D. J. Johnson

Time-resolved proton focus of a high-power ion diode

An improved understanding of the factors that control the axial focus of applied‐B ion diodes was obtained from time‐resolved diagnostics of ion‐beam trajectories. This resulted in a new selection of anode shape that produced a proton focus of 1.3‐mm diameter from a 4.5‐cm‐radius diode, which is a factor of 2 improvement over previous results. We have achieved a peak proton power density of 1.5±0.2 TW/cm2 on the 1‐TW Proto I accelerator. The radial convergence of this proton beam, defined as the ratio of the anode diameter to focused beam FWHM, is 70. Time‐resolved information about virtual cathode evolution, the self‐ and applied‐magnetic‐field bending, and the horizontal focus of the beam was also obtained. In addition, the diffusion of the magnetic field into the anode plasma is estimated by measuring the horizontal focal position as a function of time. Finally, we discuss the effects of gas cell scattering on the beam focus.

Applied‐B ion diode experiments on the Particle Beam Fusion Accelerator‐I

A series of experiments was performed with an Applied‐B ion diode on the Particle Beam Fusion Accelerator‐I, with peak voltage, current, and power of approximately 1.8 MV, 6 MA, and 6 TW, respectively. The purpose of these experiments was to explore issues of scaling of Applied‐B diode operation from the sub‐TW level on single module accelerators to the multi‐TW level on a low impedance, self‐magnetically insulated, multimodule accelerator. This is an essential step in the development of the 100‐TW level light ion beam driver required for inertial confinement fusion. The accelerator and the diode are viewed as a whole because the power pulse delivered by the 36 imperfectly synchronized magnetically insulated transmission lines to the single diode affects module addition, diode operation, and ion beam focusability. We studied electrical coupling between the accelerator and the diode, power flow symmetry, the ionic composition of the beam, and the focusability of the proton component of the beam. Scaling of...

Direct measurement of the energy spectrum of an intense proton beam

A time‐resolved magnetic spectrograph was used to measure the energy spectrum of an intense (0.7 TW/cm2) proton beam. A thin gold foil placed at the focus of an ion‐diode Rutherford‐scattered protons by 90° into the spectrograph, reducing the beam intensity to a level suitable for magnetic analysis. The scattered protons were collimated and then deflected in a samarium‐cobalt permanent magnet. The deflected protons were recorded simultaneously on CR‐39 and eight p‐i‐n diodes. A Monte Carlo computer code was used to calculate the sensitivity and resolution of the spectrograph. Data taken on the Proto I accelerator shows a 150–250‐keV‐wide proton‐energy spectrum at each instant in time.

Enhanced ion stopping powers in high-temperature targets

Light ions deposit their energy in target materials by interaction with bound and free electrons. As the target heats toward inertial confinement fusion temperatures a progression of ionization states will be encountered. The stopping power of each ion created in this process will depend upon details of the respective bound electron states. In general, the net ion stopping power will increase compared to cold matter due to the free electron contribution. We report an experimental and theoretical study of enhanced ion stopping powers in targets heated by 0.5–1.4 TW/cm2 proton beams. The experiments were performed on the Proto‐I accelerator with aluminum and nickel foil targets. The theoretical effort incorporated free and bound electron stopping terms in hydrocode simulations of the target response. At these intensities we observe and calculate stopping power enhancements of 100% for aluminum and 50% for nickel.

ICF target diagnostics on PBFA II (invited)

Particle Beam Fusion Accelerator II is a light‐ion fusion accelerator that is presently capable of irradiating a 6‐mm‐diam sphere with ∼50 kJ of 5.5‐MeV protons in ∼15 ns. An array of particle and x‐ray diagnostics fielded on proton Inertial Confinement Fusion target experiments quantifies the incident particle beam and the subsequent target response. An overview of the ion and target diagnostic setup and capabilities will be given in the context of recent proton beam experiments aimed at studying soft x‐ray emission from foam‐filled targets and the hydrodynamic response of exploding‐pusher targets. Ion beam diagnostics indicate ∼100 kJ of proton beam energy incident within a 1.2‐cm radius of the center of the diode with an azimuthal uniformity which varied between 6% and 29%. Foam‐filled target temperatures of 35 eV and closure velocities of 4 cm/μs were measured.

