Microwave electron gun theory and experiments

Rf gun development at IHEP for BFELP

A thermionic RF gun usually suffers from a rapid decrease of electron beam energy caused by a rapid increase of beam current due to the back-bombardment effect. The authors propose a new method named ldquocavity detuningrdquo to keep the electron beam energy constant. This method detunes the resonant frequency of the gun cavity to a frequency that is lower (by several hundred kilohertz) than the driving frequency. A proof-of-principle experiment was carried out using a 4.5 cell S-band RF gun. The beam energy was successfully kept constant during a macro-pulse duration of 7.5 mus using a cavity detuning of -590 kHz, even with a significant current increase from 240 to 660 mA. A numerical simulation was also conducted to evaluate the cathode temperature, the current density on the cathode surface, and the phase stability of the output beam. All results demonstrated that cavity detuning provides an easy and simple way to compensate for the energy decrease in thermionic RF guns.

The objective of this paper is to compare the state of cathode surface of different axial magnetic field (AMF) contact systems (CSs) during arcing and after arc extinction. The arcs were ignited by opening of electrodes and were fed by sine half-wave current pulses with a duration of 10 ms. Investigations were performed in the range of arc currents up to the levels of guaranteed failure. The measurements were done by a high-speed Phantom M310 video camera. The cathode surface state during arcing was studied using a set of neutral density filters. It was discovered that melting of a part of the cathode surface during arc burning takes place in high-current vacuum arcs. A changing relief in the form of hillocks and hollows forms on the melted surface. We found out that there are two components in droplet erosion of the cathode, when its surface is partially melted: in addition to droplets formed directly in the cathode spot (which can be called droplets of the first type), the liquid part of the surface also emits droplets (which can be called droplets of the second type) significantly different in their sizes and velocities. The formation mechanisms of droplets of the first and the second type are different. The cathode surface temperature distribution immediately after current zero was obtained by comparing the results of cathode surface filming to the results of calibrated band-lamp filming in identical conditions. Reflection of anode radiation from cathode surface was analyzed by mathematical simulation. The filming was carried out using an interference filter. We found that the distribution of the cathode surface temperature is substantially nonuniform. The axial region is the most heated. We conclude that in all investigated CSs with the same amplitude value of the current, the temperature of the most heated parts of the cathode surface is significantly lower than the temperature of the most heated parts of the anode surface. Analysis of obtained results shows that switching failure can be provoked by the cathode and by the anode processes depending on CS design.

Aiming at production of high brightness electron beams with longer pulses than several μsec for applications to free electron lasers by use of an RF gun with a thermionic cathode, the adverse effect of back-streaming electrons onto the cathode surface were studied both experimentally and numerically. Temperature rises of the thermionic cathode surface due to the back-streaming electrons were observed directly by use of an infrared thermometer. From the measured temperature rises, powers of the back-streaming electron beams were, then, evaluated by use of a 0-dimensional model taking into account heat transfer through thermal radiation and thermal contact to the gun body, and comparisons were made with numerical results by use of a 2-dimensional particle-in-cell simulation code we have developed. Both by the experimental evaluation and by the particle-in-cell simulation, the beam power onto the cathode surface was found to increase with the increasing RF input to the RF gun cavities, and reach a considerable power compared to the cathode heater power supply.

Low Emittance X-band Thermionic Cathode RF Gun for Medical Application

Capacitors short-circuit discharge in an explosive environment can ignite and detonate the surrounding explosive media, causing dangerous accidents. At low voltages, this kind of discharge constitutes a micro-nano discharge; because the discharge gaps here are of the order of only microns to nanometers, the discharge process, electrode energy consumption, explosive media ignition energy, and other energy relationships are unclear. To study the relationships between the capacitor storage energy and various kinds of dissipation energies under short-circuit discharge, a model comprising conical and spherical cylinder microbumps is proposed based on the cathode surface morphology obtained by three-dimensional profiling and scanning electron microscopy, respectively. Then, the second-order non-chi-squared differential equations were established based on the principle of energy conservation and heat balance to deduce the relationships between the cathode surface temperature and height of the microbump, conical angle, and spherical radius; further, the energy consumed by the anode surface is calculated based on the theory of heat transfer. Using heat conduction theory, the energy consumed by the microbumps on the cathode surface is calculated, and the energy consumed on the anode surface is deduced using the surface heat source as the loading heat source. The residual energy of the capacitor is calculated from the discharge time and voltages before and after discharge, and the effective energy of the gas is calculated using the law of conservation of energy. Finally, the discharge channel energy, electrode energy consumption, and end residual energy of the discharge capacitor are used to derive the effective ignition energy of the explosive gas. This research is of great significance for the design of intrinsically safe circuits with high power.

