R.D. Fulton

The interaction of a high irradiance, subpicosecond laser pulse with aluminum: The effects of the prepulse on x‐ray production

The conversion efficiency into kilovolt line radiation for 248‐nm light at 1017 W/cm2 on an aluminum target is measured. The x‐ray yield is found to increase with the scale length of the target plasma. The interaction is modeled as resonance absorption, and the plasma scale length is determined from the prelase energy and irradiance.

X‐ray generation by high irradiance subpicosecond lasers

We have studied the interaction of 290‐fs, 308‐nm laser pulses with aluminum targets at irradiances exceeding 5×1018 W/cm2. The x‐ray spectrum is dominated by the H‐ and He‐like lines from aluminum, with the brightest lines radiating 0.8% of the incident laser light energy. This fraction is close to that measured at 50 times less irradiance, but occurs at a slightly higher ionization stage. The x rays are emitted from a region of subcritical electron density at 3–6×1021 W cm−3. The radiance of the 1.73‐keV Lα line is measured to be 4×1012 W/cm2/sr.

Design of a driver for the Cygnus X-ray source

Cygnus is the prototype of a radiographic x-ray source leveraging existing hardware and designs to drive a rod-pinch diode at 2.25 MV. This high-resolution x-ray source is being developed to support the Sub-Critical Experiments Program (SCE) at the Nevada Test Site (NTS), and as such employs a modular technology that is scaleable to higher voltages and can be readily deployed underground. The diode is driven by three Induction Voltage Adder (IVA) cells from the Sandia SABRE [1] accelerator, threaded by a positive polarity vacuum coax that extends 2 meters to the diode and is designed to operate below electron emission on the anodized outer electrode. The /spl sim/40 ohm diode impedance requires a 40/3/sup 2/ or /spl sim/4.5 ohm source to drive the three IVA cavities in parallel; a convenient impedance for a single water coax. The water coax is designed to function as a two-step impedance transformer as well as a long, passive water cable, accommodating several bends along its length. The latter feature allows independent positioning of the pulsed power driver, IVA and diode x-ray source. The long water coax is driven by a PFL originally developed for Sandias Radiographic Integrated Test Stand (RITS) and a low-inductance commercial Marx charges the single PFL. The accelerator design is a result of a cooperative effort by Titan-PSI and Maxwell (now collectively Titan-PSD) SNLA, LANL, NRL, and Bechtel-Nevada.

Performance of the Cygnus X-ray source

Summary form only given. Cygnus is a radiographic X-ray source developed for support of the Sub-Critical Experiments Program at the Nevada Test Site. Major requirements for this application are: a dramatically reduced spot size as compared to both Government Laboratory and existing commercial alternatives, layout flexibility, and reliability. Cygnus incorporates proven pulsed power technology (Marx Generator, Pulse Forming Line, Water Transmission Line, and Inductive Voltage Adder sub-components) to drive a high voltage vacuum diode. In the case of Cygnus, a relatively new approach (the rod pinch diode) is employed to achieve a small source diameter. Design specifications are: 2.25 MeV peak energy, < 1 min source diameter, and 5-10 rads dose at 1 meter. The pulsed power and system architecture design plan has been previously presented (Weidenheimer et al., 2001). The first set of Cygnus shots are now underway and are geared to verification of electrical parameters and, therefore, use a large area diode configuration offering increased shot rate as compared to that of the rod pinch diode. Later tests incorporate the rod pinch diode and will concentrate on X-ray production with time resolved measurements of X-ray dose and spot size. In this work we present results of initial operation in terms of electrical and radiation parameters. In addition, the issues associated with static and time resolved radiographs may be included. Performance of the pulse power system is being evaluated by comparison of measured to design output parameters. This is accomplished by comparison of multiple voltage and current measurements throughout the system with various circuit model codes such as MicroCAP and T-Line.

Intense electron beam sources for flash radiography

High intensity pulsed electron beams are used to create bremsstrahlung x-ray sources for flash radiographic interrogation of dynamic experiments. Typical industrial sources operate below 200 GW/cm2 intensities, while experimental requirements can demand above 50 TW/cm2. Recent developments in pulsed power-driven high intensity electron beam systems have significantly increased these operating regimes, demonstrating 20 TW/cm2, and computations predict successful extrapolation to higher intensities. Detailed studies of electron beam configurations, both theoretical and experimental, and the prognosis for each to increase to the required levels is discussed.

