Gun Structure Research Articles

Theoretical design of the gun-cum-input cavity and focusing structure envisaged for a 350 MHz, 100 kW (CW) inductive output tube

The paper reports the theoretical approach and computer-aided design results for a 350 MHz, 100 kW (CW) inductive output tube. The design considerations and methodologies employed for electromagnetic simulation of individual electron gun and RF cavity assemblies and that of their combined assembly were presented. The electron gun parameters namely the beam radius, perveance and electrodes dimensions were optimized. The input cavity was designed and the parameters, namely the resonant frequency, radius and length of the cavity with respect to the desired propagating mode were characterized. The combined integrated model demonstrates the RF input cavity placed onto the gun structure. The interaction of electron beam (40 kV, 3.5 A) and applied input RF signal of 400 W produces an RF current of ~ 85 A in the interaction gap (i.e. cathode-grid spacing). The focusing structure of the device necessary for getting the efficient amplification has also been designed and optimized. The simulations presented in the paper are carried out using CST, MAGIC2D and TRAK codes.

The Research of Atmospheric Pressure Cold Plasma Jet Generated by Auto-Excited-Pulsed DC Discharge

In order to obtain stable cold plasma jet under atmospheric pressure condition, we designed a high-voltage DC power supply based on the principle of multilayer piezoelectric ceramic transformer (MPT). At the same time, we developed a cold plasma jet device that its shape is similar to the gun structure. The total weight of device, including the power supply and the generator, is less than 500 g. In the experiments of argon plasma under atmospheric pressure, current–voltage measurements show that the discharge actually appears periodically pulsed with a frequency of about 30 kHz. The discharge current has a pulse-width that is about 100 ns, while its peak value reaches about 32 mA. The maximum length of the plasma is about 3 cm. The further study showed that the discharge current pulse frequency is determined by the ion drift mechanism.

Thermodynamics Coupling Analysis of Grid-Controlled Electron Gun

This paper carried out temperature analysis in the working state of the electron gun structure, the thermal deformation under high temperature and thermal stress analysis. Firstly, from the steady state temperature cloud chart of the electron gun we can see the cathode temperature is 985 °C, shadow grid temperature is same as cathode temperature, and the controlled-grid temperature is as high as 540 °C. Secondly, on the basis of thermal analysis, the coupled thermo-mechanical simulation technology is broken through; further thermal stress and thermal deformation at high temperature are analyzed. Thermal stress analysis results show that the maximum stress point is at contact area of cathode and its supporting tube. And the thermal deformation result shows that at high temperature under the action of thermal deformation, the axial distance of cathode-grid distance reduced 0.078 mm. Based on thermal deformation, the development personnel of microwave tube can use simulator to analysis the influence of thermal deformation to the electron gun design parameters, which can provide theoretical basis for tube design.

Evaluation of Gun Support Structure Using Equivalent Dynamic Approach in Non Linear FEA

Robot Gun structure is an efficient way in which multiple welds can be done simultaneously. However mounting several weld guns on a single structure induces a variety of dynamic loads, especially during movement of the robot arm as it maneuvers to reach the weld locations.The primary idea employed in this paper, is to model those dynamic loads as equivalent G force loads in FEA. This approach will be on the conservative side, and will be saving time and subsequently cost efficient. The approach of the paper is towards creating a standard operating procedure (SOP) when it comes to analysis of such structures, with emphasis on deploying various technical aspects of FEA such as Non Linear Geometry, Multipoint Constraint Contact Algorithm, Multizone meshing .

Design and Experiment of a Q-band Gyro-TWT Loaded With Lossy Dielectric

Distributed lossy dielectric is of considerable interest for high-power milli- and submillimeter radiation sources, particularly for gyrotron traveling wave tubes (gyro-TWTs). The analytical design and experiment of a Q-band gyro-TWT loaded with periodic lossy dielectric rings operating in the 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 harmonic are presented. The gyro-TWT is driven by a 70-kV 10-A helical electron beam with a velocity ratio of 1.0. Hot test performs maximum peak output power of 152 kW, 41-dB saturated gain, and 21.7% efficiency at 47.6 GHz. The measured bandwidth over 100 kW is at the range of 47-49 GHz. PIC simulation and large-signal prediction show excellent agreements to the hot test results. The potential backward wave oscillation interactions involving the spurious modes TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> , TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> , and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> are effectively suppressed by loading lossy dielectric rings periodically. The Q-band gyro-TWT is zero-drive stable at the operating points, which demonstrates that loading lossy dielectric is excellent to stabilize the spurious oscillations. Moreover, the designs of magnetron injection gun, interaction circuits, and input and output structures are also achieved to satisfy the requirements.

