Transverse Velocity Spread Research Articles

Genetic Algorithm-Based Shape Optimization of Modulating Anode for Magnetron Injection Gun With Low Velocity Spread

The magnetron injection gun (MIG) is an important component in the gyro-traveling wave tube (gyro-TWT). The electron velocity spread induced to the mismatch of the electric and magnetic fields influences the performance of gyro-TWT. To improve electron beam quality, we designed a modulating anode with curved geometry. And the multi-objective genetic algorithm was employed to optimize the curved geometry. An electron beam with a velocity ratio of 1.05 and low transverse velocity spread of 0.31% in a MIG for a Q-band gyro-TWT is obtained. Compared with the previous design with a transverse velocity spread of 1.17%, the electron beam performance got improved. And the thermal analysis and parametric sensitivity were done. The curved MIG can stably provide an electron beam with a transverse spread lower than 0.8%.

Magnetron injection gun design for a Q-band 300 kW 30 A gyrotron traveling wave tube

This paper is intended to present the optimal design of a triode type magnetron injection gun (MIG) for a 300 kW, 30 A gyrotron traveling wave tube (gyro-TWT), which is operated at Q band fundamental TE01 mode. Based on the analysis of velocity ratio (VR) distribution along the emission strip (ES), a further optimization of cathode geometry on the basis of a preliminary optimized gun is performed, and a new cathode structure is proposed. Compared with initial optimal parameters, the new structure demonstrates a decline of transverse velocity spread (TVS) from 3.66% to 0.57% and longitudinal velocity spread (LVS) from 4.11% to 0.72%, while VR is maintained at 1.05. The achieved overall LVS reaches as low as 3.44% when considering cathode surface roughness and thermal temperature effect. The sensitivity study has been carried out by changing the gun parameters like anode voltage, beam current, and cathode magnetic field to ensure the practical operation stability.

Design of a large-orbit electron gun with gradual reversal magnetic field for a W-band permanent magnet peniotron

This paper demonstrates the operating characteristics of an axis-encircling large-orbit electron gun and a corresponding permanent magnet system of a W-band peniotron. The practical novel large-orbit electron beam with axial velocity spread of 5.17%, transverse velocity spread of 1.28%, guiding center deviation ratio 10.7%, and high velocity ratio 2 is designed. The main advantages of such a peniotron in the large-orbit configuration with a permanent magnet system are its compact size, reduced operational gap, and lower cost. Critical characteristics, such as acceleration voltage and the relative axial position of the magnetic system and electrode system have also been well examined and it is shown that a careful control of them is important for the device performance.

Electron Beam Emission and Interaction in 0.3-THz Gyrotron for Second Harmonic, CW Operation

This paper describes the design of a 0.3-THz gyrotron at second harmonic operation. The interaction cavity design and the beam-wave interaction computation are carried out for TE0,6 mode by using the particle-in-cell electromagnetic simulation approach. The simulation results confirm > 6-kW output power with efficiency ~ 17% by using an electron beam with accelerating voltage 20 kV and current 2.0 A. To achieve the required electron beam parameters, a triode magnetron injection gun is designed. The transverse-to-axial velocity ratio (α), 1.27 and the transverse velocity spread <; 4% are realized. Considering the fabrication and operation of the device, the parametric analysis is also performed.

Development of a Magnetic Cusp Gun for Terahertz Harmonic Gyrodevices

A magnetic cusp gun (MCG) is being developed to generate an axis-encircling electron beam, which is called the large orbit beam, which is going to drive a 0.396-THz fourth-harmonic gyrotron. Developing an MCG imposes crucial challenges on a simultaneously minimizing guiding center deviation and velocity spread of the electron beam, particularly because an ultrahigh magnetic compression ratio is unavoidable, as is the case for a terahertz (THz) gyrotron. The study of the electron dynamics in the MCG reveals that, close to the emitter, a pair of focusing electrodes are employed to construct a special focusing and accelerating electric field as a way to balance the space-charge influence and guiding center deviation. Investigation indicates that both the electron-beam generalized-angular-momentum spread and the guiding center distribution are the critical factors contributing to beam parameter spread. Intensive optimization generates a high-power MCG with a pitch factor of 1.5, the highest magnetic field of 4 T, minimum transverse velocity spread of 1.1%, and a beam current of 2 A. The key parameters exhibit excellent stability tuning over a wide range of beam current and magnetic field. These merits enable the harmonic gyrotrons or even the frequency-tunable THz gyrotrons to be developed.

