Beam Velocity Spread Research Articles

Helical Electron Beam Status Online Evaluation for Magnetron Injection Gun

The magnetron injection gun (MIG) is an essential component of the gyrotron traveling wave tube (gyro-TWT). Although the electron beam status influences the performance of the device, it cannot be measured directly in the experiment. An online evaluation module (OEM) for the experiment is developed to calculate the instant beam parameters of MIG. The OEM, by reconstructing the geometry of the MIG and related magnetic field distribution, can obtain the electron beam status under the operating parameters through the online simulation. The beam velocity spread of thermal emission with instant temperature and surface roughness are also considered. The validation is done in a W-band gyro-TWT, and the beam performance is evaluated in the experiment. With a pitch factor of 1.06 electron beam, the velocity spread affected by the electric-magnetic mismatch, thermal emission, and roughness is 1.00%, 0.56%, and 0.43%, respectively. The other beam parameters are also presented in the developed module. The OEM could guide and accelerate the testing process and ensure the safe and stable operation of the device.

Effect of Beam Velocity Spread With Spatial Variation on Mode Excitation in Gyrotrons

Effect of Beam Velocity Spread With Spatial Variation on Mode Excitation in Gyrotrons

Formation of Sheet Helical Electron Beams for High-Power Planar Gyrotrons

Planar magnetron-injection gun forming a powerful (3 MW) sheet helical electron beam (HEB) for 140 GHz gyrotron was designed. Modified scheme of an electron beam analyzer based on the retarding electric field method was developed to measure the electron beam characteristics, including pitch-factor and velocity spread. Experimental results are in good agreement with theoretical predictions and demonstrate feasibility of formation of a wide (20 mm) sheet HEB with an energy of 100 keV, a current of 30 A and a pitch-factor of above 1.2 – 1.3.

Design and Cold Test of a G-Band 10-kW-Level Pulse TE01-Mode Gyrotron Traveling-Wave Tube

Overall design and cold test of a G-band gyrotron traveling-wave tube (gyro-TWT) amplifier are presented. This gyro-TWT aims at achieving a goal of 10-kW pulse output power in the range of 210–220 GHz. It is operated in a circular TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode at the fundamental cyclotron harmonics, which is driven by a 50-kV, 3-A gyrating electron beam. Low velocity spread diode-type magnetron injection gun (MIG), multichannel TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode input coupler, broadband metasurface output window, and lossy material loaded beam–wave interaction circuit are simulated and partially measured. Particle-in-cell (PIC) simulation shows that the designed gyro-TWT can achieve a saturated output power over 10 kW in the range of 210–228 GHz with a beam velocity spread of 2.29%.

Experimental Generation of 1.1-kA Gyrating Electron Beam Current From an Explosive Emission Cathode Magnetron Injection Gun

Most high-power gyrotrons are driven by the thermal cathodes, and the output power can only reach megawatt (MW) level. In the present study, a single anode explosive emission cathode (EEC) magnetron injection gun (MIG) with the high beam current, the pitch radio of 1.2 and the controlled beam velocity spread was designed. At the peak voltage of 300 kV, the peak beam current of 1.1 kA and pulsewidth of 170 ns were emitted by a carbon fiber array (CFA) emitter. Compared to the reported EEC MIG, large beam loss was avoided in this experiment. The guiding center radius of 14.1 mm was close to the simulation results. CFA has the stronger emission capability, the lower emission threshold electric field, and the better emission uniformity than sharpened edge graphite (SEG), so it is suggested as the emitter of the EEC MIG.

RF behavior of a 35 GHz conventional cavity gyrotron using multimode analysis and PIC simulation

ABSTRACT In this paper, the RF behavior of a 35 GHz, TE 04 mode cylindrical cavity gyrotron is investigated using the nonlinear multimode theory and PIC code “CST Particle Studio”. A code is developed to realize the beam-wave interaction mechanism using time-dependent multimode theory. The cold cavity behavior is observed for the cavity operation in the designed TE 04 mode and 35 GHz frequency. The magnetic field profile is kept uniform along the length of the interaction structure. The output power is estimated for the designed mode as well as all nearby competing modes using the multimode analysis and PIC simulation. This yields the output power more than 200 kW with 33% efficiency in the designed TE 04 mode, whereas the smaller output power in the nearby competing modes. Power sustainability is also observed for the stable operation of the device. The effects of beam velocity spread and detuning of magnetic field on its performance are also observed.