Study on Performance Parameters of the Plasma Source for a Short-Conduction-Time Plasma Opening Switch

Plasma source performance parameters, including plasma ejection density and velocity, greatly affect the operation of a short-conduction-time plasma opening switch (POS). In this paper, the plasma source used in the POS of Qiangguang I generator is chosen as the study object. At first the POS working process is analyzed. The result shows that the opening performance of the POS can be improved by increasing the plasma ejection velocity and decreasing the plasma density. The influence of the cable plasma gun structure and number on the plasma ejection parameters is experimentally investigated with two charge collectors. Finally a semi-empirical model is proposed to describe the experimental phenomenon.

Brazing of photocathode RF gun structures in Hydrogen atmosphere: Process qualification, effect of brazing on RF properties and vacuum compatibility

In this paper, we report on the development of a brazing process for an ultra-high vacuum (UHV) compatible photocathode RF gun structure developed at our Centre. The choice of brazing alloy and its form, brazing clearance between parts to be joined and the brazing cycle adopted have been qualified through metallographic examination of identical joints on an OFE copper prototype that was cut open after brazing. The quality of brazed joint not only affects the UHV compatibility of the gun, but also influences the RF parameters finally achieved. A 2-D electromagnetic code, SUPERFISH, was used to predict the variation in RF parameters before and after brazing considering actual brazing clearances provided between the parts to be joined. Results obtained from low power RF measurements on the brazed gun structure confirm the integrity of the brazed joints and show good agreement with those predicted by electromagnetic simulations. The brazed gun structure has been leak-tested and pumped down to a vacuum level limited by the vacuum compatibility of the flange-fittings employed in the setup.

Design of an Electron Optics System for a $W$-Band Sheet Beam Klystron

The design of an electron optics system for a W-band sheet beam klystron was described. An elliptically cylindrical electron gun and wiggler focusing structure were adopted to produce and guide a 80-kV 4-A sheet beam with an elliptical cross section of 10 times 0.4 mm. The wiggler focusing principle was reviewed and a practical structure was presented. MAFIA was used to simulate the structure. The simulation showed that the electron beam transmission attained 100% over 100 mm in distance with a fill factor that is less than 0.6.

MITIGATION OF RF GUN BREAKDOWN BY REMOVAL OF TUNING RODS IN HIGH FIELD REGIONS

The π-mode resonant frequency of the 1.6 cell SLAC/BNL/UCLA style RF photoinjector electron gun is conventionally tuned using cylindrical copper tuning pieces that extend into the full-cell cavity through holes in the side of the gun. This design begins to fail in many versions of this popular gun design at higher voltage levels, when the cavity undergoes electric breakdown in the vicinity of the tuners. In order to remove the tuners from the region of high electric field, mitigating this problem, one must change the full cell geometry significantly. We have investigated a method for accomplishing this, in which we stretch the gun structure to tune the resonant frequency up by over 2 MHz. We constructed a device to perform this stretching and tested the modified photoinjector in an RF test bed. We succeeded in putting approximately 8.4 MW of RF power into the gun, an improvement over the 4 MW routinely achieved with a similar gun using conventional tuning methods installed at the UCLA Neptune laboratory. Recent results in testing this gun with a magnesium cathode insert are reported as well.

