C.W. Mendel

Experiments on a current‐toggled plasma‐opening switch

Plasma‐opening switches have been used to improve pulsed‐power wave shapes for over a decade. These switches have used the inertia of the plasma to hold the switch closed. This results in conflicting requirements when long hold‐off time and fast opening are required, and also results in variation in opening current due to variation in initial plasma fill. The current‐toggled plasma‐opening switch attempts to overcome these problems by using external magnetic fields rather than inertia to control the plasma conductor. Data will be presented showing several features of the operation of this switch. These data will be compared to models used to design the switch. The comparisons indicate that the mass can be measured approximately from fast coil data and that the slow coil flux does set the opening level of the current. They also indicate that the opening current is somewhat dependent upon plasma mass, and that the design of the field coils that provide the control fields must be done more carefully to provi...

Time‐resolved voltage measurements in terawatt magnetically insulated transmission lines

We have developed two voltage diagnostics that circumvent many of the difficulties of measuring voltage in magnetically insulated transmission lines driven by terawatt (megavolt and megampere) electrical pulsers. Two versions of simple vacuum capacitive probes use strong magnetic fields to deflect electrons from the anode‐mounted displacement current collector. We then introduce the electron launching voltage monitor as a novel way to measure voltage. This device uses a relationship between anode electric field and electron current launched from a cathode‐mounted perturbation. The electron launching voltage monitor has a large number of advantages over methods commonly used to measure voltage, including large signal level, tolerance to poor vacuum, and nanosecond temporal response. This article shows designs for all these monitors, and presents data from experiments done on the SuperMite pulser at Sandia National Laboratories.

Design of a command-triggered plasma opening switch for terawatt applications

The crucial element of an inductive energy storage system is the opening switch. In microsecond and nanosecond pulsed power systems the plasma opening switch has been in use for many years. The development of the triggered switch addresses three important areas: complete de-coupling of the closed phase and the opening phase will allow improved performance, especially at longer conduction times; the simplified physics allows for easier modeling because of a better-defined geometry; and triggering will reduce jitter of the output pulse. Improving performance will allow longer conduction time, and triggering will negate the naturally increased self-operating jitter at longer conduction time. The triggered switch system is based on moving the plasma switch armature with a magnetic field. Up until the time the armature is pushed away, it is held in place against the drive current magnetic pressure by a second magnetic field. Our system is designed to deliver 1-2 terawatts of usable load power at multi-megavolt potentials. We define usable load power as the product of load voltage and load cathode current. The length of the vacuum storage inductor defines the 35 ns pulse length. This paper shows the design of the switch and trigger system, which is conservatively designed to provide a wide range of trigger signals. The trigger power for this system is important for cost reasons. The first experiments will use a trigger level of ten percent of the output pulse; we describe design features intended to reduce the amount of trigger power needed. Particle-in-cell simulations of the active trigger are also shown.

Observation of reflected waves on the Sabre positive polarity inductive adder MITL

We are studying the coupling of extraction applied-B ion diodes to ignetically Insulated Transmission Lines (MITLs) on the SABRE mdia Accelerator and Beam Research Experiment, 6 MV, 300 kA) sitive polarity inductive voltage adder. Our goal is to determine nditions under which efficient coupling occurs. The best total power iciency for an ideal ion diode load (i.e., without parasitic losses) is tained by maximizing the product of cathode current and gap voltage. [TLs require that the load impedance be undermatched to the selflited line operating impedance for efficient transfer of power to ion )des, independent of transit time isolation, and even in the case of a iltiple cathode system with significant vacuum electron flow. We serve that this undermatched condition results in a reflected wave iich decreases the line voltage and gap electron sheath current, and :reases the anode and cathode current in a time-dependent way. The :TL diode coupling is determined by the flow impedance at the adder t. We also show that the flow impedance increases along the ension MITL on SABRE. Experimental measurements of current i peak voltage are compared to analytical models and TWOQUICK 8-D PIC code simulations.

Losses at magnetic nulls in pulsed-power transmission line systems

Pulsed-power systems operating in the terawatt regime must deal with large electron flows in vacuum transmission lines. In most parts of these transmission lines the electrons are constrained by the self-magnetic field to flow parallel to the conductors. In very low impedance systems, such as those used to drive Z-pinch radiation sources, the currents from multiple transmission lines are added together. This addition necessarily involves magnetic nulls that connect the positive and negative electrodes. The resultant local loss of magnetic insulation results in electron losses at the anode in the vicinity of the nulls. The lost current due to the magnetic null might or might not be appreciable. In some cases the lost current due to the null is not large, but is spatially localized, and may create a gas and plasma release from the anode that can lead to an excessive loss, and possibly to catastrophic damage to the hardware. In this paper we describe an analytic model that uses one geometric parameter (aside...

