J. N. Olsen

Ion‐expansion energy spectra correlated to laser plasma parameters

The analysis of LiH and LiD nanosecond — created laser plasmas reveals correlations between the various parameters of each plasma event. Relationships are found involving the energy spectra of the ion species, the target reflectivity, neutron production, hard and soft x‐ray emission, the electron temperature, and the input laser energy. Ion — expansion spectra of nonthermal origin are observed under the condition of high target reflectivity. High target reflectance is correlated to neutron and hard x‐ray production as well as an apparent decrease in electron thermal content for laser energies of 18–70 J. The ion — expansion spectra are obtained with mass and charge separation by the parallel electric and magnetic fields of a Thomson parabola ion analyzer. A novel feature of this device is the conversion of the analyzed ion beam to a visible image via an electron multiplier array, providing complete information on a single — shot basis. The Nd+3 glass laser used in these experiments delivers up to 70 J to ...

Ion beam transport in laser-initiated discharge channels

Inertial‐confinement fusion reactors with light ion‐beam drivers will require that several intense ion beams propagate a few meters from separate diodes to a single target. Each beam would be confined and guided by a laser‐initiated z discharge in a background gas. In this experiment we have studied the transport of a proton beam in such a laser channel. A pulsed, line‐tuned, CO2 laser triggers the discharge in 4–20 Torr of ammonia gas. When the discharge current reaches peak value, an ion beam is injected from a pinch‐reflex ion diode on the Hydra accelerator. The transported proton beam is detected by nuclear activation of carbon targets. Total current transport efficiencies of up to 50% were achieved in 13–45 kA, one‐meter‐long discharges. The most reproducible results were found for discharge currents of less than 30 kA; framing photography shows that the channel is unstable for kink modes at higher currents. Conditions in the discharge which led to this kink instability are examined, and the implicat...

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.

Laser‐initiated channels for ion transport: Breakdown and channel evolution

The electrical breakdown and discharge evolution in CO2 laser‐heated molecular gases has been studied. With the laser tuned to a vibrational mode of NH3, C2H4, CH2CHCN, or CH3OH the breakdown potential decreases as much as 10‐fold for laser pulse energies up to 35 J/cm2. The subsequent 50–142‐cm discharges are straight, stable, and reproducible. Analogous tests in D2 and air yield only a small alteration of breakdown potential and do not cause a straight discharge. The expansion of the initial laser‐heated gas has been modeled by the CHARTB hydrocode with the addition of the NH3 equation of state in tabular and analytic form to that code. The breakdown characteristics and initial expansion stage confirm the earlier calculation of laser heating to 1900–2100 °K. Experimental observations of the discharge evolution in NH3 have measured (1) the radial expansion velocity by streak‐camera photography of the Hβ emission zone, (2) the plasma temperature by the Niv/Niii line‐ratio method, and (3) the electron‐dens...

Laser‐initiated channels for ion transport: CO2‐laser absorption and heating of NH3 and C2H4 gases

Initiation and guiding of an electrical discharge by CO2‐laser heating of a molecular gas can provide a channel suitable for the transport of a light ion beam for inertial‐confinement fusion. We report absorption measurements for a CO2 laser tuned to the molecular vibrations of NH3 and C2H4 as a function of gas pressure, laser frequency, and laser energy density. The SATUR laser‐gas‐interaction code models the details of the absorption saturation process for NH3, calculating gas temperature in the process. The calculation is normalized to absorption data at 0.04 J/cm2 and tested with transmission measurements up to 15 J/cm2 into a 50‐cm cell. These calculations are the basis for understanding the electrical breakdown and discharge evolution as observed and simulated in the following report. Of particular importance is the calculation of gas temperature of 1900–2100 °K for 15–35‐J/cm2 incident laser energy density.

Self‐magnetic‐field‐enhanced ion diode

An intense ion source has been developed utilizing an ion diode that partially suppresses electron flow using the self‐magnetic field of the diode current. This is an extension of a diode class known as pinch reflex ion diodes. In this case the diode was coupled to the particle beam fusion accelerator and was configured in two different cylindrical designs. The first pinch ion diode was a straight, 42‐cm‐diam cylinder and the second, Obi, was a focusing, 26‐cm‐diam, aspheric barrel. In the Obi diode a central gas cell provided current‐neutralized beam transport. In addition, the accelerator was run in a low‐voltage, 0.8 MV, and a high‐voltage, 2.0 MV, mode. The best results showed that the Obi diode produced 3 TW of protons at 34% efficiency in the high‐voltage mode. We present an analytic model of ion efficiency, compare various diode impedance models, and discuss beam divergence mechanisms. The limitation of this ion source as a fusion driver is presently the 2–3° divergence that we measure using a shad...

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...

Fuel preconditioning studies for e‐beam fusion targets

Fuel temperature and density conditions, achieved during the preheat phase of electron‐beam fusion compression experiments, must be accurately known to understand experimental results via numerical simulations. We present studies of discharge preheating in a simplified cylindrical geometry which compare measured quantities with results from the one‐dimensional Lagrangian CHARTB magnetohydrodynamic code. Experimental measurements included schlieren photography and ultraviolet through visible time‐ and space‐resolved spectroscopy in various configurations. It is seen that an 8‐kA 500‐ns heating pulse in 100 Torr of D2+10% O2 produces 10–12 eV temperatures, 1018 cm−3 electron densities, and 7×105 cm/s expansion velocities in the heated discharge channel. These results are consistent with previous claims for neutron‐producing targets, although the target geometry is different.

Nanosecond and picosecond laser‐produced CD2 plasmas

A preliminary comparison is made of plasmas created by irradiation of CD2 slab targets by single high‐energy Nd3+ glass laser pulses of 3.5‐nsec (35 J max) and 3–10‐psec (10 J max) duration. For the nanosecond irradiations we find electron temperatures of 350–400 eV and appreciable C5+ ions at 20‐keV expansion energy. Ion expansion energies for a picosecond pulse are about one‐third those for a nanosecond irradiation of comparable energy. Up to 50% of the incident pulse energy may be accounted for in plasma expansion in either case. In each case, the specularly reflected energy back into the input f/5.0 lens is small: 0.4% and 0.8% for the nanosecond and picosecond times, respectively. Detailed ion analysis shows the target composition to be CD2−xHx, where 0.2

Electromagnetic particle simulations of self‐magnetic‐field‐enhanced ion diodes

Electromagnetic particle simulations of equatorial self‐magnetic‐field‐enhanced ion diodes have been performed. These simulations complement recent experiments on the particle beam fusion accelerator using the pinch ion diode (pid) and the Obi diode. Those experiments are detailed in a companion paper. The simulations have provided useful information concerning the operation of the diode. This paper will describe many of the simulation findings in such areas as current and charge neutralization, virtual cathode formation, diode impedance, ion divergence, and electron temperatures. In addition, a model for ion efficiency has been obtained that is based upon the simulation results. This model is compared to various other efficiency models.