A dc proton injector for use in high-current cw linacs

A 75-keV dc and pulsed-mode proton injector is being developed for beam testing of a 100-mA, 6.7-MeV cw radio frequency quadrupole (RFQ) at Los Alamos. A microwave proton source operating at 2.45 GHz produces 130-mA hydrogen-ion beam currents with > 85% proton fraction, yielding the 110-mA proton current required at the RFQ injection point. Doppler-shift spectroscopy confirms previously measured proton fractions. The discharge may be pulsed by current modulation of the magnetron power supply. A 1-MHz coherent oscillation observed in the extracted ion beam was eliminated by selecting proper magnetron operation. Transport and matching of the proton beam to the RFQ is accomplished by a two-solenoid, beam space-charge neutralized low-energy beam transport (LEBT) system. The injector was temporarily reconfigured to operate at 50 keV for injector matching studies into a 1.25 MeV cw RFQ. A maximum current of 100-mA has been accelerated through the RFQ in cw mode

Development and test results of the low-energy demonstration accelerator (LEDA) proton injector on a 1.25 MeV cw radio frequency quadrupole

The low-energy demonstration accelerator (LEDA) 75-keV proton injector is being developed for tests of high-current (100-mA) cw linacs. The injector comprises a microwave proton source and a space-charge neutralized magnetic low-energy beam-transport system (LEBT). The LEDA injector has been configured to provide flexible 50-keV beam matching into a cw 1.25-MeV radio-frequency quadrupole (RFQ) brought from Chalk River Laboratories (CRL). The LEBT has two solenoid focus magnets separated by 117 cm. Between the solenoids are two steering magnets and diagnostic stations for measuring the beam current, profile, and position. The ion-source extraction system was modified to a 50-keV triode to test the injector/RFQ system. Beam-matching tests showed that injector-RFQ transmission is 90% for 50-mA RFQ current. At the RFQ design current of 75 mA the beam transmission decreased to 80--85%. Optimized injector tuning led to 100-mA beam accelerated through the RFQ

Development of a broadband reflectivity diagnostic for laser driven shock compression experiments

A normal-incidence visible and near-infrared shock wave optical reflectivity diagnostic was constructed to investigate changes in the optical properties of materials under dynamic laser compression. Documenting wavelength- and time-dependent changes in the optical properties of laser-shock compressed samples has been difficult, primarily due to the small sample sizes and short time scales involved, but we succeeded in doing so by broadening a series of time delayed 800-nm pulses from an ultrafast Ti:sapphire laser to generate high-intensity broadband light at nanosecond time scales. This diagnostic was demonstrated over the wavelength range 450-1150 nm with up to 16 time displaced spectra during a single shock experiment. Simultaneous off-normal incidence velocity interferometry (velocity interferometer system for any reflector) characterized the sample under laser-compression and also provided an independent reflectivity measurement at 532 nm wavelength. The shock-driven semiconductor-to-metallic transition in germanium was documented by the way of reflectivity measurements with 0.5 ns time resolution and a wavelength resolution of 10 nm

Two-dimensional Imaging Velocity Interferometry: Technique and Data Analysis

We describe the data analysis procedures for an emerging interferometric technique for measuring motion across a two-dimensional image at a moment in time, i.e. a snapshot 2d-VISAR. Velocity interferometers (VISAR) measuring target motion to high precision have been an important diagnostic in shockwave physics for many years Until recently, this diagnostic has been limited to measuring motion at points or lines across a target. We introduce an emerging interferometric technique for measuring motion across a two-dimensional image, which could be called a snapshot 2d-VISAR. If a sufficiently fast movie camera technology existed, it could be placed behind a traditional VISAR optical system and record a 2d image vs time. But since that technology is not yet available, we use a CCD detector to record a single 2d image, with the pulsed nature of the illumination providing the time resolution. Consequently, since we are using pulsed illumination having a coherence length shorter than the VISAR interferometer delay ({approx}0.1 ns), we must use the white light velocimetry configuration to produce fringes with significant visibility. In this scheme, two interferometers (illuminating, detecting) having nearly identical delays are used in series, with one before the target and one after. This produces fringes with at most 50% visibility, but otherwise has the same fringe shift per target motion of a traditional VISAR. The 2d-VISAR observes a new world of information about shock behavior not readily accessible by traditional point or 1d-VISARS, simultaneously providing both a velocity map and an 'ordinary' snapshot photograph of the target. The 2d-VISAR has been used to observe nonuniformities in NIF related targets (polycrystalline diamond, Be), and in Si and Al

