Search Results (12 total)

A numerical model of the ion-hose instability for long-pulse electron linacs is presented, where the ion motion is represented by fluid parameters. In order to gain extra numerical stability, the fluid behavior of the ions is evolved via particle-in-cell (PIC) techniques. This methodology provides a much faster simulation than a full PIC calculation, allowing for end-to-end simulations of the ion-hose instability in actual linear accelerator configurations. After the description of the simulation model and some simple test cases, the instability is analyzed for a variety of nominal accelerator transport conditions. Simulations of the instability are provided for sections of the DARHT long-pulse accelerator that show different growth regimes of the instability. We find that large-amplitude growth is possible in accelerator and transport regions lacking uniform external focusing, for electron pulse lengths of 2   μsec and longer.

Currently ongoing at Los Alamos National Laboratory is a program to develop high-power, planar 100–300 GHz traveling-wave tubes. A necessary part of this effort is a sheet electron beam source. Previously, we have described a novel asymmetric solenoid lens concept for transforming the circular beam from a high-perveance electron gun to a planar configuration. The lens is a standard electromagnetic solenoid with elliptical, instead of circular, pole apertures. The elliptical pole openings result in asymmetric focusing, which in turn forms an elliptical sheet beam suitable for our planar structures. Here we report the first experimental demonstration of this lens.

Two-plane focusing of sheet electron beams will be an essential technology for an emerging class of high-power, 100 to 300 GHz rf sources [Carlsten , IEEE Trans. Plasma Sci. 33, 85 (2005)]. In these devices, the beam has a unique asymmetry in which the transport is emittance dominated in the sheet’s thin dimension and space-charge dominated in the sheet’s wide dimension. Previous work has studied the stability of the transport of beams in the emittance-dominated regime for both wiggler and periodic permanent magnet (PPM) configurations with single-plane focusing, and has found that bigger envelope scalloping occurs for equilibrium transport, as compared to space-charge dominated beams [Carlsten , this issue, Phys. Rev. ST Accel. Beams 8, 062001 (2005)]. In this paper, we describe the differences in transport stability when two-plane focusing is included. Two-plane wiggler focusing degrades the transport stability slightly, whereas two-plane PPM focusing greatly compromises the transport. On the other hand, single-plane PPM focusing can be augmented with external quadrupole fields to provide weak focusing in the sheet’s wide dimension, which has stability comparable to two-plane wiggler transport.

A sheet-beam traveling-wave amplifier has been proposed as a high-power generator of rf from 95 to 300 GHz, using a microfabricated rf slow-wave structure [Carlsten , IEEE Trans. Plasma Sci. 33, 85 (2005)], for emerging radar and communications applications. The planar geometry of microfabrication technologies matches well with the nearly planar geometry of a sheet beam, and the greater allowable beam current leads to high-peak power, high-average power, and wide bandwidths. Simulations of nominal designs using a vane-loaded waveguide as the slow-wave structure have indicated gains in excess of 1   dB/mm, with extraction efficiencies greater than 20% at 95 GHz with a 120-kV, 20-A electron beam. We have identified stable sheet-beam formation and transport as the key enabling technology for this type of device. In this paper, we describe sheet-beam transport, for both wiggler and periodic permanent magnet (PPM) magnetic field configurations, with natural (or single-plane) focusing. For emittance-dominated transport, the transverse equation of motion reduces to a Mathieu equation, and to a modified Mathieu equation for a space-charge dominated beam. The space-charge dominated beam has less beam envelope ripple than an emittance-dominated beam, but they have similar stability thresholds (defined by where the beam ripple continues to grow without bound along the transport line), consistent with the threshold predicted by the Mathieu equation. Design limits are derived for an emittance-dominated beam based on the Mathieu stability threshold. The increased beam envelope ripple for emittance-dominated transport may impact these design limits, for some transport requirements. The stability of transport in a wiggler field is additionally compromised by the beam’s increased transverse motion. Stable sheet-beam transport with natural focusing is shown to be achievable for a 120-kV, 20-A, elliptical beam with a cross section of 1 cm by 0.5 mm, with both a PPM and a wiggler field, with magnetic field amplitude of about 2.5 kG.

For short electron bunches in accelerators, the radial ion density due to residual-gas ionization faithfully reproduces the radial electron bunch distribution for time scales similar to the electron bunch length. If the electron bunch length is sufficiently long, however, the ions focus and, even for radially uniform electron beams, tend to form a very nonuniform equilibrium distribution. This ion distribution, in turn, leads to nonlinear focusing forces on the electron bunch itself. In this paper, we find the equilibrium distribution when the electron distribution is uniform, and calculate the emittance growth for axial slices in the electron bunch later in time. A regime is found in which the emittance growth is quadratic with both residual gas pressure and electron bunch length.