Towards a 1 MW, 170 GHz gyrotron design for fusion application

The electrical design of different components of 1MW, 170GHz gyrotron such as, magnetron injection gun, cylindrical interaction cavity and collector and RF window is presented in this article. Recently, a new project related to the development of 170GHz, 1MW gyrotron has been started for the Indian Tokamak. TE34,10 mode is selected as the operating mode after studied the problem of mode competition. The triode type geometry is selected for the design of magnetron injection gun (MIG) to achieve the required beam parameters. The maximum transverse velocity spread of 3.28% at the velocity ratio of 1.34 is obtained in simulations for a 40A, 80kV electron beam. The RF output power of more than 1MW with 36.5% interaction efficiency without depressed collector is predicted by simulation in single-mode operation at 170GHz frequency. The simulated single-stage depressed collector of the gyrotron predicted the overall device efficiencies>55%. Due to the very good thermal conductivity and very weak dependency of the dielectric parameters on temperature, PACVD diamond is selected for window design for the transmission of RF power. The in-house developed code MIGSYN and GCOMS are used for initial geometry design of MIG and mode selection respectively. Commercially available simulation tools MAGIC and ANSYS are used for beam–wave interaction and mechanical analysis respectively.

Design of a Triode Magnetron Injection Gun for a 1-MW 170-GHz Gyrotron

A triode magnetron injection gun (MIG) is designed for a 1-MW 170-GHz gyrotron, which will be used in an electron cyclotron resonance heating system for the International Thermonuclear Experimental Reactor. The initial design of MIG is performed by using the in-house code MIGSYN. For the electron-beam emission and propagation, the 2-D code EGUN is implemented. The transverse-to-axial velocity ratio α is 1.37, and the transverse velocity spread less than 4% is realized. The results are also validated against two other 2-D and 3-D codes, i.e., TRAK and CST-Particle Studio, respectively, which are in close agreement. From the fabrication and operation point of view, the parametric analysis and misalignment study of electron beam are also performed.

Electron beam emission and interaction of double-beam gyrotron

This paper presents the numerical simulation of a double-beam magnetron injection gun (DB-MIG) and beam-wave interaction for 60GHz, 500kW gyrotron. The beam-wave interaction calculations, power and frequency growth estimation are performed by using PIC code MAGIC. The maximum output power of 510kW at 41.5% efficiency, beam currents of 6A and 12A, electron beam velocity ratios of 1.41 and 1.25 and beam voltage of 69kV are estimated. To obtain the design parameters, the DB-MIG with maximum transverse velocity spread less than 5% is designed. The computer simulations are performed by using the commercially available code EGUN and the in-house developed code MIGANS. The simulated results of DB-MIG design obtained by using the EGUN code are also validated with another trajectory code TRAK, which are in good agreement.

Design of Magnetically Tunable Magnetron Injection Guns for Gyrotrons at Multiple Frequencies

In this paper, the design of a triode-type magnetically tunable magnetron injection gun (MT-MIG) is presented for 1-MW 110-, 120-, and 127.5-GHz-frequency gyrotrons with the various operating modes. Some basic equations relevant to the problem available in the literature have been used to obtain the preliminary design. Computer simulation has been performed by using the commercially available code EGUN. The triode-type MT-MIG with the accelerating voltage of 80 kV, the beam current of 40 A, the average transverse-to-axial velocity ratio of the electron beam of 1.26-1.35, and the maximum transverse velocity spread of less than 5% has been designed. Tuning of the MT-MIG is performed by varying the magnetic field.