Comparison of beam simulations with measurements for a 1.25-MeV, CW RFQ

The Low-Energy Demonstration Accelerator (LEDA) injector is tested using the Chalk River Injector Test Stand (CRITS) radio-frequency quadrupole (RFQ) as a diagnostic instrument. Fifty-keV, dc proton beams are injected into the 1.25-MeV, CW RFQ and transported to a beamstop. Computer-simulation-code predictions of the expected beam performance are compared with the measured beam currents and beam profiles. Good agreement is obtained between the measurements and the simulations at the 75-mA design RFQ output current

Recovery of Metastable Dense Bi Synthesized by Shock Compression

X-ray free electron laser (XFEL) sources have revolutionized our capability to study ultrafast material behavior. Using an XFEL, we revisit the structural dynamics of shock compressed bismuth, resolving the transition sequence on shock release in unprecedented details. Unlike previous studies that found the phase-transition sequence on shock release to largely adhere to the equilibrium phase diagram (i.e., Bi-V → Bi-III → Bi-II → Bi-I), our results clearly reveal previously unseen, non-equilibrium behavior at these conditions. On pressure release from the Bi-V phase at 5 GPa, the Bi-III phase is not formed but rather a new metastable form of Bi. This new phase transforms into the Bi-II phase which in turn transforms into a phase of Bi which is not observed on compression. We determine this phase to be isostructural with β-Sn and recover it to ambient pressure where it exists for 20 ns before transforming back to the Bi-I phase. The structural relationship between the tetragonal β-Sn phase and the Bi-II phase (from which it forms) is discussed. Our results show the effect that rapid compression rates can have on the phase selection in a transforming material and show great promise for recovering high-pressure polymorphs with novel material properties in the future

Coordination changes in liquid tin under shock compression determined using in situ femtosecond x-ray diffraction

Little is known regarding the liquid structure of materials compressed to extreme conditions, and even less is known about liquid structures undergoing rapid compression on nanosecond timescales. Here, we report on liquid structure factor and radial distribution function measurements of tin shock compressed to 84(19) GPa. High-quality, femtosecond x-ray diffraction measurements at the Linac Coherent Light Source were used to extract the liquid diffuse scattering signal. From the radial distribution function, we find that the structural evolution of the liquid with increasing pressure mimics the evolution of the solid phase. With increasing pressure, we find that the liquid structure evolves from a complex structure, with a low coordination number, to a simple liquid structure with a coordination number of 12. We provide a pathway for future experiments to study liquids at elevated pressures using high-energy lasers to shock compress materials beyond the reach of static diamond anvil cell techniques

Measurement of RF surface efficiency at cryogenic temperatures using a resonant cavity

Exploiting the potential efficiency gain of a normal conducting rf accelerator operated at cryogenic temperatures requires careful preparation of the rf conducting surface. Experimental apparatus has been assembled to study the surface conductivity to rf currents at 425 MHz and 850 MHz through a temperature range from room temperature to 14 K. The apparatus is built around an open-ended coaxial cavity with the cavity tubular ends below the cutoff frequency at resonance. The center conductor in the coaxial cavity is the test sample, and the use of a dielectric stand-off for the center conductor precludes the need for an rf contact joint and facilitates sample changes. The rf testing is conducted under vacuum with low-power rf. A CTI-Cryogenics cryopump coldhead is used for cryogenic temperature cycling of the test cavity. A detailed description of the apparatus and measurement procedures are presented

Nonisentropic Release of a Shocked Solid

We present molecular dynamics (MD) simulations of shock and release in micron-scale tantalum crystals that exhibit post-breakout temperatures far exceeding those expected under the standard assumption of isentropic release. We show via an energy-budget analysis that this is due to plastic-work heating from material strength that largely counters thermoelastic cooling. The simulations are corroborated by experiments where the release temperatures of laser-shocked tantalum foils are deduced from their thermal strains via in situ x-ray diffraction, and are found to be close to those behind the shock