A continuous electron beam with a correlated emittance will eventually thermalize. Initially, the beam emittance for an intense, high-brightness, space-charge-dominated beam will oscillate, but after a sufficiently long time, it will reach an equilibrium value. The emittance oscillations are due to coherent transverse plasma oscillations in the beam and are a manifestation of periodic energy exchange between potential and kinetic energies. The beam eventually attains an equilibrium emittance, as the beam equipartitions the kinetic and potential energies. This equipartioning is reached as the beam thermalizes due to a form of Landau damping of the radial oscillations at different radial positions within the beam. Slight differences in the transverse plasma oscillation frequency for different radial positions lead to incoherence in the oscillations. In this paper, we calculate the equilibrium time scales required for equipartioning. We show that the equilibrium emittance scalings and magnitude can be predicted by conservation of energy considerations. In addition, we show that, in the space-charge dominated regime, there is a correspondence between the energy-conservation approach and the kinematic approach.

The nonlinear free-energy concept has been particularly useful in estimating the emittance growth resulting from any excess energy of electron beams in periodic and uniform channels. However, additional emittance growth, that is geometrical rather than thermodynamic in origin, is induced if the particles have different kinetic energies and axial velocities, which is common for mildly relativistic, very intense electron beams. This effect is especially strong if particles lose or gain significant kinetic energy due to the beam’s potential depression, as the beam converges and diverges. In this paper we analyze these geometric emittance growth mechanisms for a uniform, continuous, intense electron beam in a focusing transport channel consisting of discrete solenoidal magnets, over distances short enough that the beam does not reach equilibrium. These emittance growth mechanisms are based on the effects of (1) energy variations leading to nonlinearities in the space-charge force even if the current density is uniform, (2) an axial velocity shear radially along the beam due to the beam’s azimuthal motion in the solenoids, and (3) an energy redistribution of the beam as the beam compresses or expands. The geometric emittance growth is compared in magnitude with that resulting from the nonlinear free energy, for the case of a mismatched beam in a uniform channel, and is shown to dominate for certain experimental conditions. Rules for minimizing the emittance along a beamline are outlined.

The centrifugal space-charge force of a continuous electron beam, focused in a lens of an accelerator, is numerically calculated. As in the case of an electron beam in a dipole magnetic field, the effect on the transverse motion of the particles from the centrifugal space-charge force in a focusing element tends to cancel the effect from the potential depression of the beam; however, the cancellation is not exact, as it is for the dipole case. The centrifugal space-charge force in the focusing case arises from a nonzero axial derivative of the transverse vector potential due to the beam's space charge, as the beam is transversely accelerated in the focusing element. The transverse equation of motion for particles in the beam is used to quantify the partial cancellation of the nonlinear transverse acceleration from the centrifugal space-charge force and from the beam's potential depression, for focusing both with a magnetic quadrupole lens and a solenoid.

The space-charge forces for short electron bunches in circular motion can be very different from the space-charge forces for short electron bunches undergoing straight-line motion. The two major effects introduced by the circular motion are an off-axis, so-called ‘‘noninertial space-charge’’ effect, in which there is essentially no net energy loss of the bunch, and a coherent synchrotron radiation effect, in which the bunch radiates coherent energy. The consequence of these effects is a potentially large growth in the electron bunch’s transverse emittance. We derive an expression for these forces from a Green’s function approach, starting with the definitions of the retarded scalar and vector potentials. In particular, we find an expression for the total electric field along the direction of motion from a short line of charge in circular motion. These expressions in turn can be used in numerical particle simulations to estimate the amount of emittance growth, including the effects of suppressing the coherent synchrotron radiation by reducing the beam pipe dimensions. © 1996 The American Physical Society.

After compressing electron bunches with a half-meter long, four-dipole chicane at 8 MeV, we have measured full width at half maximum (FWHM) bunch lengths of less than 1 ps for charges from 0.1 to 1.1 nC. The uncompressed FWHM bunch lengths varied from 10 to 20 ps, and we achieved compression ratios in excess of 40 and peak currents greater than 1 kA. Bunch lengths for low charges were measured using a transversely deflecting rf cavity; bunch lengths for high charges were inferred from the energy spread induced in the beam by its longitudinal space-charge force as it drifted from the end of the compressor to the spectrometer.

Talman [Phys. Rev. Lett. 56, 1429 (1986)] has proposed a novel relativistic effect that occurs when a charged particle beam is bent in the magnetic field from an external dipole. The consequence of this effect is that the space-charge forces from the particles do not exhibit the usual inverse-square energy dependence and some part of them are, in fact, independent of energy. This led to speculation that this effect could introduce significant emittance growth for a bending electron beam. Subsequently, it was shown that this effect’s influence on the beam’s transverse motion is canceled for a dc beam by a potential depression within the beam (to first order in the beam radius divided by the bend radius). In this paper, we extend the analysis to include short bunch lengths (as compared to the beam pipe dimensions) and find that there is no longer the cancellation for forces both transverse to and in the direction of motion. We provide an estimate for the emittance growth as a function of bend angle, beam radius, and current, and for magnetic compression of an electron bunch.