Simulation for Aberration of Chromium Atomic Beam Focusing and Deposition

The image distortion which comes from aberration is analyzed and the effects on focal line features are also discussed, which are resulted from the spherical aberration, chromatic aberration and beam spread. The simulation results have show that source imperfection, especially the transverse velocity spread, plays a critical role in broadening the feature width.

A bright, slow cryogenic molecular beam source for free radicals

We demonstrate and characterize a cryogenic buffer gas-cooled molecular beam source capable of producing bright beams of free radicals and refractory species. Details of the beam properties (brightness, forward velocity distribution, transverse velocity spread, rotational and vibrational temperatures) are measured under varying conditions for the molecular species SrF. Under typical conditions we produce a beam of brightness 1.2 × 10(11) molecules/sr/pulse in the X(2)Σ(+)(v = 0, N(rot) = 0) state, with 140(m/s) forward velocity and a rotational temperature of ≈ 1 K. This source compares favorably to other methods for producing beams of free radicals and refractory species for many types of experiments. We provide details of construction that may be helpful for others attempting to use this method.

Design of 84 GHz 500 kW Gyrotron for Technological Millimeter Wave Applications

In this paper, the design and the numerical simulation of the 84 GHz, 500 kW gyrotron at first harmonic operation for technological applications is described. The operating mode of the gyrotron is TE10, 4. Various in-house developed and commercially available computer codes are used for the design purpose. The triode-type magnetron injection gun with the accelerating voltage 70-kV, the beam current 10-A and the maximum transverse velocity spread less than 5% is designed. The preliminary design is obtained by using some trade-off equations. The mode selection, the cold cavity and the beam-wave interaction analysis are discussed for the design of weakly tapered open resonator type of the interaction cavity. The parametric analysis of the interaction cavity and the electron gun is also presented.

Design of Electron Gun for 1.5 MW, 140 GHz Gyrotron

This paper presents the design of the triode type electron gun for a 140 GHz, 1.5 MW gyrotron with the transverse to the axial velocity ratio of the beam 1.4 and the transverse velocity spread 1.28%. The operating mode of the gyrotron is TE24,8 and it is operated in the fundamental harmonic. The analytic trade-off equations for the electron gun design have been used to estimate the initial gun parameters. The electron trajectory tracing program has been used to optimize the electron gun design. The parametric dependences of modulating anode voltage, beam voltage and cathode magnetic field on the beam quality has also been studied.

A slow gravity compensated atom laser

We report on a slow guided atom laser beam outcoupled from a Bose-Einstein condensate of 87Rb atoms in a hybrid trap. The acceleration of the atom laser beam can be controlled by compensating the gravitational acceleration and we reach residual accelerations as low as 0.0027 g. The outcoupling mechanism allows for the production of a constant flux of 4.5x10^6 atoms per second and due to transverse guiding we obtain an upper limit for the mean beam width of 4.6 \mu\m. The transverse velocity spread is only 0.2 mm/s and thus an upper limit for the beam quality parameter is M^2=2.5. We demonstrate the potential of the long interrogation times available with this atom laser beam by measuring the trap frequency in a single measurement. The small beam width together with the long evolution and interrogation time makes this atom laser beam a promising tool for continuous interferometric measurements.

Numerical Simulation of a Double-anode Magnetron Injection Gun for 110 GHz, 1 MW Gyrotron

A 40 A double-anode magnetron injection gun for a 1 MW, 110 GHz gyrotron has been designed. The preliminary design has been obtained by using some trade-off equations. The electron beam analysis has been performed by using the commercially available code EGUN and the in-house developed code MIGANS. The operating mode of the gyrotron is TE22,6 and it is operated in the fundamental harmonic. The electron beam with a low transverse velocity spread (\( \delta {\beta_{ \bot \max }} = 2.26\% \)) and the transverse-to-axial velocity ratio of the electron beam (α) = 1.37 is obtained. The simulated results of the MIG obtained with the EGUN code have been validated with another trajectory code TRAK. The results on the design output parameters obtained by both the codes are in good agreement. The sensitivity analysis has been carried out by changing the different gun parameters to decide the fabrication tolerance.