Experimental Investigation of a Novel Circuit for Millimeter-Wave TWTs

A novel folded waveguide circuit that features thick iron pole pieces with hollow centers was built as part of a periodic-permanent-magnet-focused W-band single-stage test-vehicle traveling-wave tube (TWT). These hollow centers, which comprise part of the slow wave circuit, increase the rms axial field and significantly reduce the unwanted transverse field imbalance. For this TWT, a tetrode gun that creates an ultralaminar 20-kV 0.25-A nominal electron beam was used. It was demonstrated that this gun and magnetic structure can provide greater than 97% beam transmission for peak beam power levels as high as 9.25 kW (25 kV, 0.37 A). The unplated circuit, operating around 91 GHz on the edge of a passband, exhibits between 10 dB and 12 dB gain that compares favorably with results of device modeling utilizing the 3D particle-in-cell code Magic3D. Using a feedback approach to characterize large-signal operation, the tube generated 40 W of regenerative oscillator power. Design-optimized versions of this circuit show promise of enabling W-band TWT amplifiers that provide up to 300 W of peak RF output power

Model for intensity calculation in electron guns

The calculation of the current in an electron gun structure is one of the main investigations involved in the electron gun physics understanding. In particular, various simulation codes exist but often present some important discrepancies with experiments. Moreover, those differences cannot be reduced because of the lack of physical information in these codes. We present a simple physical three-dimensional model, valid for all kinds of gun geometries. This model presents a better precision than all the other simulation codes and models encountered and allows the real understanding of the electron gun physics. It is based only on the calculation of the Laplace electric field at the cathode, the use of the classical Child–Langmuir’s current density, and a geometrical correction to this law. Finally, the intensity versus voltage characteristic curve can be precisely described with only a few physical parameters. Indeed, we have showed that only the shape of the electric field at the cathode without beam, and a distance of an equivalent infinite planar diode gap, govern mainly the electron gun current generation.

Thermal analysis of electron gun for travelling wave tubes

Thermal analysis of a pierce type electron gun using the FEM software ANSYS and its experimental validation are presented in this paper. Thermal analysis of the electron gun structure has been carried out to find out the effect of heater power on steady state temperature and warm-up time. The thermal drain of the supporting structure has also been analyzed for different materials. These results were experimentally verified in an electron gun. The experimental results closely match the ANSYS results.

Design of a gridded gun and PPM-focusing structure for a high-power sheet electron beam

We describe the design of a gridded electron gun and periodic permanent magnet (ppm)-focusing structure to create and guide a 415-kV, 250-A sheet beam with a cross section of 100 mm /spl times/ 8 mm. The gun is intended for use with a 40-MW X- band sheet beam klystron for driving accelerators. Using the three-dimensional (3-D) code suite AMAZE, we designed a ppm structure that guided the beam over 80 cm with 98% transmission.

Measurement and simulation of the UMER beam in the source region

As the beam propagates in the University of Maryland Electron Ring (UMER), complex transverse density structure including halos has been observed. A primary objective of the experiment is to understand the evolution of a space-charge-dominated beam as it propagates over a substantial distance. It is therefore important to understand which details of the beam structure result from propagation of the beam in the ring and which characteristics result from the specific details of the initial distribution. Detailed measurements of the initial beam characteristics have therefore been performed. These include direct measurement of the density using a phosphor screen, as well as pepper-pot measurements of the initial transverse distribution function. Detailed measurements of the distribution function have also been obtained by scanning a pinhole aperture across a beam diameter, and recording phosphor screen pictures of the beam downstream of the pinhole. Simulations of the beam characteristics in the gun region have also been performed using the WARP P.I.C. code. From these simulations, the observed behavior is attributed to a combination of perturbations to the transverse distribution by a cathode grid that is used to modulate the beam current, as well as the complex transverse dynamics that results from the combination of the nonlinear external focusing fields of the gun structure and the nonlinear space charge forces.

Revision of (sub)nanosecond pulser for IRI Van de Graaff electron accelerator aided by field propagation calculations

The shorted air line stub used for subnanosecond pulsing of the grounded-grid cathode gun structure of the IRI 3MV Van de Graaff electron accelerator is revised. Three-dimensional high-frequency field propagation calculations provide better insight into the performance of different geometrical shapes. Effects on rise- and decay time, and ringing on the output pulses are considered. Practical possibilities for improvement are discussed. Comparison with sampling measurements on several device modifications confirms the reliability of the calculations. The calculation method is subsequently used as design aid for the construction of a “1ns” device using a quartz loaded shorted stub to fit into the geometry of the existing variable pulse length unit. Capabilities for short pulsing of the accelerator are improved and extended by application of the results obtained.

