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 magneticField confinement)

Contents1. Initial gun design based on existing cathode and simulation results2. Redesigning the MIG type cathode3. 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 -68.5kV.

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

-67.85kV

-70.185kV(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

setup2. Reduce the beam width while also reducing the beam scalloping3. Ensure the magnitude of B field does not exceed the saturation limit

of the iron alloy which we use to cover the solenoid(1.6-2.2 T)

Present MIG CAD Diagram(Cross sectional view)

Blue boxes indicates the space where we can place the solenoids

Vacuum pumptube

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.

• Now that we have aligned the field lines the next step is to vary the magnitude of the B field in order to reduce the scalloping

• We first find the magnetic potential required to eliminate or reduce the scalloping.

• A magnetic field potential of roughly 6.3x Wb/m would be enough to confine the beam perfectly along the A field lines and reduce the scalloping to 0.

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 -60.5kV was used. The extracted beam current is 15A.

1/Gamma = 0.971

1/Gamma 1

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.