Paul Allison

An Emittance Scanner for Intense Low-Energy Ion Beams

An emittance scanner has been developed for use with low-energy H- ion beams to satisfy the following requirements: (1) angular resolution of ± 1/2 mrad, (2) small errors from beam space charge, and (3) compact and simple design. The scanner consists of a 10-cm-long analyzer containing two slits and a pair of electric deflection plates driven by a ±500-V linear ramp generator. As the analyzer is mechanically driven across the beam, the front slit passes a thin ribbon of beam through the plates. The ion transit time is short compared with the ramp speed; therefore, the initial angle of the ions that pass through the rear slit is proportional to the instantaneous ramp voltage. The current through the rear slit then is proportional to the phase-space density d2i/dxdx. The data are computer-analyzed to give, for example, rms emittance and phase-space density contours. Comparison of measured data with those calculated from a prepared (collimated) phase space is in good agreement.

A Direct Extraction H- Ion Source

A direct extraction H- ion source of the Penning design described by Dudnikovl has been built and tested. To date a beam of 108 mA has been extracted from a 10 × 0.5 mm2 slit and transported through a 90° bending magnet into a Faraday cup at a duty factor of ~7 Hz × 700 ¿s (0.5%). The duty factor limitation is imposed by operating temperature requirements so that at reduced arc current a dc beam of 4 mA has been reached.

Cell design for the DARHT linear induction accelerators

The dual-axis radiographic hydrotest facility at Los Alamos National Laboratory will employ two linear induction accelerators to produce intense, bremsstrahlung X-ray pulses for flash radiography. The accelerator cell design for a 3-kA, 16-20-MeV, 60-ns flattop, high-brightness electron beam is presented. The cell is optimized for high-voltage stand-off while also minimizing the transverse impedance. Measurements of high-voltage RF characteristics are summarized.<<ETX>>

H− and D− scaling laws for Penning surface‐plasma sources

The small‐angle source (SAS), 4X source, and 8X source are Penning surface‐plasma sources that produce high‐current, high‐brightness H− ion beams for accelerator applications. The scaling from the SAS (1X source) to the 4X source, and from the 4X source to the 8X source is at least as good as predicted by the scaling laws. In many instances, the scaling is better than predicted, particularly in the critical area of the efficiency with which H− ions are produced per unit of discharge power. Using ζ=jH−/FC, where FC is the cathode power density load, ζ4X≊2ζSAS and ζ8X≊1.5ζ4X. The jH− scaling is at least as good as predicted by the scaling laws—we have been able to produce the predicted H− current in both the 4X and the 8X sources. The SAS was scaled up in size to the 4X source, and the 4X source was scaled up in size to the 8X source, on the assumption that the effective kTH−=5 eV. We also assume that the effective kTH−=the effective kTD−. These temperature scalings appear to be obeyed. The D− current scali...

Beam injector and transport calculations for ITS

The Integrated Test Stand at Los Alamos National Laboratory (LANL) is addressing issues in high-brightness electron beam generation, acceleration, and transport. The machine consists of a 3 kA, 3.5 MV injector, eight induction acceleration gaps, a drift section and a final-focus magnet which focuses the beam onto diagnostic targets. One of the goals of the program is to test and improve the predictive capability of numerical models. We have carried out detailed simulations of the diode and initial drift region with the particle-in-cell codes IVORY and SPROP, obtaining good agreement with experimental streak-camera data at several axial locations. Transport through the accelerating cells to the target 10 m from the cathode is modeled with the envelope codes LAMDA and XTR. The magnet settings for minimum spot-size are close to the experimental values. From the measured value of the minimum spot-size we infer a normalized Lapostolle emittance of about 0.2 cm-rad. We have characterized the sensitivity of the spot-size to variation in the machine parameters.

Some comments on emittance of H− ion beams

Some properties of emittance, emittance distributions, and measurement techniques are reviewed. In comparing the results of measurements with several different types of H− sources with each other and with emittance formulae, it is concluded that the emittance of surface‐type sources is dominated by the effective ion temperature. Other effects, such as ion‐optical distortions, may account for the emittance of volume‐type sources.

Operating experience with a 100‐keV, 100‐mA H− injector

According to beam dynamics calculations it should be possible to accelerate a high‐perveance beam in a radio‐frequency quadrupole (RFQ) accelerator with low emittance growth and nearly 100% capture efficiency. A 100‐mA, 100‐keV H− ion injector with a 5‐Hz, 1‐ms duty factor was built for use with this accelerator, but the beam emittance at 100 keV was found to be two to four times the value previously determined at 20 keV. This emittance growth was traced to the 20‐keV beam transport, where an instability occurred in the background plasma created by beam ionization of the residual gas. The injector has been rebuilt with a shorter transport length, resulting in greatly reduced emittance growth.

Transverse impedance measurements of prototype cavities for a dual-axis radiographic hydrotest (DARHT) facility

The authors describe the results of wire measurements of several prototype DARHT cavities. The measurements reported cover MOD0 with ferrites removed, measurements with drive rods in place with various terminations, and measurements with compensation cans. Measured impedances are compared with AMDS code predictions. The effects of various rod terminations and compensation resistors on cavity impedances are shown.<<ETX>>

The 4X Source

Our Penning surface-plasma source (SPS) discharge chamber was enlarged 4X in two dimensions. To date, three pulsed discharge modes have been studied: two with noisy arc (¿20% H- current fluctuations) and one with quiescent arc (¿1% H- current fluctuations). Lower arc magnetic field and higher H2 gas flow allow switching from the noisy to the quiescent mode. The noisy modes yield up to 120 mA of 29-keV H- beam; for 110 mA at 29 keV, the two-dimensional normalized rms emittance is 0.017 x 0.018 ¿·cm·mrad. The quiescent mode yields 75 mA of 29-keV H- beam; for 67 mA at 24 keV, the emittance is 0.011 x 0.012 ¿·cm·mrad.

Spectroscopic investigation of H− and D− ion source plasmas

Several H I (Balmer), Cs I, Cs II, and Mo I lines emitted by the small‐angle source and 4X source plasmas are studied. After correcting for Stark broadening, the Hα line width gives the H‐atom temeprature kTHO. After correcting for Doppler broadening, the Hβ and Hδ line widths give the electron density ne. For pulsed operation of both sources, kTHo is 1.5 to 2 eV and ne is 1 to 2×1014/cm3,with kTHo and ne scaling approximately with the square root of the discharge current. For the 4X source operated on D2, kTDo and ne are near the values of kTHo and ne obtained for H2 operation. Assuming that the H−/D− ion temperature equals the H/D‐atom temperature, we deduce a lower limit to the H−/D− beam emittance.