Multiframe ion pinhole camera for intense ion beam transport and focusing experiments

We have developed a new ion pinhole camera to obtain energy density profiles of a focused multiterawatt ion beam as a function of ion energy. Beam ions that are elastically scattered from a thin gold foil are imaged through six baffled pinholes onto six separate areas of a sheet of CR‐39 nuclear track detector. Each of the areas is covered by an aluminum range filter and records an image with a different lower bound on the ion energy. Subtracting images with adjacent cutoff energies results in images with upper and lower bounds on the ion energy. Ion images are read from the CR‐39 with VERA, a fully automatic, image‐processor‐based nuclear track counting system. Images of the Particle Beam Fusion Accelerator‐II (PBFA‐II) proton beam have been obtained and show that the beam has been focused to achieve a horizontal FWHM of 5.7 mm and has delivered 9 kJ/cm2 to a target.

Simulation and interpretation of ion beam diagnostics on PBFA‐II

Ion diode and beam focusing experiments are in progress on PBFA‐II working toward an ultimate goal of significant burn of an ICF pellet. Beam diagnostics on these experiments include a Thomson parabola, Kα x‐ray pinhole cameras, filtered ion pinhole cameras, and a magnetic spectrometer. We are developing two new computer programs to simulate and interpret the data obtained from these diagnostics. vida is a VAX‐based program that manipulates and unfolds data from digitized particle and x‐ray diagnostic images. vida operations include: image display, background subtraction, relative‐to‐absolute coordinate transformations, and image projection into the beam reference frame. picdiag allows us to study the effects of time‐dependent ion focusing on the performance of ion beam diagnostics.

Observation of K α. X-ray satellites from a target heated by an intense ion beam

We have made the first observation of K α X-ray satellites from a target heated by an intense ion beam. The satellites are produced when thermal ionization due to beam heating is accompanied by inner-shell ionization from beam ion impact. The Particle Beam Fusion Accelerator II was used to irradiate a conical aluminum target with a proton beam. The nominal beam parameters were 50–75 kJ in a 1-cm spot, 15–20-ns pulse length, and 4–5-MeV protons at peak power. An elliptical crystal X-ray spectrograph inside a 1000-kg tungsten shield was used to record the spectra. The peak ion stage reached by the aluminum target was +8. Collisional radiative calculations were performed, which indicate a peak electron temperature of 20–60 eV.

Intense ion beam diagnostics for particle beam fusion experiments on PBFA II (invited)

A review of the diagnostics used at Sandia National Laboratories to measure the parameters of intense proton and lithium beams generated on the PBFA‐II accelerator will be presented. These diagnostics consist of several types, namely: Kα x‐ray pinhole cameras, a multiframe dE/dx ion pinhole camera, a p‐i‐n array ion pinhole camera, Thomson parabola spectrographs, a Rutherford magnetic spectrograph, plasma visible spectroscopy, and several nuclear activation diagnostics. These components, when taken together, enable a rather thorough description of the 5‐MV, 10‐TW ion beams presently being produced. A unique feature of these diagnostics is that they are capable of operating in hard (several MeV) x‐ray bremsstrahlung backgrounds of some 109–1010 rad/s. Details of each diagnostic, its integration, data reduction procedures, and recent PBFA‐II data will be discussed.

Electron-Anode Interactions in Particle-in-Cell Simulations of Applied-B Ion Diodes

Particle-in-cell simulations of applied-B ion diodes using the QUICKSILVER code have been augmented with Monte Carlo calculations of electron-anode interactions (reflection and energy deposition). Extraction diode simulations demonstrate a link between the instability evolution and increased electron loss and anode heating. Simulations of radial and extraction ion diodes show spatial non-uniformity in the predicted electron loss profile leading to hot spots on the anode that rapidly exceed the 350-450 {degree}C range, known to be sufficient for plasma formation on electron-bombarded surfaces. Thermal resorption calculations indicate complete resorption of contaminants with 15-20 kcal/mole binding energies in high-dose regions of the anode during the power pulse. Comparisons of parasitic ion emission simulations and experiment show agreement in some aspects; but also highlight the need for better ion source, plasma, and neutral gas models.