MAGY Simulations of Mode Interaction in a Coaxial Gyrotron

Mode interactions in coaxial gyrotrons are studied numerically using the Maryland gyrotron code (MAGY), which has recently been modified to treat coaxial structures. Simulations are focused on the study of the FZK 170 GHz coaxial gyrotron. This device is being considered for electron cyclotron plasma heating in International Thermonuclear Experimental Reactor; however, current performance is below expectations based on previous simulations. The mode competition process during the voltage rise includes at least six modes in two groups (corotating and counterrotating with an electron beam). Simulations show that the sequence of modes achieving a measurable level of output power during start-up depends on several electron beam parameters: voltage, current, beam radius, beam thickness, pitch angle, and spread in transverse velocity. MAGY simulations made with nominal parameters confirm the results of previous simulations. Two beam parameters are not well characterized: pitch angle and spread in transverse velocity. We found that the measured results are consistent with a decrease in the pitch angle and a 3% transverse velocity spread. Also, the effect of modes at the cyclotron harmonic on the fundamental cyclotron harmonic modes is studied and discussed.

Analysis of Aberrations in Laser-Focused Nanofabrication

Based on the semi-classical model,we analyse the motion equation of chromium atoms in the laserstanding wave field under the condition of low intensity light fieldusing fourth-order Adams–Moulton algorithm. The trajectory of theatoms is obtained in the standing wave field by analytical simulation.The image distortion coming from aberrations is analysedand the effects on focal beam features are also discussed. Besidesthese influences, we also discuss the effects on contrast as well asthe feature width of the atomic beam due to laser power and laser beamwaist. The simulation results have shown that source imperfection,especially the transverse velocity spread, plays a critical role inbroadening the feature width. Based on these analyse, we present somesuggestions to minimize these influences.

Numerical Simulation Study of Double-Beam Magnetron-Injection Guns for High-Power Gyrotrons

This paper describes the detailed design of the double-beam coaxial magnetron-injection gun (DBMIG) for use in Ka-band coaxial gyrotron with two beams operating in the harmonic mode. The DBMIG operating voltage is 70 kV with a total beam current of 20 A, evenly divided among the beamlets, and provides two 70-kV 10-A beams with a velocity ratio of 1.5. The minimum of the transverse velocity spread and the axial velocity spread are about 3% and 5%, respectively. The emitter current density has been kept below 10 Amiddotcm-2 (space-charge limited). The primary computational tools used in the design process were the self-consistent particle-in-cell simulation code MAGIC

Experimental and theoretical investigation into the effect of the electron velocity distribution on chaotic oscillations in an electron beam under virtual cathode formation conditions

The effect of the electron transverse and longitudinal velocity spread at the entrance to the interaction space on wide-band chaotic oscillations in intense multiple-velocity beams is studied theoretically and numerically under the conditions of formation of a virtual cathode. It is found that an increase in the electron velocity spread causes chaotization of virtual cathode oscillations. An insight into physical processes taking place in a virtual-cathode multiple-velocity beam is gained by numerical simulation. The chaotization of the oscillations is shown to be associated with additional electron structures, which were separated out by constructing charged particle distribution functions.

A novel simplified two-dimensional magneto-optical trap as an intense source of slow cesium atoms

We describe the design and performance of a slow atom source based on a 2D magneto-optical trap (MOT) that uses an innovative simple optical configuration. Metal-coated retro-reflecting prisms replace mirrors and quarter-wave plates so the optical power of the cooling laser beam is recycled. This source has been characterised for three different configurations: with and without transverse magnetic field gradient, and with a pusher beam to obtain a 2D + -MOT. The longitudinal velocity is of the order of 25 m·s −1 , with a transverse velocity spread ≤1 m·s −1 , while the typical atomic flux density obtained is up to 1.3×10 14 at ·s −1 ·m −2 for a cesium vapour pressure of ~4×10 −8 mbar in the source. We use this slow atom beam, instead of cesium vapour, to load a 3D moving optical molasses that feeds a continuous cold atom fountain. We obtain a gain of a factor ~20 in the atomic flux launched by the fountain.