Performance of plasma opening switches for the particle beam fusion accelerator II (PBFA II)

During 1987 and 1988, Plasma Opening Switch (POS) experiments have been continued with the goal of providing voltage and power gain on the PBFA II ion beam accelerator at Sandia National Laboratories. The experiments have developed a POS that has a rugged plasma source, will open rapidly, and will couple to a high-impedance load. The initial erosion switch design with improved plasma uniformity does not couple to these loads. Therefore, we have abandoned further development of this switch for voltage and power gain. Three alternate designs have been developed, tested, and are found to have better performance with the high impedance loads. These new switches employ magnetic fields to control and confine the injected plasma. A summary of the switch configurations, their theory of operation, and the experimental results is presented and discussed.

Pbfa II applied b-field ion diode proton beam characteristics

An applied B-field ion diode on PBFA II has produced a 17 TW proton beam for investigation of beam generation and transport physics pertinent to inertial confinement fusion experiments. Power was led to the diode via two conical self-magnetically-insulated transmission lines that incorporated plasma opening switches. The diode utilized a pair of B-field coils in disc shaped cathodes to produce a 3 T axial B-field that insulated the 16 mm anode-cathode gap from electron loss. The 15-cm-radius anode was configured with a 5.5-cm-tall curved ion emitting region. A 2.6 MA ion beam originated from this region, was accelerated to 6 MV in the anode-cathode gap, and then transported ballistically toward the axis in a current neutralizing gas cell. The best transport (75%) occurred with narrow 5.5-cm-tall anode sources in which a 180 kJ proton beam was observed within 1.2 cm of the diode centerline. The FWHM of the beam focused at the centerline of the diode was 5 to 7 mm. This beam gave a peak proton power density of approximately 5 TW/cm/sup 2/.

Experiments on insulation of relativistic electron flows in oblique magnetic fields

Several experiments on magnetically insulated electron flows are reported. These include experiments in combined axial and azimuthal magnetic fields like those seen in powerful ion diodes, and flows in which the degree of insulation is changed along the flow path. Conservation of transverse electron canonical momentum implies that the flows cannot be exactly in the E*B direction. Measurements of the field pitch (the ratio of the axial to the azimuthal magnetic field) show a tendency to flow in the E*B direction with very little current along the magnetic field. However, it was also found that the flow tended to find an equilibrium where electrons in the E*B flow have only small transverse canonical momentum. Other data show a surprising degree of electron recapture at the cathode under some conditions. This has important implications for multiterawatt devices. >

Passive Mitigation of Load Debris in a Magnetically Insulated Transmission Line

The Z driver at Sandia National Laboratories delivers one to two megajoules of electromagnetic energy inside its ~10 cm radius final feed in 100 ns. The high current (~20 MA) at small diameter produces magnetic pressures well above yield strengths for metals. The metal conductors stay in place due to inertia long enough to deliver current to the load. Within milliseconds however, fragments of metal escape the load region at high velocity. Much of the hardware and diagnostics inside the vacuum chamber is protected from this debris by blast shields with small view ports, and fast-closing valves. The water-vacuum insulator requires different protection because the transmission line debris shield should not significantly raise the inductance or perturb the self- magnetically insulated electron flow. This report shows calculations and results from a design intended to protect the insulator assembly.

Plasma Diagnostic Techniques

Bremsstrahlung Radiation resulting from free-electron transitions which occur when one particle suffers a deceleration on encountering another particle. It is an important component of continuum radiation. Cerenkov radiation Radiation that results when a charged particle loses energy while moving through a medium at a speed greater than the wave velocity in the medium. Cyclotron radiation Radiation emitted by a charged particle moving in a circular orbit in a magnetic field. It occurs at the cyclotron frequency and its harmonics. Also called synchrotron radiation. Doppler shift A shifting of a spectral line due to the relative velocities of radiating or light-scattering particles. Electromagnetic scattering Process of radiation emission due to the acceleration of charged particles by electromagnetic waves. Gridded analyzer Device that utilizes biased grids to measure the flux of ions as a function of their energy. Interferometer Device that is arranged to compare the relative phases of two paths of coherent radiation, one of which passes through the plasma. Ion spectrometer Device that can resolve different ion species and record their currents. Usually these involve deflection of the ions by electric and/or magnetic fields. Langmuir probe Relatively small electrode exposed to a plasma where it collects electrons or ions when a potential is applied. Laser-induced fluorescence Excitation of atoms or ions by a laser tuned to a selected transition of the atomic species being detected. The resultant radiation is called fluorescence. Line broadening Increase in the width of a spectral line due to some physical process in the plasma. Rogowski coil Multiturn toroidal inductor that encircles a distributed current which is to be measured. Stark shift Shifting of a spectral line due to electric fields in the vicinity of the radiating particle. Thompson scattering Classical limit of light scattering by free charges. Zeeman effect Splitting of a spectral line in the presence of a magnetic field.