 1.75 kW / CW has been attained at the beam voltage of 15 kV and the beam current of 1A. The maximum window power efficiency of 15.5% has been obtained at the cathode voltage slightly lower than 15 kV. This gyrotron is designed to deliver a Gaussian beam after mode conversion from the oscillation mode TE_22,8 in the cavity with a complex of an internal radiator and beam shaping mirrors. The detailed measurement with an infrared camera has confirmed that a Gaussian beam is radiated when the magnetic field strength B _c at the cavity is adjusted at a proper value. However, within a range of B _c , the output power is emerged into multiple directions, which suggests simultaneous oscillation of competing cavity modes.     

A long-pulse,CARM oscillator experiment

Results of a long-pulse, cyclotron autoresonance maser (CARM) oscillator experiment are reported. A Stanford accelerator (SLAC 5045) klystron gun produced a 1 pis electron beam at 250-300 kV and 10-25 A at a repetition rate of up to 4 Hz. A beam α≡β⊥/β₁, variable from 0 to 1, was produced on this electron beam with a wiggler magnetic field near guide field resonance. The experiments were carried out with two different Bragg reflection resonators designed for the TE₁₁ mode. Using the first resonator, many harmonic gyrotron modes were observed in the 28-40GHz frequency range in the TE₂₁ and TE₀₁ modes and, for the first time, a second harmonic upshifted (CARM) TE₁₁ mode at 74-5 GHz. Using the second resonator, fundamental CARM operation was observed for many parameter settings, and for frequencies ranging from 29 to 32 GHz. Output powers ranged from ～01 kW to l00kW, resulting in efficiencies of 01% to 2%. Identification of CARM and gyrotron modes is made by comparison of measured frequencies with dispersion theory and measurement of the farfield radiation pattern. Comparison with theory indicates that the device efficiency is reduced by finite spread in the axial electron beam momentum.     

Analyses of two-cavity gyroklystron with feedback

Making use of the linear kinetic theory and the perturbation approach, a gyroklystron with feedback, in which a low order (TE₀₁₁) mode is presented in the bunching cavity by the feedback of the output cavity, and the low order mode in the bunching cavity is coupled by the electron beam to a high order (TE₀₂₁) mode in the output cavity, has been investigated in detail. And electron beam to wave interaction in each cavity and electron bunching mechanism in the bunching cavity have been analysed. The formulas of electron beam to wave interaction power and starting current etc. have been derived. Then some calculations are carried out.     

An Ultrahigh-order-mode, Higher-harmonic Coaxial Gyrotron Oscillator in Sub-terahertz Wave Range

A coaxial cavity gyrotron oscillator at a frequency of 0.34 THz is studied, which operates with a quite low magnetic field of 4.55 Tesla at the third cyclotron harmonic of the ultrahigh-order mode TE_43,4. Properly choosing the depth of the longitudinal corrugations on the inner rod and optimizing the electron-beam position significantly suppress the mode competition. Nonlinear multimode simulations show the feasibility of the single-mode operation with an output power of 163 kW by using an electron beam with a voltage of 70kV and a current of 30A, which corresponds to an interaction efficiency of 9.2 % with maxim density of ohmic losses 2.9 kW/cm².     

Design of a CW 1 THz Gyrotron (Gyrotron Fu Cw III) Using a 20 T Superconducting Magnet

Design of a CW 1 THz gyrotron at second harmonic operation using a 20 T superconducting magnet has been described. The mode competition analysis is employed to investigate operation conditions of second harmonic mode, which is being excited at the frequency ranging from 920 GHz to 1014 GHz. The output power up to 250 watt corresponding to the efficiency of 4.16 percent could be achieved by using an electron beam with accelerating voltage 30 kV and current 200 mA. The important advantage of this gyrotron is that the single mode excitation at second harmonic, and extremely high frequency of the radiation, could be maintained even at high currents. It opens possibility to realize a high power radiation source at 1 THz. Such gyrotron is under construction at FIR Center, University of Fukui.     

Quantitative Auger Electron Spectroscopic Analysis of Hg1−x Cd x Te

Auger electron spectroscopy of Hg_1?x Cd _x Te has been investigated for quantitative analysis. HgCdTe is known to be a very sensitive material which easily suffers from ion beam and electron beam interactions. Two methods were compared for accurate and reproducible quantification of the chemical composition. The first strategy was to reduce as much as possible the damage caused by surface preparation and the effect of the incident electron beam on the material. Quantification was then achieved by use of relative sensitivity factors estimated by use of calibration reference samples. This method was rejected because of unavoidable beam damage which resulted in different chemical changes for the reference CdTe and the HgCdTe sample of interest. The second strategy was to precisely control the experimental conditions to ensure reproducible degradation. Quantification was achieved by analysis of reference HgCdTe samples with different Cd composition. Successful quantification was achieved on stoichiometric material of composition of 0.2 < X _Cd < 0.3 with a X _Cd discrimination limit of ΔX _Cd = 0.02 and an analysis step of 10 nm.     

