High-perveance Beams Research Articles

Experimental performance of an E×B chopper system

Beam operation of a novel $E\ifmmode\times\else\texttimes\fi{}B$ chopper system has started in the low-energy beam transport (LEBT) section of the accelerator-driven neutron source FRANZ. The chopper is designed for low-energy high-perveance beams and high repetition rates, and will finally operate with 120 keV protons. It combines a static magnetic deflection field with a pulsed electric compensation field in a Wien filter-type $E\ifmmode\times\else\texttimes\fi{}B$ configuration. The chopper was designed, manufactured and successfully commissioned at the required repetition rate of 257 kHz using a 14 keV helium beam with up to 3.5 mA of beam current. Beam pulses with rise times of $(120\ifmmode\pm\else\textpm\fi{}10)\text{ }\mathrm{ns}$, flat-top lengths of $(85\ifmmode\pm\else\textpm\fi{}10)\text{ }\mathrm{ns}$ to $(120\ifmmode\pm\else\textpm\fi{}10)\text{ }\mathrm{ns}$ and full width at half maximum (FWHM) between $(295\ifmmode\pm\else\textpm\fi{}10)\text{ }\mathrm{ns}$ and $(370\ifmmode\pm\else\textpm\fi{}10)\text{ }\mathrm{ns}$ were experimentally achieved.

Scientific issues in future induction linac accelerators for heavy-ion fusion

The worldwide heavy-ion fusion effort has made a large amount of progress over almost three decades of research on the intense beam physics and concepts needed for an inertial fusion driver. In the 1970s, ‘80s, and ‘90s in the US, most experiments used driver parameters scaled to low energy and low line charge density in order to study, most often with a single beam, the physics of high-perveance beams. More recently, the US program has introduced higher line charge density experiments, where the beam space charge potential is significant. This allows investigation of the production, and effect on the beam, of stray electrons. Increasingly, the possibilities for neutralizing plasmas are also being investigated, both just upstream and in the target chamber. This talk will outline the scientific issues that remain to be examined. These include issues of longitudinal compression, dynamic aperture, electron/ion instabilities, effect of electrons and gas on the beam, longitudinal emittance evolution, halo production, inductive effects at high energy, multiple beams, innovative final focus approaches, and driver issues for solenoidal transport. An outline of future experiments to address these issues will be presented and discussed.

Circular-to-planar transformations of high-perveance electron beams by asymmetric solenoid lenses

The generation and transport of planar electron beams is a topic of increasing interest for applications to high-power, high-frequency microwave devices. This paper describes the use of solenoid lenses with elliptical pole apertures to transform a circular high-perveance beam to a planar configuration. The resulting transformation system has a short axial length and is easily fabricated. Furthermore, the approach allows use of a conventional electron gun. We designed lenses and simulated the dynamics of a beam system to drive a 95 GHz, traveling-wave tube experiment at Los Alamos National Laboratory. The three-dimensional self-consistent calculations indicate that a $0.5\text{ }\ensuremath{\mu}\mathrm{perv}$ beam can be compressed to a height of less than 0.5 mm with sufficiently low emittance for transport through periodic-permanent-magnet or wiggler arrays.

Progress in heavy ion fusion research

The U.S. Heavy Ion Fusion program has recently commissioned several new experiments. In the High Current Experiment [P. A. Seidl et al., Laser Part. Beams 20, 435 (2003)], a single low-energy beam with driver-scale charge-per-unit-length and space-charge potential is being used to study the limits to transportable current posed by nonlinear fields and secondary atoms, ions, and electrons. The Neutralized Transport Experiment similarly employs a low-energy beam with driver-scale perveance to study final focus of high perveance beams and neutralization for transport in the target chamber. Other scaled experiments—the University of Maryland Electron Ring [P. G. O’Shea et al., accepted for publication in Laser Part. Beams] and the Paul Trap Simulator Experiment [R. C. Davidson, H. Qin, and G. Shvets, Phys. Plasmas 7, 1020 (2000)]—will provide fundamental physics results on processes with longer scale lengths. An experiment to test a new injector concept is also in the design stage. This paper will describe the goals and status of these experiments, as well as progress in theory and simulation. A proposed future proof-of-principle experiment, the Integrated Beam Experiment, will also be described.

