Voltage pre-pulse as a tool for lowering anode electron losses in magnetically insulated transmission line*

The development of magnetically-insulated transmission lines (MITL) was a singular achievement in pulsed power. On Z today at Sandia National Laboratories, MITLs routinely deliver tens of terawatts of electric power to a small volume for inertial-confinement fusion, radiation physics, astrophysics, and other HEDP experiments. MITL designs have largely relied on demonstrated experimental performance and, most recently, on validated circuit simulations to keep the power flow losses within acceptable levels. 

 

One of the desired parameters for Z and for all future pulsed-power installations is to keep the anode heating in MITLs below 400°C. When a high-voltage pulse enters the MITL and the cathode starts to spontaneously emit electrons at about 200 kV/cm, these electrons are not initially magnetically insulated and deposit their energy to the anode. With present MITL designs on Z, these onset electron losses last for several tens of nanoseconds and do not deposit a significant amount of heat into the anode. However, when scaled to larger high-current drivers, these initial electron losses can lead to significant anode heating that may degrade MITL performance and limit the total power delivered to the load.

 

We have demonstrated in simulations, for the first time, that the introduction of a voltage pre-pulse into the MITL significantly reduces or even eliminates these initial electron losses. The voltage pre-pulse, when properly designed, keeps the MITL voltage below the emission threshold while enabling the gradual buildup of MITL current and magnetic field. By the time the main voltage pulse enters the MITL, the emitted electrons experience more cathode current and become magnetically insulated in a shorter amount of time. This effectively results in less current being deposited into the anode, allowing for more robust MITL operation. The proposed method may increase the total current delivered to the load on Z today by safely redesigning the existing 4-level MITL and could have a significant impact on the design of future high-current drivers.