A thermionic-cathode RF gun for compact FEL use

Coupling boundary condition for high-intensity electric arc attached on a non-homogeneous refractory cathode

Summary form only given, as follows. The effect of increasingly high sodium contents on the attachment conditions of a DC mercury-sodium (Hg-Na) arc discharge on a tungsten (W) cathode is studied. A physical model based on the collisionless assumption is used for the description of the cathode sheath. The solution of this model; i.e. the plasma heat flux versus the cathode surface temperature, is coupled to a thermal model for the cathode bulk for a self-consistent solution of the problem under diffuse attachment conditions. Domains of existence for self-sustaining operation of the system in the electron temperature at the sheath edge - cathode surface temperatures space (T/sub e/-T/sub s/ space) are obtained versus the Na content in the arc discharge and reveal the higher cathode surface temperature requirements as the Na content increases. For a typical set of HPS lamp operating conditions (p=1 atm, p/sub Hg/=0.9P, p/sub Na/=0.1P, I=3.2 A), the model predicts melting of the pure W cathode tip. The substantial increase of the operating tip temperature at increasingly high Na content is attributed to the increasingly high density of Na/sup +/ ions, itself associated with the much lower ionization potential of Na with respect to Hg, which must be sustained by the electron emission processes. Lowering the W cathode tip work function brings the operating temperatures to lower values in agreement with well-known experimental facts.

Performance of the thermionic RF gun injector for the linac-based IR free electron laser at the FEL-SUT

First operation of cesium telluride photocathodes in the TTF injector RF gun

A recently developed unified theory of arcs and their electrodes, with cathodes which are thermionic emitters, has been simplified so that there is a reduction in computation times by approximately a factor of 100. Electrode and arc regions are treated together and points at the surface of the electrodes are treated in a special way to account for electrode effects; no assumptions are made concerning the current density at the cathode surface. The theory is used to make predictions of arc and electrode temperatures and arc voltages for arcs in argon as a function of current in the range 50 - 400 A. The maximum temperatures of the arc and the cathode and also the current - voltage characteristics are in reasonable agreement with experimental results for cathodes with a included angle. For a cathode with a included angle, we predict a maximum in the temperature several millimetres from the tip of the electrode, in approximate agreement with experiment. Temperatures of the cathode tip are predicted to be much higher for tungsten cathodes than for thoriated tungsten cathodes and are in reasonable agreement with experimental results. Tungsten electrodes are hotter than thoriated tungsten electrodes, partly due to increased ion heating, but largely due to greater heat conduction from the arc to the electrode due to the arc plasma covering a greater area of the electrode surface for tungsten.

For arcs at atmospheric pressure which have cathodes that are thermionic emitters, it is possible to calculate the major properties of the arc and the electrodes as a function of current, by accounting for electrode shape and the heat transfer processes occurring at the surface of the electrodes. Such processes occur due to electron and ion emission and absorption and also from radiation emission and absorption. Electrical resistance of the plasma near the electrodes is calculated either by taking account of ambipolar diffusion or simply using the local plasma value, with mesh sizes sufficiently large to account for ambipolar diffusion. Derived temperature profiles are in fair agreement with experiment. Results of electrode temperatures and arc melting effects, including such phenomena as the transition from globular to spray modes in arc welding, are also in good agreement with experiment. Prediction of properties for non-thermionic cathodes still constitute a major problem. Approximate calculations indicate that electrons at the surface of the non-thermionic cathodes may be produced by photo ionisation of neutral atoms rather than by field emission.

独立调谐热阴极微波电子枪电子反轰效应

The system of equations relating the electrode surface temperature in the cathode spot of normal glow discharge at atmospheric pressure to the discharge cathode sheath parameters is formulated. An analytical expression for the cathode surface temperature in the spot has been obtained, and the parameters of the spot and discharge cathode sheath in helium have been calculated.