Enhanced-efficiency, narrow-band gigawatt microwave output of the reditron oscillator

Experiments are described which confirm theoretical predictions of higher-efficiency, narrower-bandwidth microwave output of the reditron oscillator over simple vircator high-power microwave devices. The authors produced 1.6 GW of microwave power at 2.46 GHz. The conversion efficiency from beam power to microwave power was 5.5 to 6%, exceeding the usual 1 to 3% efficiency obtained with conventional vircators. The 1% bandwidth was about a factor of 10 less than the usual value for virtual cathode oscillators. Results of computer calculations in reasonable agreement with the experimental findings are presented. >

Cygnus Dual Beam Radiography Source

The subcritical experiment (SCE) program was initiated after the 1992 moratorium on underground nuclear testing in support of stockpile stewardship. The dynamic material properties of plutonium are a major topic of exploration for the SCE program. In order to provide for a multilayered containment of plutonium, the SCEs are executed in the Ula underground tunnel complex at the Nevada Test Site (NTS). Cygnus is a new radiographic X-ray source developed for diagnostic support of the SCE Program at NTS. Typically, SCEs have been limited to surface diagnostics. Cygnus radiography was developed to complement the existing surface diagnostics, provide a more extensive spatial view (albeit temporally limited), and provide internal (penetrating) measurements. The Stallion series of SCEs consists of the following four shots listed in chronological order: Vito, Rocco, Mario, and Armando. Armando was the initial experiment for Cygnus radiography. The Rocco, Mario, and Armando tests use identical physics packages, permitting the correlation of Armando radiographic results with surface results from all three shots. The main X-ray source requirements for an SCE involve spot size, intensity, penetration, and duration. To this end Cygnus was designed to satisfy the following specifications: ~1 mm source diameter, 4 Rads dose at a distance of 1 meter, ~2.25 MeV endpoint energy, and < 100 ns pulse length. Two Cygnus sources (Cygnus 1, Cygnus 2) were fielded at NTS providing two views separated in space by 60deg and in time by 2 mus. Cygnus performance as a dual beam radiography source at NTS is highlighted in this paper.

Performance of the Cygnus x-ray source

Cygnus is a radiographic x-ray source developed for support of the Sub-Critical Experiments Program at the Nevada Test Site. Major requirements for this application are: a dramatically reduced spot size as compared to both Government Laboratory and existing commercial alternatives, layout flexibility, and reliability. Cygnus incorporates proven pulsed power technology (Marx Generator, Pulse Forming Line, Water Transmission Line, and Inductive Voltage Adder sub-components) to drive a high voltage vacuum diode. In the case of Cygnus, a relatively new approach (the rod pinch diode [1]) is employed to achieve a small source diameter. Design specifications are: 2.25 MeV endpoint energy, &#60; 1 mm source diameter, and >3 rads dose at 1 meter. The pulsed power and system architecture design plan has been previously presented [2]. The first set of Cygnus shots were geared to verification of electrical parameters and, therefore, used a large area diode configuration offering increased shot rate as compared to that of the rod pinch diode. In this paper we present results of initial rod pinch operation in terms of electrical and radiation parameters.

The Atlas High-Energy Density Physics Project

Atlas is a pulsed-power facility under development at Los Alamos National Laboratory to drive high-energy density experiments. Atlas will be operational in the summer of 2000 and is optimized for the study of dynamic material properties, hydrodynamics, and dense plasmas under extreme conditions. Atlas is designed to implode heavy-liner loads in a z-pinch configuration. The peak current of 30 MA is delivered in 4 µs. A typical Atlas liner is a 47-gram-aluminum cylinder with ∼ 4-cm radius and 4-cm length. Three to five MJ of kinetic energy will be delivered to the load. Using composite layers and a variety of interior target designs, a wide variety of experiments in ∼ cm3 volumes will be performed. Atlas applications, machine design, and the status of the project are reviewed.

Pegasus II experiments and plans for the Atlas pulsed power facility

Atlas will be a high-energy (36 MJ stored), high-power (/spl sim/10 TW) pulsed power driver for high energy-density experiments, with an emphasis on hydrodynamics. Scheduled for completion in late 1999, Atlas is designed to produce currents in the 40-50 MA range with a quarter-cycle time of 4-5 /spl mu/s. It will drive implosions of heavy liners (typically 50 g) with implosion velocities exceeding 20 mm//spl mu/s. Under these conditions, very high pressures and magnetic fields are produced. Shock pressures in the 50 Mbar range and pressures exceeding 10 Mbar in an adiabatic compression will be possible. By performing flux compression of a seed field, axial magnetic fields in the 2000 T range may be achieved. A variety of concepts have been identified for the first experimental campaigns on Atlas. Experimental configurations, associated physics issues, and diagnostic strategies are all under investigation as the design of the Atlas facility proceeds. Near-term proof-of-principle experiments employing the smaller Pegasus II capacitor bank have been identified, and several of these experiments have now been performed. This paper discusses a number of recent Pegasus II experiments and identifies several areas of research presently planned on Atlas.