Arian L. Pregenzer

Measurement of electron energy deposition necessary to form an anode plasma in Ta, Ti, and C for coaxial bremsstrahlung diodes

Measurements are made of surface doses necessary to initiate an anode plasma by electron bombardment of Ta, Ti, and C anodes for coaxial geometries characteristic of high‐power electron‐beam diodes. Measured lower and upper bounds of doses necessary to form an anode plasma are 54±7–139±16 J/g in Ta, 214±23–294±71 J/g in Ti, and 316±33–494±52 J/g in C. Within these bounds, probable values for the threshold are given under specific assumptions. The measurements are consistent with a thermal desorption model for plasma formation.

Electrohydrodynamically driven large-area liquid ion sources

A large-area liquid ion source comprises means for generating, over a large area of the surface of a liquid, an electric field of a strength sufficient to induce emission of ions from a large area of said liquid. Large areas in this context are those distinct from emitting areas in unidimensional emitters.

Liquid lithium ion source: nonlinear behavior of liquid surface in electric field

Calculations have shown that electrohydrodynamic instabilities can produce surface distortions that may locally enhance the electric field to the extent that field evaporation of ions can occur from a liquid metal surface. Linear analysis of the electrohydrodynamic equations of motion has yielded the regions of instability as well as the wavelengths and growth times of the dominant unstable mode as a function of applied electric field. In the calculation presented here, the liquid surface is followed well into the nonlinear regime using a surface integral method. The results agree with the linear theory where appropriate, and add further support to the feasibility of a large‐area liquid‐lithium ion source for inertial confinement fusion experiments on a light ion‐beam driver.

Accuracy of Coupled Monte-Carlo/Next-Event-Estimator for Bremsstrahlung Dose Predictions

The accuracy of an efficient hybrid Monte Carlo/ analytic method for predicting the spatial dependence of x-ray energy deposition in common dosimetry is determined by comparing its predictions with those of full Monte Carlo Transport. For x-ray simulators, the hybrid model underpredicts energy deposition in thermoluminescent dosimetry by failing to account for Compton buildup and overpredicts deposition in gold calorimetry by failing to account for the lack of electron equilibration and fluorescence escape. For gamma simulators, the method accurately predicts the deposition in thermoluminescent dosimetry. We also find that the 2.16-mm Al electron equilibrators used with our standard thermoluminescent dosimeters are excessive for x-ray simulators and insufficient for gamma simulators.

Monte Carlo calculations of low energy electron backscatter coefficients

Abstract Experimental values of the electron backscatter coefficient, η, are compared with results obtained from a one-dimensional, multiple scattering Monte Carlo transport code, for normally incident electrons in the 3–100 keV range. For electron energies less than 10 keV the th

Ion production from LiF-coated field emitter tips

Ion emission has been obtained from a LiF‐coated tungsten field‐emitter tip. Ion formation is thought to be caused by the high electric field experienced by the LiF. At the time of emission the electric field at the surface of the LiF is calculated to be on the order of 100 MV/cm. Inside the LiF the field is on the order of 10 MV/cm. These fields exceed the value needed to produce bulk dielectric breakdown in LiF. The surface field is of sufficient magnitude to produce ion emission by field evaporation from the crystal surface. Even prior to dielectric breakdown, precursor processes can lead to ion formation. Electric‐field‐stress fragmentation of the LiF layer is thought to occur, followed by ionization of the fragments.

Ion sources for light‐ion fusion (invited)

Light‐ion drivers offer a potentially efficient and low‐cost method to compress and heat an inertial‐confinement fusion (ICF) target. In order to produce a significant thermonuclear burn of the fuel, focused ion‐beam intensities of 100 TW/cm2 are required. The ion sources for these pulsed‐power drivers should produce a single‐ion species and should be capable of providing current‐density levels up to 5 kA/cm2, in pulse widths of 10–20 ns, at voltages up to several tens of megavolts. Most ion sources used in the past have produced multiple‐ion species, including protons and heavier ions, such as carbon and oxygen. In the last few years there has been a substantial research effort to produce single‐species sources. The major effort since 1983 has been directed towards developing a pure, high‐current Li+ source for the light‐ion fusion accelerator, PBFA II. There are two types of ion sources being used for intense ion‐beam generation: (1) passive sources which are produced directly or indirectly by the diode...

Electrohydrodynamically driven, large‐area liquid metal ion source for inertial confinement fusion

Analysis of the electrohydrodynamic (EHD) equations of motion of a planar liquid‐lithium surface in the presence of a normal electric field suggest that liquid lithium may provide a large‐area ion source for intense ion‐beam diodes. Such sources are being developed for the Particle Beam Fusion Accelerator II at Sandia National Laboratories. In this paper, theoretical and experimental studies of the planar EHD ion source will be reviewed. When a planar liquid surface is subjected to an electric field of sufficient magnitude, EHD instabilities produce an array of cusps on the surface. The electric field enhancement at the apex of each cusp is sufficient to permit field evaporation of ions. The time delay between application of the electric field and ion emission depends on the magnitude and rate of increase of the applied electric field, and on the initial amplitude of the surface perturbation. Above 10 MV/cm, theory indicates that field emission will occur on a nanosecond time scale and that the characteri...

Measurements of temporal and spatial characteristics of electrohydrodynamic instabilities

Since an electrohydrodynamically (EHD) driven liquid‐lithium ion source is being developed for experiments on the Particle Beam Fusion Accelerator II at Sandia National Laboratories, EHD experiments have been started with several liquids. In this paper, we present results of experimental studies of the behavior of the surface of water and ethanol in the presence of 15–40‐kV/cm dc electric fields. Fields of this magnitude can produce unstable growth of surface perturbations with wavelengths in the 0.1–1.0 cm range. There is excellent agreement between theory and experiment for values of the wavelength of the dominant mode as a function of the applied electric field E0. Theoretical predictions about the temporal behavior of cusp formation are consistent with experiment. The experimental results are applicable, via scaling laws, to liquids in the presence of electric fields in the 1–50 MV/cm range, where unstable surface perturbations may produce field enhancements sufficient to field evaporate ions from the...

Radiation Field Results from the Speed Triax Diode, a Large Area, High Dose Rate, Short Pulse 1 MV Bremsstrahlung Source

A flash X-ray source with a high dose rate and fast risetime, but which has a low concurrent total dose, is described. Monte Carlo code predictions of dose are compared with measurements of both radiation field patterns and cross-correlation of dose in thermoluminescent detectors (TLDs) and PIN diodes. Reasonable agreement between predictions and measurements is observed for low-Z materials. Large discrepancies exist, however, for dose enhancement effects in Si from a thin Au layer.