C. W. Mendel

A fast‐opening switch for use in REB diode experiments

The operating characteristics of a plasma‐filled fast‐opening switch and its performance in pinching experiments on Sandia’s Proto I accelerator (double sided, 2 MV, 7.2 Ω, 24 nsec each side) are described. The growth of a sheath between the plasma and the cathode of the switch is the mechanism responsible for the switch impedance rising from 0.1 to 50 Ω in 10 nsec. The effect of this switch on diode prepulse and machine turn‐on transient suppression is discussed.

Analytic model of Applied‐B ion diode impedance behavior

An empirical analysis of impedance data from Applied‐B ion diodes used in seven inertial confinement fusion research experiments was published recently. The diodes all operated with impedance values well below the Child’s‐law value. The analysis uncovered an unusual unifying relationship among data from the different experiments. The analysis suggested that closure of the anode‐cathode gap by electrode plasma was not a dominant factor in the experiments, but was not able to elaborate the underlying physics. Here we present a new analytic model of Applied‐B ion diodes coupled to accelerators. A critical feature of the diode model is based on magnetic insulation theory. The model successfully describes impedance behavior of these diodes and supports stimulating new viewpoints of the physics of Applied‐B ion diode operation.

Planar one‐dimensional magnetically insulated electron flow for arbitrary canonical‐momentum distribution

Equations are derived for finding the current and voltage profiles of electron flows from their momentum distributions. It is found that many of the features of the flow are independent or quasi‐independent of the distribution functions. For this reason, it is concluded that the potential difference across the flow can usually be found from the line‐current measurements.

Performance of a plasma‐filled, series‐field‐coil ion beam diode

An ion diode having many characteristics desirable for ion beam fusion is described. These features include bunching, excellent impedance match to the pulser, self‐magnetic insulation of electrons, and selectable ion species. The diode accomplishes this by employing a series magnetic field coil and an initial plasma fill. The series field coil is configured in such a way as to give uniform electron loss and ion current density over the entire anode. Data are presented to show experimental agreement with the model.

Production and postacceleration of intense ion beams in magnetically insulated gaps

Experiments are described pertaining to the development of very high‐current pulsed linear ion accelerators utilizing electron neutralization. A novel magnetically insulated gap using radial magnetic fields has been tested. It provides stable electron cloud confinement over microsecond time scales with no detectable leakage current. The gap can act as an ion injector when used in conjunction with a plasma source. Control of the electron cloud dynamics allows the injector to operate in an enhanced current density mode (10–50 times the Child‐Langmuir limit) with high efficiency and with plasma source control of the current flow. Currents up to 20 kA at 100 kV applied voltage resulted when using a light‐ion flashboard plasma source. Carbon beams were produced by extraction from a flowing plasma from a gun array. A 3‐kA beam with equal fractions of C+ and C++ was extracted over a microsecond time scale with little proton contamination. The use of active plasma injection into the high‐intensity magnetically in...

Near-field measurements of subnanosecond-created laser plasmas

The near field of expansion for plasmas created by a 40‐psec 1.0–1.5‐J laser has been probed to measure the electron density and local electric and magnetic fields. Holographic interferometry was used to measure the electron density in the range 1017 to 6×1018 cm−3 at delays of 2.5–22 nsec. Space‐charge electric fields of up to 1900 V/cm were detected by deflection of a fast He+ ion beam probe as late as 26 nsec after irradiation. Spontaneous magnetic fields of up to 50 G, rising in 10 nsec, were seen 4 mm from the target, decaying rapidly and often reversing at 8 mm. The temperature in the blowoff estimated from the combination of local electric field and electron‐density scale length is high compared to predictions based on a peak Te=160 eV inferred from a multichannel soft x‐ray spectral analysis. An anisotropic fast ion group appears in the interferometric density profiles and ejection of ∼109 fast electrons is observed in the electric field measurements. These effects are compared to the early time i...

Series‐field‐coil ion beam diode experiment and numerical simulation

An experiment with a series‐field‐coil intense ion beam diode operating at the 1.6‐MV, 0.8‐TW level is described. The diode operates in the extraction mode with a 20‐cm focal length. The diode design procedure is described together with some of the essential ion diode theory used in this design process. The experiment produced a well‐focused ion beam while verifying some of the theoretical assumptions about ion diodes of this type. The experimental results are compared with numerical computer simulations and excellent agreement is obtained.

15‐cm hybrid ion diode on PBFA‐I

Diode experiments on the PBFA‐I pulser using a magnetically insulated ion diode are described. The insulating magnetic field is supplied by self‐field due to the ion current plus the field generated by a series field coil. In the experiments described here, the diode operated at the 10‐TW, 2.5‐MV level with over 300 kJ going to the diode on many shots. The operation of the diode, the dielectric anode, and the proton beam focusing are described.

Conical Z‐Pinch Gun

A pulsed plasma gun is described which has several advantages over other commonly used guns. Among these are simplicity of operation, construction and analysis, and stability of operation. The output has a sharply peaked velocity distribution. The initial model, described herein, emits a burst of 5×1018 Ar ions moving at 9 cm/μsec. A snowplow analysis predicts operating characteristics generally in agreement with observations.

PBFA II ion diode theory and implications

Fully electromagnetic, relativistic, two-dimensional, particle-in-cell (PIC) simulations of barrel-type and extractor-type Applied-B ion diodes have increased our confidence in the design of present and future diodes for the Particle Beam Fusion Accelerator II (PBFA II). In addition, the data from various experiments on Pro to I, Proto II, and PBFA I Applied-B ion diodes are inconsistent with previous models of diode operation, based on anode-cathode gap closure from expanding plasmas. A new model has been devised and applied to the PBFA II diode to explain the diode impedance and its time history, and to suggest methods for controlling the impedance.