John G. Leopold

Optimizing the performance of flat-surface, high-gradient vacuum insulators

High-gradient insulators (HGI) are periodic assemblies of conducting and insulating layers that have been shown to withstand higher pulsed voltages in vacuum than homogeneous insulators of the same length. We carried out calculations and experimental studies to understand the effect of geometry on the performance of well-conditioned, flat-surface HGI assemblies. We tested stacks with several different values of I/M (where I is the axial length of an insulating layer and M is the length of a metal layer). The experiments showed that HGI performance was substantially better than conventional insulators for I/M<3 and somewhat worse for I/M>3. Numerical calculations of electron orbits showed: 1) that the electric fields in HGI assemblies may have the favorable property of sweeping charged particles away from the surface and 2) that electron multiplication on the surface is suppressed when I/M<3.

Time- and space-resolved light emission and spectroscopic research of the flashover plasma

The results of an experimental study of the evolution of surface flashover across the surface of an insulator in vacuum subject to a high-voltage pulse and the parameters of the flashover plasma are reported. For the system studied, flashover is always initiated at the cathode triple junctions. Using time-resolved framing photography of the plasma light emission the velocity of the light emission propagation along the surface of the insulator was found to be ∼2.5·108 cm/s. Spectroscopic measurements show that the flashover is characterized by a plasma density of 2–4 × 1014 cm−3 and neutral and electron temperatures of 2–4 eV and 1–3 eV, respectively, corresponding to a plasma conductivity of ∼0.2 Ω−1 cm−1 and a discharge current density of up to ∼10 kA/cm2.

Numerical Experiments on Matching Vacuum Transmission Lines to Loads

To efficiently connect a high-current-density load to a magnetically insulated vacuum transmission line, we require that the regularity of the electron flow be conserved. Regularity, i.e., Brillouin flow, can be preserved by keeping the invariants of the motion constant along the flow and requiring that small changes in these be adiabatic. It is shown that by matching the vacuum impedances of the various transmission line sections and assuring smooth transition between them, Brillouin flow of sheath electrons continues almost unperturbed to the load without losses. This idea is numerically demonstrated for a new type of diode which we name Brillouin diode.

Vacuum surface flashover: experiments and simulations

It is conjectured that vacuum dielectric surface flashover can be avoided by preventing its initiation. Assuming that the flashover process is initiated by the impact of charged particles on the insulator surface, by deflecting these away from the surface, the development of a flashover avalanche can be repressed. As evidence of this, we present the results of experiments where high-voltage fast-rising microsecond timescale voltage pulses are applied on vacuum insulator samples where we methodically manipulate the magnitude and the orientation of the electric fields at the cathode and anode triple junctions and along the insulator surface.

On the dynamics of the flow along cylindrical self magnetically insulated vacuum transmission lines

We study by PIC simulation the dynamics of the electron flow along self magnetically insulated negative polarity cylindrical transmission lines (MITL) of fixed vacuum impedance. We find that the theoretical models of magnetic insulation predict correctly the steady state macroscopic measurable voltages and currents but we also observe that the flow is governed by regular quasi-cycloidal electron motion of fixed spatial period which scales with MITL radius. While this is not completely surprising, such regularity has not been observed before. When retrapping due to load mismatch is present, the flow shows additional interesting features.

The flow dynamics along non-uniform self magnetically insulated transmission lines

We investigate the situation where an upstream large radius MITL is connected to a downstream small radius MITL of the same vacuum impedance through a magnetically insulated coaxial conical section transmission line of fixed vacuum impedance. When the conical section is long and the load is matched, stable quasi-cycloidal electron motion persists [1]. As the length of the conical section is reduced its vacuum impedance decreases relative to that of the adjacent cylindrical MITLs and the electron sheath flow becomes more complex. When both the load and the conical section MITL are under-matched the electron flow is perturbed to the extent that considerable and persistent vortices appear. For this relatively simple system the complexities of the flow and the onset of vortices are characterized.

Applying a different approach to pulsed high-voltage insulation

Different approach to pulsed high-voltage vacuum insulation has been recently presented [1]. These design guidelines have been successfully applied in a 2MV pulsed power machine where we replaced a complicated stacked ring insulator, which sometimes failed through catastrophic breakdown, with a simple monolithic insulator designed as proposed [1]. No vacuum insulator surface breakdown occurred since installation.

Regular Charged Particle Flow in Pulsed-Power-Driven Nonuniform Transmission Lines

In electron-emitting nonuniform transmission lines at currents sufficient to support magnetic insulation, it is possible to maintain regular Brillouin electron flow by matching the vacuum impedances of various parts of the transmission line and by ensuring adiabatic transition between them. This idea, first demonstrated numerically in the design of the Brillouin diode (BD), is extended to the structure of the pair known as knob and dustbin. Rather than shunting current to the walls, a ldquodesignerrdquo knob-dustbin allows regular flow of the entire current to the BD.

More on High-Gradient Insulators

High-Gradient Insulators (HGI) are periodic structures made up of metal and insulator layers. HGIs are superior to ordinary insulators as they break down in vacuum at 2- 4 times higher applied voltages. Recently a model has been proposed which explains flat surface HGI behavior. The model and experiments have been extended here to coned flat surface HGIs. We have constructed such HGIs and found that their behavior follows our model predictions.

Flow Dynamics Along Complex Magnetically Insulated Transmission Lines

We study by PIC simulations the simple model of a magnetically insulated transmission line fed by a matched voltage source and driven additionally by a single transverse source adding voltage to it at a certain point along its length. For best power flow, an impedance transition is required after this point. We study the effect of the parameters of this geometrical transition on the power flow and the dynamics of the sheath flow. We also find conditions for the onset of unstable flow resulting in the formation of strong electron vortices.