Search Results (9 total)

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.

A parity-nonconserving asymmetry in the total cross section for 6-GeV/c polarized protons on a water target has been measured. The experiment used two independent detector systems to measure the helicity dependence of the transmission through the target. Special attention was paid to possible sources of systematic errors. The result is A_L=(2.65±0.60±0.36)×10^-6 where A_L=(σ₊-σ_-) / (σ₊+σ_-) is the fractional difference of the total cross sections for positive- and negative-helicity protons on an unpolarized target. The first error is statistical and the second is an estimate of systematic uncertainties. This result is much larger than predictions based on meson-exchange calculations but consistent with a recent QCD calculation.

We have measured differential cross sections for both π⁺p and π^-p elastic scattering at incident-pion kinetic energies of 30, 50, 70, and 90 MeV in the center-of-mass angular range between 50° and 150°. The experiment detected pions scattered from a liquid-hydrogen target with multiwire proportional chambers and scintillation-counter range telescopes. The relative accuracy of each angular distribution is better than 5%, while the absolute cross sections have uncertainties of 4% to 25%. Our results for the absolute cross section for π⁺p scattering at 30 and 90 MeV are inconsistent with previous measurements. Our π^-p measurements comprise the first extensive set of precision differential cross sections below 90 MeV.

A parity-nonconservation asymmetry has been measured in the total cross section for 6-GeV/c polarized protons on a water target. The asymmetry A_L=(σ₊-σ_-) / (σ₊+σ_-), defined as the fractional difference of total cross sections for positive- and negative- helicity protons on an unpolarized target, is A_L=(2.65±0.60)×10^-6. The quoted error includes both statistical and systematic contributions.

We measure a limit on a parity-nonconserving asymmetry in the p-Be cross section at 6 GeV/c. The result is (σ₊-σ_-) / (σ₊+σ_-)=(5±9)×10^-6 where σ₊(σ_-) is approximately the p-Be total cross section for positive (negative) beam helicity. Systematic errors are determined to be <5×10^-6.

We report a result of (1±4)×10^-7 for the parity-nonconserving component in the p-p nuclear cross section at 15 MeV. Our experiment uses rapid spin reversal of a longitudinally polarized proton beam and an unpolarized H₂ target. Sources of systematic error are discussed and found to be <10^-7.