Design and simulation of a W-band gyrotron traveling wave amplifier

The design of a W-band gyrotron traveling wave tube amplifier is presented. The device with a lossy ceramic interaction circuit operates in the fundamental harmonic TE01 circular electric mode. The 70-kV, 6-A electron beam is produced by a double anode magnetron injection gun with an average perpendicular-to-parallel velocity ratio of 1.2 and a parallel velocity spread of less than 3%. The beam-wave interaction is investigated by using a particle-in-cell code. The simulations predict that the amplifier can produce an output peak power of over 100 kW, 28% efficiency, 42 dB gain, and a 3 GHz bandwidth, respectively under the assumption of beam velocity spread 3%.

Suppression of the Parasitic Oscillations in anX-Band Relativistic Coaxial Gyrotron Driven by an Intense Beam Current

In 100-MW level relativistic gyrotrons driven by explosive emission cathodes (EECs), significant parasitic oscillations are caused by the high beam current and the large beam velocity spread. In this study, the parasitic oscillations in an X-band coaxial relativistic intense current gyrotron were explored. The oscillations were suppressed in particle-in-cell (PIC) simulations through modifications of design. Lossy ceramics were inserted to suppress the parasitic modes in the beam tunnel. The beam collector was optimized to keep the electron beam from the cyclotron resonance region of the parasitic modes. In the 3-D PIC simulation of the whole tube, the output power of 35 MW and efficiency of 20.6% were achieved under the voltage of 300 kV, the beam current of 567 A, the pitch factor of 1.24, and the velocity spread of 25.6%. A single TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode operation was achieved in this relativistic coaxial gyrotron.

Theory Analysis of Nonlinear Excitation of Second Harmonic THz Gyrotron for DNP-NMR

As an important parameter, start current reveals the excitation condition of cavity modes. Normally, the method of successive iterations with one mode associated in the calculation is used to get the start current, which just reflects the self-excitation conditions in the presence of single mode. Nevertheless, in many cases, the prior-excited mode may affect the start current of the subsequent ones. In this article, we re-derived electron motion equations with two modes involved in the calculation and calculated the start current numerically based on those equations. We analyzed start currents of operation mode TE26+ and parasitic modes of a 394-GHz gyrotron with the two methods. In the calculation using the conventional method, TE26+ satisfies self-excitation condition while TE26− does not. In the calculation using the latter method, the start current of TE26− decreases under the influence of TE26+. This nonlinear oscillation of TE26− is further observed in time domain multimode simulation. In practical application, the beam velocity spread, ohmic loss, and high-frequency loss are inevitable. We studied their effects on mode interaction and then optimized the operation parameters. When the equivalent conductivity is $1.7e^{7}~\text {s/m}$ and beam velocity spread is 0.03, the optimized output power is 149.2 W.

Modeling of parasitic oscillations in smooth-wall circular symmetric dielectric-loaded gyrotron beam ducts

The self-consistent simulation of the electron beam interaction was performed in the presence of a lossy dielectric layer in the smooth-wall beam duct of a gyrotron oscillator. For this purpose, the linear and spectral code TWANGlinspec was extended. In this code, the local transverse structure of the TE mode is adapted to the solution of the complex cold dispersion relation of an infinite, homogeneous, dielectric coated cylindrical waveguide. Before considering the realistic situation, the validity of the TE pure mode (Ez = 0) assumed in TWANGlinspec had to be assessed for SiC or BeOSiC materials. The effect of the dielectric layer on the parasitic starting current is large for parasitic oscillations localized at the end of the beam duct and in the so-called spacer region. For the realistic case, the geometry of the dual frequency gyrotron for the Tokamak à Configuration Variable was considered, and the effect of a dielectric layer was numerically investigated. The electron beam velocity spread was also included in the simulations. The result is that in such conditions, the parasitic oscillation starting currents are increased above the operating beam current. The observation of these spurious modes would therefore not be expected.