Simulations of intense heavy ion beams propagating through a gaseous fusion target chamber

In heavy-ion inertial confinement fusion (HIF), an ion beam is transported several meters through the reactor chamber to the target. This standoff distance mitigates damage to the accelerator from the target explosion. For the high perveance beams and millimeter-scale targets under consideration, the transport method is largely determined by the degree of ion charge and current neutralization in the chamber. This neutralization becomes increasingly difficult as the beam interacts with the ambient chamber environment and strips to higher charge states. Nearly complete neutralization permits neutralized-ballistic transport (main-line HIF transport method), where the ion beam enters the chamber at roughly 3-cm radius and focuses onto the target. In the backup pinched-transport schemes, the beam is first focused outside the chamber before propagating at small radius to the target. With nearly complete charge neutralization, the large beam divergence is contained by a strong magnetic field resulting from roughly 50-kA net current. In assisted-pinched transport, a preformed discharge channel provides the net current and the discharge plasma provides nearly complete charge and current neutralization of the beam. In self-pinched transport, the residual net current results solely from the beam-driven breakdown of the ambient gas. Using hybrid particle-in-cell simulation codes, the behavior of HIF driver-scale beams in these three transport modes is examined. Simulations of neutralized ballistic transport, at a few-mTorr flibe pressure, show excellent neutralization given a preformed or photoionized (from the heated target) plasma. Two- and three-dimensional simulations of assisted-pinch transport in roughly 1-Torr Xe show the importance of attaining >1-μs magnetic diffusion time to limit self-field effects and achieve good transport efficiency. For Xe gas pressures ranging from 10–150 mTorr, calculations predict a robust self-magnetic force sufficient for self-pinched transport. The latest simulation results are presented and the important remaining issues for each transport scheme are discussed.

Simulation techniques for heavy ion fusion chamber transport

Heavy ion-driven inertial confinement fusion requires efficient transport of the ion beam over several meters from the entrance of the accelerator to the target. Currently considered transport modes with high perveance beams require significant charge and current neutralization that will likely require pre-existing dense plasmas. We have developed a hybrid implicit algorithm to treat dense plasmas accurately. Using this algorithm, simulations of neutralized ballistic transport show improved transport efficiency for a 4-GeV, 4-kA Pb ion beam by including a 10 13 cm −3 plasma at initialization.

Fabrication of gated nanosize Si-tip arrays for high perveance electron beam applications

Nanosize Si-tip arrays with gated electrodes have been fabricated using the self-aligned method. In order to have a parallel electron beam (high perveance beam) toward the anode plate, we have designed a nanosize tip array with heights of the tip slightly less than that of a gate electrode. A high perveance beam is supposed to provide better focusing of the electron beams. Hence, it is important to have a high perveance electron beam for nanolithographic application. The fabricated procedures for nanoscale Si-tip array are reactive ion etching, sharpening, and oxidation followed by a 7:1 BHF oxide etch. The metal gate fabrication procedures are performed with self-aligned techniques using plasma oxide deposition, metal sputter deposition, and photoresist spin coating. The self-aligned methods allow for a controlling gate aperture less than 1.0 μm. The structure of the fabricated gated electron source was designed to have a 1.5 μm gate aperture, a 1.5 μm SiO2 insulating layer, and a 0.3 μm Mo volcano-type gate electrode. The Fowler–Nordheim and current–voltage characteristics of the fabricated tip arrays after seasoning the tip in a high vacuum chamber (<5×10−8 Torr) were examined carefully and its turn-on voltage was found to be ∼25 V. The observed bright electron spots on the anode screen was measured to be ∼300 μA. The total area for the (300×300) Si-tip array was ∼1.5 mm2. A charge coupled device camera photographed the bright area on the anode phosphor plate from electron bombardments and the size of electron bombardment spot was almost the same as the original tip array area (1.8 mm2). In addition, we have also fabricated the gated Si-tip arrays with a focusing electrode for angular confinement of the electron beam emission and high perveance for the beam trajectory.