Eigenfrequency Shift of Higher-Order Modes in a Coaxial Cavity with Eccentric Inner Rod

The eigenvalues of a coaxial cavity must be modified by the structural eccentricity (for instance, by the misalignment of the inner rod), which causes an eigenfrequency shift. In this paper the eigenfrequency shift of the higher-order modes is numerically investigated in terms of the eigenvalue equation. Taking the TE_31, 17, 1, TE_32, 17, 1, TE_33, 17, 1 and TE_34, 17, 1 modes as examples, calculations show that the eigenfrequency of a mode may have a down-shift or up-shift, which depends on the ratio of the outer conductor radius to the inner rod radius R _out/R _in. For a higher-order mode, the greater the value of R _out/R _in, the smaller the influence of the structural eccentricity on the eigenfrequency shift. Moreover, the structural eccentricity may have a weaker influence if the azimuthal index of the mode is higher.     

Hysteresis in Mode Competition in High Power 170 GHz Gyrotron for ITER

Hysteresis in mode competition in high power 170 GHz gyrotrons for ITER is studied. Specific examples of hysteresis with respect to variation of magnetic field, current, electron beam radius and beam voltage are given.     

System Development and Performance Testing of a W-Band Gyrotron

A high-power W-band gyrotron has been designed and performance tested in Korea, with an output power in the range of tens of kilowatts. The gyrotron consists of a diode-type electron gun operating at 40 kV, a TE6,2 mode interaction cavity, and a mode converter for producing a highly Gaussian output mode beam. Presented here are the detailed component design procedure and the experimental results of the gyrotron’s performance evaluation. A maximum power of 62 kW was achieved with an efficiency of 22 %, and a highly Gaussian output beam was observed. The gyrotron’s output beam is analyzed, and its transmission through an oversized waveguide is discussed. This gyrotron is the first gyrotron developed in Korea with high power greater than 10 kW and high frequency greater than 90 GHz.     

Continuously Frequency Tunable High Power Sub-THz Radiation Source—Gyrotron FU CW VI for 600 MHz DNP-NMR Spectroscopy

A high frequency gyrotron with a 15 T superconducting magnet named Gyrotron FU CW VI has achieved continuous frequency tuning through the relatively wide range of 1.5 GHz near 400 GHz. The operation is at the fundamental cyclotron resonance of the TE₀₆ cavity mode with many higher order axial modes. The output power measured at the end of the circular waveguide system ranges from 10 to 50 watts at the low acceleration voltage of 12 kV for beam electrons. The beam current is also low. It is around 250 mA. This gyrotron is designed as a demountable radiation source for the 600 MHz DNP-NMR spectroscopy. The design and operation results of the gyrotron FU CW VI are presented.     

Design of a 300 GHz Band TWT with a Folded Waveguide Fabricated by Microelectromechanical Systems

For future broadband wireless links, we have designed a 300 GHz band traveling wave tube (TWT) with a folded waveguide fabricated by microelectromechanical systems (MEMS). The TWT operates at a beam voltage of 12 kV and a beam current of 8.3 mA. The classical large signal simulation code predicts the output power greater than 1 W and gain larger than 20 dB over the bandwidth from 280 to 300 GHz.     

A cyclotron resonance frequency multiplier

The current density available from known thermionic cathodes imposes a serious limitation on the power output obtainable from electron tubes operating at frequencies above about 10 Gc. This paper describes a proposal for a frequency multiplier which has the interesting property that the RF energy imparted to the electron beam by the signal source may be one or two orders of magnitude greater than the dc energy supplied by the battery. Thus it is expected that the power output at a particular frequency will be much higher for a given cathode current density than that obtained from conventional types of electron-beam oscillators and amplifiers. The multiplier operates by converting the input energy at one radio frequency into rotational energy of an electron beam by means of a Cuccia coupler. The beam then couples power to the output load via a multicavity circuit containing2ncavities wherenis the frequency multiplication to be obtained. The paper includes details of experimental work on a low-power tube multiplying from 600 Mc to 3600 Mc. In this tube 1.3 watts of RF input power is coupled on to a 50 volt, 260-µa beam and the power at 3600 Mc is coupled out via a 12-cavity magnetron anode structure. Photographs are given of the trace of the rotating beam impinging on a fluorescent screen and these demonstrate the degree of focussing achieved along the length of the tube. The paper concludes with a study of the feasibility of a tube multiplying from 10 Gc to 100 Gc with an output of tens of watts.