Biomedical subjects

H P Freund

Publications and source records attributed to H P Freund.

5 recordsLinked to original sources

PubMed2005

Nonlinear harmonic generation in free-electron lasers with helical wigglers.

It is widely believed that harmonics are suppressed in helical wigglers. However, linear harmonic generation (LHG) occurs by an azimuthal resonance that excites circularly polarized, off-axis waves, where the hth harmonic varies as exp((ihtheta). Nonlinear harmonic generation (NHG) is driven by bunching at the fundamental and has different properites from LHG. While NHG has been studied in planar wigglers, there has been no analysis of NHG in helical wigglers. The 3D simulation code medusa has been modified for this purpose, and it is shown that NHG is substantial in helical wigglers and that the even and odd harmonics have comparable intensities.

Journal Article↗

PubMed2004

Phase-matching segmented wigglers in free-electron lasers.

Since many free-electron lasers use a segmented undulator, it is important to understand the requirements on phase matching between the undulators. A simulation that self-consistently determines the phase slippage between the light and the electrons is used to study these effects, The simulation is found to be in agreement with an analytic formulation of the phase slippage. A seeded x-ray free-electron laser is studied which makes use of a segmented undulator with quadrupoles in the gaps to provide strong focusing. Optimal performance is found for gap lengths corresponding to a phase slippage within about 20% of a wavelength through a unit cell consisting of an undulator and gap.

Journal Article↗

PubMed2003

Efficiency enhancement in free-electron lasers by means of concurrent rf acceleration.

Efficiency enhancement in free-electron lasers (FELs) using rf beam acceleration in the wiggler is described. Since the beam tube is a waveguide, there are low and high frequency resonances. Injection of low frequency power can act as an inverse-FEL accelerator concurrently with high frequency power extraction. Simulation of a FEL using this technique shows that substantial efficiency enhancements are possible without significant increases in the beam energy spread, which facilitates the use of energy recovery schemes. The technique is applicable to amplifier and oscillator configurations.

Journal Article↗

PubMed2001

Free-electron lasers. Status and applications.

A free-electron laser consists of an electron beam propagating through a periodic magnetic field. Today such lasers are used for research in materials science, chemical technology, biophysical science, medical applications, surface studies, and solid-state physics. Free-electron lasers with higher average power and shorter wavelengths are under development. Future applications range from industrial processing of materials to light sources for soft and hard x-rays.

Biochemical Phenomena↗

PubMed1990

Determination of the spread of HIV from the AIDS incidence history.

The relation between the incidence of HIV in the general population, the number of AIDS cases, and the incubation period for the disease is examined. The number of AIDS cases can be expressed in terms of a convolution integral over the incubation period distribution and the temporal history of HIV incidence. In order to determine the level of HIV incidence it is necessary to invert the convolution. In this manner, it is possible to determine the spread of HIV up to the present time from knowledge of the AIDS incidence history and the incubation period. We describe the inversion of the convolution in terms of a Laplace transform technique that is applicable for any given incubation period distribution. Substantial simplifications in the technique are found in the case of an Erlang distribution for the probability density. The spread of HIV infections in the United States is charted through 1988 using AIDS incidence data that are corrected for both the revised AIDS case definition and reporting time delays. The results are consistent with current estimates of the HIV incidence in the United States and show no evidence of saturation in the rate of new infections. Indeed, the rate of new infections still appears to be climbing as of that date. While the technique is unable to predict the future course of the epidemic, it may provide a useful benchmark for comparison with mathematical models of the epidemic. The techniques are conceptually applicable to diseases other than AIDS.

Acquired Immunodeficiency Syndrome↗