The uniform-field magnicon: an architecture more suitable for high-perveance beams

This paper provides the details and modeling results of a 700-MHz, 2.5-MW, continuous wave, non-conventional magnicon architecture. It then compares the new architecture to the conventional design. The new magnicon architecture proves less sensitive to beam size yet provides the very high dc-to-RF conversion efficiency demonstrated in the initial magnicon development. The magnicon developments to date have utilized longitudinal, dc magnetic fields with different field strengths in the deflection system and output cavity. While this transition between field strengths was used in the initial magnicon to assist in the conversion of the initial longitudinal beam energy into the transverse direction, it also spreads the beam bunch (for finite beam size), reducing conversion efficiency. The new magnicon architecture presented in this paper utilizes a constant focusing field over the entire device. Conventional and new magnicon designs are presented for the 700-MHz, 2.5-MW requirements. Large-signal modeling techniques are used to compare magnicon performance of these designs for large-diameter beams. The modeling techniques used to generate the self-consistent solutions for the beam-driven magnicon cavities are discussed, and the relationship between the magnicon efficiency and electron-beam diameter is presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Low-energy beam transport of intense and partially space-charge-neutralized ion beams

High-perveance ion beams are required for heavy-ion inertial-fusion drivers and cannot be transported without a reduction of space charge forces by space charge neutralization. The rearrangement of beam ions due to the self-field of partly neutralized beams is one source of r.m.s. emittance growth in low-energy sections. Calculations including the influence of compensating electrons have been carried out. A low-energy beam line is being set up to investigate the influence of partial space charge neutralization on the transverse beam emittance of high-perveance beams (typically 9keV, 3.5 mA He+). Theoretical assumptions and results of numerical simulations are presented and compared with first experimental results.

Electromagnetic scanning systems

Rated beam currents of ion implantation systems have increased by three orders of magnitude over the last decade. Scanning systems employing time varying electric fields which are entirely adequate for low and intermediate current applications present significant problems when they are applied to high current low energy beams of heavy dopants. The successful handling of these high perveance beams depends upon electron space charge neutralization to preserve the optical quality of the beam. A new hybrid mechanical-magnetic scanning system with parallel scan correction as applied to a high current implanter is analyzed and the principles governing the design of this hybrid system are outlined. Data on the operation of this system is presented to illustrate typical performance at energies up to 80 keV and currents as high as 10 mA for ion beams scanned over a 30 cm lateral dimension at frequencies of the order of 0.1 Hz. Dopant uniformity data shows that under favorable conditions hybrid magnetic scanning can achieve uniformities as good as 2σ < 1%.

A high current ion implanter with hybrid scanning

An ion implanter delivering currents above 1 mA at 200 keV is described. Wafers are supported by a carousel; the beam is electrostatically scanned in a direction perpendicular to the wafers' displacement. We show how electrostatic scanning can be used even for high perveance beams. The advantage of fast (electrostatic) scanning on both dose uniformity and wafer temperature is demonstrated. As an example, it has been possible to implant 10 16 at.cm −2 over 6000 cm 2 during 90 min at an energy near 200 keV without exceeding 120°C on the wafer surface. Representative measured inhomogeneities range in the order of α = 0.5% for 3″ wafers at doses of 10 13 −10 16 cm −2.

Space-charge wavelengths in electron beams

The discrepancies between measured and calculated small-signal space-charge wavelengths are discussed. It is shown that conventional methods of calculation are inadequate, causing errors up to 40 percent in a typical case. It is demonstrated that these errors cannot be explained by beam scalloping action, and that they are most severe when a high-perveance beam is used at low voltages with a high magnetic field under conditions of low interception. Although these effects would not be noticed in klystrons with relatively short drift lengths, they may in many cases explain the critical variation of gain and efficiency with magnetic field. A more accurate method of calculating space-charge wavelengths is developed, and the effectiveness of this method is demonstrated by a comparison with measurements.

Effect of Velocity Distribution on Traveling-Wave Tube Gain

The density-function method is applied to the one-dimensional model of the traveling-wave tube to determine the effect of velocity distribution on traveling-wave tube performance. For narrow velocity spreads, a term is introduced into the root equation for the incremental propagation constants which is like the space charge term 4QC and directly added thereto. This term is proportional to the ratio of the variance of the distribution to the square of the gain parameter C. For a typical tube, thermal velocities are found to reduce the gain by less than one part in 104. In a tube employing confined magnetic focusing, and employing a high perveance beam, the velocity spread resulting from space charge lowers the gain by about 1 percent.