Itamar Navon

Real-time measurement of the focal-spot intensity-distribution for megavolt flash X-ray machines

A new method for measuring the 2D intensity-distribution of the focal-spot of megavolt flash X-ray machines is demonstrated. A position-sensitive plane detector views the radiation source through an X-ray-opaque large square aperture. The square aperture was introduced in a previous publication [1] and it was shown that in this case, the focal-spot intensity-distribution is given by the second order mixed Cartesian partial derivative of the penumbral image intensity distribution. This technique is demonstrated by measuring the intensity distribution of the focalspot of a pulsed X-ray machine (1 MV, 40 kA, 50 ns). The results are compared with the physical damage visible on the metal target.

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.

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.

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.

Instabilities in the electron flow along magnetically insulated transmission lines

A magnetically insulated vacuum transmission line (MITL) is a means by which high currents can be carried from generators to loads. In addition to the current flowing in the negative conductor, the source high voltage propagating along the transmission line causes electron emission into the vacuum which forms into an electron sheath restrained by self magnetic forces to flow along the cathode surface. The dynamics of this high current sheath flow is complicated, in particular when the transmission line is non-uniform or when additional power is added along its length. In the present paper we present PIC calculations which show that the circumstances causing the electron sheath to become turbulent, forming persistent vortices flow along non-uniform transmission lines are the same as those for driven MITLs. Under-matched terminal loads or intermediate under-matched sections of a non-uniform MITL suppress emission by a retrapping wave which propagates upstream. When the sheath along a uniform MITL section encounters an overmatched section it overflows downstream where it can suppress emission too. We show that vortices form only when upstream advancing retrapping, downstream overflow and local emission compete.

Pulsed high-voltage vacuum-insulation design

An approach to pulsed high-voltage vacuum-insulation is presented, whereby it is proposed to prevent vacuum surface breakdown before it develops. This is achieved by shaping the electro-magnetic environment in such a way that charged particles are prevented from impacting the surface of the insulator and secondary electrons, if emitted, cannot re-impact the insulator surface.

The Insulator at the Front of a Pulsed Power Machine

Summary form only given. We have recently shown that under certain circumstances the stacked-ring insulator at the front of a pulsed power machine can be replaced with a single monolithic insulator tube. We have used detailed charged particles tracing techniques to design such a monolithic insulator so that vacuum surface flashover is avoided. This technique has been first introduced for the explanation of the mechanism responsible for improved voltage holdoff in high gradient insulators (HGI) and their optimization with respect to vacuum surface breakdown. The success of this method was experimentally demonstrated in HGI. Here we present PIC simulation results which support our earlier ray tracing predictions. Moreover, in machines with a central tube carrying a current, the magnetic field due to this current flow deflects electrons further away from the surface of the monolithic insulator, decreasing even more the possibility for vacuum surface flashover.