Investigation of the Effect of Electron Beam Quality on a 0.68-THz Second Harmonic Gyrotron

The lack of high-power terahertz sources is a major impediment to the development of terahertz science and technology. The gyrotron has great potential to generate high-power terahertz radiation, and so attracts extensive attention. In order to alleviate the problems of limited power capacity and wall loading, the high-order modes are often chosen to be the operating modes. Due to the dense spectrum of the modes, it will cause intense mode competition, which is harmful for the stable operation of a gyrotron. Currently, a 0.68-THz second harmonic gyrotron is under development in the Terahertz Research Center of University of Electronic Science and Technology (UESTC). The experiment shows that the measured output power is 2.75 kW with the efficiency of 9.05%. In order to determine whether the operating mode is the desired mode TE−15,2, the calculation of the starting current should be more accurate. In this article, we generalize the existing linear theory for the starting current, considering the effects of beam thickness and velocity spread. The starting currents of the operating mode and its competing modes are calculated using our designed gyrotron. We find out that the velocity spread and the beam thickness have significant influence on the starting current. The calculations also show that the desired mode TE−15,2 can be triggered first at the operating point. Besides, the measured output power from the experiment is much lower than that of the simulation result. Therefore, the effects of the velocity spread and beam thickness on beam–wave interactions are also analyzed; our results show that the efficiency is sensitive to the beam quality.

Parasitic Oscillations in Smooth-Wall Circular Symmetric Gyrotron Beam Ducts

In order to study parasitic oscillation that may occur in a realistic beam duct upstream to the gyrotron cavity, the self-consistent linear and spectral code TWANGlinspec has been modified. The large inhomogeneities in the smooth-wall beam duct geometry or in the magnetic field profile required the implementation of a numerical approach using a hybrid finite element method. The new model permits to characterize a large number of potentially spurious TE modes. Compared to previous studies on gyrotron beam duct instabilities, an extended interaction space including also the gyrotron cavity has been considered. The role of the connecting part between the beam duct and the cavity, called spacer, is highlighted and it is shown that the gyro backward-wave TE modes excited in this region generally have their minimum starting current. The sensitivity of the minimum starting current on electron beam velocity spread is also evaluated.

Investigation on the optimal magnetic field of a cusp electron gun for a W-band gyro-TWA

High efficiency and broadband operation of a gyrotron traveling wave amplifier (gyro-TWA) require a high-quality electron beam with low-velocity spreads. The beam velocity spreads are mainly due to the differences of the electric and magnetic fields that the electrons withstand the electron gun. This paper investigates the possibility to decouple the design of electron gun geometry and the magnet system while still achieving optimal results, through a case study of designing a cusp electron gun for a W-band gyro-TWA. A global multiple-objective optimization routing was used to optimize the electron gun geometry for different predefined magnetic field profiles individually. Their results were compared and the properties of the required magnetic field profile are summarized.

The effect of velocity spread of pseudospark-sourced electron beam to Y-band extended interaction oscillator

Extended interaction oscillator (EIO) operation in the terahertz range puts greater demand on the current density and brightness of an electron beam. The pseudospark (PS)-sourced electron beam is a good candidate for driving such high frequency EIOs as it has a very high combined beam current density and brightness. However, the PS-sourced electron beam can have an inherent velocity spread unless some form of post acceleration is used. Before a new EIO device in the Y-band (220 GHz–325 GHz) based on a PS-sourced electron beam can be realized, it is first necessary to analyze the influence of the beam velocity spread on performance. This paper presents the numerical studies of the EIO performance with the inclusion of the beam velocity spread. It was found that the Y-band EIO circuit can operate in a relatively wide velocity spread range when a high beam current density is used. For an electron beam current density of 1 kA/cm2, the output power is not less than 0.9 times of the power obtained with an electron beam with no velocity spread when the velocity spread is within the range of 0%–15%. We also present our latest results that demonstrate the successful design and optimization of a Y-band EIO driven by a 36 kV, 255 mA (800 A/cm2) PS-sourced electron beam. A peak power over 1.67 kW at 283.7 GHz was achieved using an electron beam, which can be generated by PS discharge, with a velocity spread of 12.5%.

Detection of a coherent population trapping resonance in a beam of atoms by the Ramsey method

Formation of a coherent population trapping (CPT) resonance is studied in the interaction of a beam of atoms with two spatially separated domains of the dichromatic field. Various resonance excitation schemes are compared depending on the choice of operation transitions and type of the polarisation scheme. In the case of a single-velocity atomic beam, the dependence of the CPT resonance profile is studied as a function of principal parameters of the system: beam velocity, distance between optical fields, laser beam dimensions and intensities, and applied permanent magnetic field. Influence of the atomic beam angular divergence and residual beam velocity spread on the resonance quality parameter is estimated.

