J. G. Leopold

High power microwave source for a plasma wakefield experiment

The results of the generation of a high-power microwave (∼550 MW, 0.5 ns, ∼9.6 GHz) beam and feasibility of wakefield-excitation with this beam in under-dense plasma are presented. The microwave beam is generated by a backward wave oscillator (BWO) operating in the superradiance regime. The BWO is driven by a high-current electron beam (∼250 keV, ∼1.5 kA, ∼5 ns) propagating through a slow-wave structure in a guiding magnetic field of 2.5 T. The microwave beam is focused at the desired location by a dielectric lens. Experimentally obtained parameters of the microwave beam at its waist are used for numerical simulations, the results of which demonstrate the formation of a bubble in the plasma that has almost 100% electron density modulation and longitudinal and transverse electric fields of several kV/cm.

Revisiting Power Flow and Pulse Shortening in a Relativistic Magnetron

The behavior of a six-vane relativistic magnetron with a single radial output slot has been studied in detail by 3-D particle in cell simulations. It is found that a delicate power imbalance caused by the pulsed-power generator and magnetron impedance mismatch is responsible for the shortening of the radiated power pulse. Pulse shortening can be avoided completely, and the radiated power can be considerably increased by decreasing the emitted current when it is too large to be sustainable. When the magnetron current is too high, the voltage decreases, which causes the radiated power to drop and shortens the pulse. This relatively simple electrical power balance is the result of intricate dynamics involving the electron flow within the magnetron, out of the interaction cavity through the axial flow and the electromagnetic modes supported by the structure.

The interaction of intense, ultra-short microwave beams with the plasma generated by gas ionization

Results of the non-linear interaction of an extremely short (0.6 ns) high power (∼500 MW) X-band focused microwave beam with the plasma generated by gas ionization are presented. Within certain gas pressure ranges, specific to the gas type, the plasma density is considerably lower around the microwave beam axis than at its periphery, thus forming guiding channel through which the beam self-focuses. Outside these pressure ranges, either diffuse or streamer-like plasma is observed. We also observe high energy electrons (∼15 keV), accelerated by the very high-power microwaves. A simplified analytical model of this complicated dynamical system and particle-in-cell numerical simulations confirm the experimental results.

Wakefield in a waveguide

The feasibility of an experiment which is being set up in our plasma laboratory to study the effect of a wakefield formed by an ultra-short (≤10−9 s) high-power (∼1 GW) microwave (10 GHz) pulse propagating in a cylindrical waveguide filled with an under-dense [(2–5) × 1010 cm−3] plasma is modeled theoretically and simulated by a particle in cell code. It is shown that the radial ponderomotive force plays a circular key role in the wakefield formation by the TM mode waveguide. The model and the simulations show that powerful microwave pulses produce a wakefield at lower plasma density and electric field gradients but larger space and time scales compared to the laser produced wakefield in plasmas, thus providing a more accessible platform for the experimental study.

A six vane, single radial output slot relativistic magnetron revisited

The behavior of a six vane relativistic magnetron with a single radial output slot has been studied in detail by 3D PIC simulations. We find that a delicate electro-dynamic imbalance caused by impedance mismatch is responsible for the shortening of the radiated power pulse. When the power pulse drops the voltage reduces and the current increases which points towards under-matched impedance of the magnetron relative to the generator providing its input power. We demonstrate this here by changing the radius of the cathode. When the impedances match the radiated power pulse no longer shortens and it can be increased considerably. This relatively simple electrical power balance is the result of intricate dynamics involving the electron flow within the magnetron, out of the interaction cavity through the axial flow and the electro-magnetic modes supported by the structure.

Self-channeling of a powerful microwave beam in a preliminarily formed plasma

The self-channeling of a high power (≤500 MW) sub-nanosecond microwave beam in the plasma formed by a neutral gas (>103 Pa) ionization was demonstrated by Shafir et al. [Phys. Rev. Lett. 120, 135003 (2018)]. In the present research, this effect is observed and studied in detail in a plasma, preliminarily formed by an rf discharge, in a low ( 103 Pa) ionization was demonstrated by Shafir et al. [Phys. Rev. Lett. 120, 135003 (2018)]. In the present research, this effect is observed and studied in detail in a plasma, preliminarily formed by an rf discharge, in a low (<150 Pa) pressure gas. The results of analytical modeling and numerical particle-in-cell simulations show that ionization-induced channeling can be realized at a significantly lower power of the microwave beam and gas pressure if the preliminarily formed plasma is radially non-uniform with minimal on axis density.

Over-injection and self-oscillations in an electron vacuum diode

We demonstrate a practical means by which one can inject more than the space-charge limiting current into a vacuum diode. This over-injection causes self-oscillations of the space-charge resulting in an electron beam current modulation at a fixed frequency, a reaction of the system to the Coulomb repulsive forces due to charge accumulation.

Revisiting the relativistic A6 magnetron

We have recently been studying the relativistic A6 magnetron with a single radial output by 3D-PIC simulations. It was found that a delicate electro-dynamic imbalance, resulting in an impedance mismatching between the magnetron and the generator supplying its power, is responsible for microwave pulse-shortening prior to beginning of cathode plasma expansion.1 Also, it was shown that pulse shortening can be eliminated by affecting the relevant impedances through changes in the emitted cathode electron current.

Investigating the power flow in a relativistic magnetron with radial output

Summary form only given. While impedance matching with a pulsed generator is well noted in the relativistic magnetron literature as a pre-requisite for avoiding pulse shortening and radiated power loss1, its effect has not been investigated. The impedance of a 3D radiating structure, such as a magnetron, is complex, dynamically non-linear and in most cases too difficult to predict analytically. It depends on spatial distribution of drifting electrons interacting with e/m waves in magnetron. This time-depending spatial distribution of electrons is determined by the external magnetic field, applied voltage, and the magnetron geometry. We have investigated by 3D PIC simulations a 6-vane magnetron with a single radial output iris and found that power flow efficiency, frequency stability, and pulse shortening are indeed a function of the relative dimensions of the magnetron cathode, anode radii, and radiator depth which determine the impedance of the structure. In particular, we find that when the magnetron impedance is under-matched relative to the power generator, it cannot support the high current supplied by the input voltage. Then, first the axial current is re-trapped, causing the voltage to decrease and the radial current to increase. The increased radial current cannot be supported by the magnetron unless the radiated power drops, resulting in pulse shortening. This unstable situation is also accompanied by mode competition. The impedance of the magnetron can be increased by reducing the cathode radius or the electron emission region thus reducing the emitted current. By doing this, pulse shortening and mode competition no longer occur and the radiated power can be optimized varying the magnetic field within the Hull cutoff and the Buneman-Hartree limit.