Collective space-charge phenomena in the source region

For many devices space-charge-dominated behavior, including the excitation of space-charge collective modes, can occur in the source region, even when the downstream characteristics are not space-charge-dominated. Furthermore, these modes can remain undamped for many focusing periods. Traditional studies of the source region in particle beam systems have emphasized the behavior of averaged beam characteristics, such as total current, rms beam size, or emittance, rather than the details of the full beam distribution function that are necessary to predict the excitation of collective modes. A primary tool for understanding the detailed evolution of a space-charge-dominated beam in the source region has been the use of simulation in concert with detailed experimental measurement. However, “first-principle” simulations beginning from the emitter surface have often displayed substantial differences from what is measured. This is believed to result from sensitivities in the beam dynamics to small changes in the mechanical characteristics of the gun structure, as well as to similar sensitivities in the numerical methods. Simulations of the beam in the source region using the particle-in-cell WARP code and comparisons to experimental measurements at the University of Maryland are presented to illustrate the complexity in beam characteristics that can occur in the source region. In addition, direct measurement of the beam characteristics can be limited by lack of access to the source region or by difficulties in obtaining enough data to completely characterize the distribution function. Methods are therefore discussed for using simulation to infer characteristics of the beam distribution from the data that can be obtained.

NEA photocathode for SEM application

Negative electron affinity (NEA) photocathodes were examined as monochromatic electron sources for achieving low acceleration voltage in SEMs. An electron gun structure was developed that makes use of transmission type NEA photocathodes for SEM application, and measurements showed it had electron beam divergence with narrow half angle of 4.2 mrad in FWHM at 3-kV acceleration. The low value of the angle is expected to make up for the large source size to obtain brightness as high as that of a Zr-O/W thermal field emitter (TFE) source. Obtained resolution was 50 nm at 1-kV acceleration voltage.

Electron Beam Sources for Semiconductor Lithography System.

This paper describes several kinds of electron beam sources and their gun structures for a semiconductorlithography system. In general, high brightness LaB6 thermal-emission type is very common due to the lowworkfunction advantage. However, for an electron-beam projection lithography system (EPL), the high emittance (low brightness) gun should be required in order to achieve excellent illumination uniformity even in theone large exposure field. Furthermore, the direct-bombardment Ta cathode and a unique illumination opticsare needed. They result in excellent pattern line-width fidelity. For the future node devices, a mask-less lithographysystem is also expected. For this purpose, various kinds of multiple beam sources are described. Insummary, optimized electron sources and gun structures should be required along with the suitable electronbeam lithography for every node device.

Comparison between monomethyl hydrazine and ECR plasma activated nitrogen as a nitrogen source for CBE growth of GaN

Monomethyl hydrazine (MMHy) was investigated as a nitrogen source for GaN CBE growth in comparison to ECR plasma excited nitrogen sources. The growth rate of GaN on (0 0 1)GaAs was comparable with the highest growth rate by the ECR plasma excited nitrogen source, though the growth rate depends on the plasma gun structure. Unexpectedly, the carbon and oxygen contamination was more serious for the ECR plasma nitrogen source than for MMHy, especially for the ECR plasma without an ion removal magnet.

Dependence of GaN MOMBE growth on nitrogen source: ECR plasma gun structure and monomethyl-hydrazine

GaN layers were grown on (001)GaAs substrates by metal-organic molecular beam epitaxy (MOMBE), using electron cyclotron resonance (ECR) plasma activated nitrogen and monomethyl hydrazine (MMHy) as a nitrogen (N) source. We investigated dependence of GaN crystalline quality, growth rate and contamination, on the N source. It was found that the growth rate for a borosilicate glass guide tube was higher than that for quartz guide tube, suggesting that the quartz guide tube kills N radicals. Unexpectedly, higher oxygen and carbon contamination was detected for the layer grown with the ECR plasma gun without the ion extraction magnet and quartz, than those for the layer grown with MMHy.