Effects of electron beam parameters and velocity spread on radio frequency output of a photonic band gap cavity gyrotron oscillator

In this paper, the effects of electron beam parameters and velocity spread on the RF behavior of a metallic photonic band gap (PBG) cavity gyrotron operating at 35 GHz with TE041–like mode have been theoretically demonstrated. PBG cavity is used here to achieve a single mode operation of the overmoded cavity. The nonlinear time-dependent multimode analysis has been used to observe the beam-wave interaction behavior of the PBG cavity gyrotron, and a commercially available PIC code “CST Particle Studio” has been reconfigured to obtain 3D simulation results in order to validate the analytical values. The output power for this typical PBG gyrotron has been obtained ∼108 kW with ∼15.5% efficiency in a well confined TE041–like mode, while all other competing modes have significantly low values of power output. The output power and efficiency of a gyrotron depend highly on the electron beam parameters and velocity spread. The influence of several electron beam parameters, e.g., beam voltage, beam current, beam velocity pitch factor, and DC magnetic field, on the PBG gyrotron operations has been investigated. This study would be helpful in optimising the electron beam parameters and estimating accurate RF output power of the high frequency PBG cavity based gyrotron oscillators.

Conformal Cross-Flow Axis-Encircling Electron Beam for Driving THz Harmonic Gyrotron

In a magnetic cusp gun, the canonical-angular- momentum (CAM) spread of the initially emitted electrons is crucial in generating substantial beam velocity spread. A new method called electron beam flow-feature compensation is proposed to build an axis-encircling electron beam with zero velocity spread by optimizing the cross-flow trajectories to compensate for the initial CAM spread. This method provides an effective solution to the velocity-spread problem in terahertz gyrotrons and increases the emission current to a level that is several times higher than the level that can be obtained using current technology.

Erratum to: Effects of electron beam velocity spread, temperature, and self-fields on dispersion relation and growth rate in a Cherenkov radiation

We study the effects of electron beam velocity spread, temperature, and self-fields on dispersion relation and growth rate in a Cherenkov radiation when an axial magnetic field is present. A generalized dispersion relation is derived and shown that in addition to cyclotron, FEL, and beam modes there are two other modes which are induced by thermal velocity of electron beam. For cyclotron mode, the effects of electron beam self-fields on the growth rate are investigated and shown that the growth rate in the presence of self-fields has a maximum value which is considerably greater than the saturation value of growth rate in the absence of self-fields. The optimum value of axial magnetic field at which the maximum growth can take place is determined for different electron beam densities. Then the effects of electron beam velocity spread and temperature on growth rate are studied and it is found that the electron beam temperature causes an increment in growth rate, while the velocity spread of electron beam has an opposite effect.

Effects of electron beam velocity spread, temperature, and self-fields on dispersion relation and growth rate for a Cherenkov radiation

We study the effects of electron beam velocity spread, temperature, and self-fields on dispersion relation and growth rate in a Cherenkov radiation when an axial magnetic field is present. A generalized dispersion relation is derived and shown that in addition to cyclotron, FEL, and beam modes there are two other modes which are induced by thermal velocity of electron beam. For cyclotron mode, the effects of electron beam self-fields on the growth rate are investigated and shown that the growth rate in the presence of self-fields has a maximum value which is considerably greater than the saturation value of growth rate in the absence of self-fields. The optimum value of axial magnetic field at which the maximum growth can take place is determined for different electron beam densities. Then the effects of electron beam velocity spread and temperature on growth rate are studied and it is found that the electron beam temperature causes an increment in growth rate, while the velocity spread of electron beam has an opposite effect.

Simulation Study of the Effects of Localized Electron Temperature Disturbance on the Quasilinear Dynamics of Beam Propagating Through Solar Corona

The effects of spatial variations of the coronal electron temperature (spatially localized disturbances in Te) on the propagation of a cloud of hot electrons through solar corona plasma and generation of Langmuir waves are investigated using numerical simulations of the quasilinear equations. It is found that the level of Langmuir waves decreases (increases) in the presence of localized enhancement (suppression) in Te. The average velocity of the beam propagation at the points where Te has disturbances decreases (increases) in the presence of localized enhancement (suppression) in Te and is constant at otherwhere as in a plasma with constant temperature. Also the influence of the presence of localized Te disturbances on the evolution of gas-dynamical parameters is investigated. The spatially localized disturbances in Te doesn’t affect the upper boundary of plateau while lower boundary increases (decreases) in the presence of localized enhancement (suppression) in Te. It is shown that the self-similar characteristic of the system is preserved in the presence of fluctuations in the electron temperature. The local velocity of the beam (velocity spread of the beam) increases (decreases) in the presence of localized enhancement (suppression) in Te.