Abstract
This paper describes a method for reducing the energy of the cyclotron motion for charged particles in coherent beams using a nonadiabatic magnetic field. For the purpose of reducing the cyclotron motion, a local magnetic field reduction of the main guiding field should be situated in the region of the descending phase of the beam oscillation. The required local magnetic field depression can be produced with a soft iron ring or with a magnet coil. The nonadiabatic element can be positioned on the descending part of any period of the beam oscillation where the beam still remains sufficiently coherent. The effect of reducing the cyclotron motion with such nonadiabatic magnetic field is independent of the electric field of the cathode-anode gap and seems to be a universal method for the cyclotron motion control in coherent beams. Therefore, it can be used for reducing the cyclotron motion of electron beams produced with different kinds of guns with different perveances and sizes. For instance, the method is capable of creating ripple-free, laminar beams even for magneto-immersed guns positioned in a magnetic field of only a few hundred Gs and with a cathode emission current density exceeding $30\text{ }\text{ }\mathrm{A}/{\mathrm{cm}}^{2}$. It can also be applied for guns producing tubular beams, as demonstrated by our simulations. The results of computer simulations are presented, which demonstrate the capability of effective cyclotron motion control with nonadiabatic magnetic field.
Highlights
Electron guns with the cathode immersed in the magnetic field are attractive because their design is simple, and the electron beams produced with these guns are relatively easy to tune
This paper describes a method for reducing the energy of the cyclotron motion for charged particles in coherent beams using a nonadiabatic magnetic field
While the existing cathodes are capable of producing high emission current densities, the cyclotron motion, which is responsible for the electron beam oscillations, limits the minimum
Summary
Electron guns with the cathode immersed in the magnetic field (dubbed magneto-immersed electron guns) are attractive because their design is simple, and the electron beams produced with these guns are relatively easy to tune. Producing laminar electron beams with relatively low emission current density from the cathode (like in the electron coolers) can be done with conventional guns by optimizing mostly the electrostatic field of the gun. While the existing cathodes are capable of producing high emission current densities, the cyclotron motion, which is responsible for the electron beam oscillations, limits the minimum. Producing an electron beam with small radial oscillations using a magneto-immersed gun with cathode emission current density over 20 A=cm in a magnetic field of few hundred Gs becomes a serious challenge and is the original goal of this study. The successful production of laminar, i.e., ripple-free, beams for an emission current density higher than 20 A=cm and for magnetic field in the electron gun region lower than 1 kGs, had not been verified. Such a beam necessarily presents an equilibrium of focusing and defocusing forces and its radius is determined by the equilibrium conditions, as given by e.g., Herrmann [9]
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