Designing a Magnetron Injection Gun and magnetic field to confine the electron beam. Jayakrishnan A. Karakkad, Brian L.Beaudoin, Thomas M.Antonsen Jr.

Introduction For effectively studying ionospheric modifications, the RF source must be capable of Megawatt (MW) power levels. The current gun design based on an existing cathode can propagate a beam only for beam currents less than 5 A. This PPT will present the development of a gun design which can handle up to 15A of beam current while still being tightly confined. Offers the possibilities of high efficiency and gridless operation. Grid heating due to intercepted electrons in the beam current can severely reduce the performance and operational range of the gun. This issue can be rectified using a MIG type emitter which produces a hollow beam. The small anode which is local to the thin hollow cathode can be used to switch the beam on and off, thus effectively eliminating the need for a grid. The electron trajectories of a 15A beam current( with magnetic Field confinement)

Contents 1.Initial gun design based on existing cathode and simulation results 2.Redesigning the MIG type cathode 3.Designing a magnetic field to optimize beam confinement and reduce its scalloping. 4.Preliminary simulation results with decelerating gap for the redesigned MIG type gun in the presence of magnetic field. 5.Conclusion

Initial gun design based on existing cathode and simulation results Geometry of emitter, focus, gap and mod-anode of the initial MIG- type gun, (b) – Particle trajectories of a 70kV – 15A beam(B field confinement present) A zoomed view of the electron trajectory. We can see that the electron start defocusing as soon as it leaves the emitter region for high values of beam current(15A or more). Defocusing Scalloping

Designing the MIG( Outline) Redesign the electrode surfaces Redesign the emitter-focus shape and geometry

Redesigning the electrodes The potential plot can help us further optimize the shape of various surfaces that affect the beam trajectories. Ideally the electron trajectory should be perpendicular to the equipotential surfaces. Moving the curved mod-anode region to the right while shifting the slanted anode region upwards we can tailor the equipotential surfaces in such a way that the beam defocusing is reduced. The potential plot (in V) obtained using MICHELLE, The focus and emitter were maintained at -70kV while the mod-anode is fixed at kV.

Redesigning the emitter-focus shape and geometry The other parameter which we can change is the emitter-focus shape and geometry. We make the emitter convex thus acting like a lens. We obtained the maximum beam width value for the R beam profile data for both the MIG type gun. The maximum beam width of the new MIG type gun is roughly 80% of the original value. (a) – Geometry of emitter, focus, gap and mod-anode of the new MIG-type gun, (b) – Particle trajectories of a 70kV – 15A beam(B field confinement present)

As the beam current increases the chances of beam scrapping at the mod-anode region(shown in figure) increases, however with the changes implemented this can be avoided. One down side of the new design is the increased difference between emitter and mod-anode voltage to extract 15A. The emitter and focus were maintained at -70kV. The voltage difference between emitter and mod-anode required is 2500V. (a) Particle trajectories of a 70kV -15 A beam in presence of magnetic field for the initial geometry (b) – Particle trajectories of a 70kV – 15A beam in the presence of magnetic field for the convex shaped emitter a)b) Increase in emitter-mod anode voltage

Time-Domain simulation We simulated the pulse mode operation of the redesigned MIG. The emitted current peaks at 15A and has a duration of roughly 60ns. We can see from the figure that the beam current follows the square wave input pulse. 15A 0A kV kV (Cutoff Voltage)

Designing the magnetic field The next step is to design the solenoids which would influence how well we can confine our beam. We need to keep the following points in mind while designing the solenoids. 1.Size and placement of solenoids is constrained by the actual gun setup 2.Reduce the beam width while also reducing the beam scalloping 3.Ensure the magnitude of B field does not exceed the saturation limit of the iron alloy which we use to cover the solenoid( T)

Present MIG CAD Diagram(Cross sectional view) Blue boxes indicates the space where we can place the solenoids Vacuum pump tube Beam - Out Anode Emitter

A simple B field confinement We use an idealized magnetic field confinement which does not take the space restrictions into picture. The aim is to align the field lines perfectly with the electron beam(shown in pink) trajectory to obtain a good confinement of the beam.

Reducing the scalloping

Preventing the saturation of the Iron alloy cover One of the factors which limits how strong we can make the magnetic field is the saturation limit of the iron alloy sheathe. We can see that the magnitude of the B field exceeds the limits at the sharp edges. Rounding of these sharp edges and increasing the thickness of the solenoid cover should help us preventing this issue.

The maximum B field magnitude is now 0.61 T. This is well within acceptable range.

Simulation results with decelerating gap The decelerating gap simulation obtained. The extracted beam current is 15 A. A decelerating potential of - 50kV was used. The decelerating gap simulation when a decelerating potential of kV was used. The extracted beam current is 15A. 1/Gamma = 0.971

Conclusion The MIG was redesigned for better beam confinement. The shape and geometry of the emitter-focus was changed to reduce the beam defocusing. An ideal magnetic field was designed to effectively confine the beam and reduce the beam scalloping(space constraints were not taken into account). The decelerating gap simulation using the redesigned MIG type cathode and the ideal magnetic field were obtained.

Things to do next Design and place the solenoid while taking the actual space constraints and the saturation threshold of the iron alloy cover into picture. Simulate the electron beam trajectories with a decelerating gap using the new magnetic field confinement.