Demonstration of a 17-GHz, high-gradient accelerator with a photonic-band-gap structure.

We report the testing of a high gradient electron accelerator with a photonic-band-gap (PBG) structure. The photonic-band-gap structure confines a fundamental TM(01)-like accelerating mode, but does not support higher-order modes (HOM). The absence of HOM is a major advantage of the PBG accelerator, since it suppresses dangerous beam instabilities caused by wakefields. The PBG structure was designed as a triangular lattice of metal rods with a missing central rod forming a defect confining the TM(01)-like mode and allowing the electron beam to propagate along the axis. The design frequency of the six-cell structure was 17.14 GHz. The PBG structure was excited by 2 MW, 100 ns pulses. A 16.5 MeV electron beam was transmitted through the PBG accelerator. The observed electron beam energy gain of 1.4 MeV corresponds to an accelerating gradient of 35 MV/m, in excellent agreement with theory.

Time- and frequency-domain models for Smith-Purcell radiation from a two-dimensional charge moving above a finite length grating.

Smith-Purcell radiation (SPR), formed by an electron beam traveling above a grating, is a very promising source of coherent radiation from the THz to the optical regime. We present two theoretical calculations of the SPR from a two-dimensional bunch of relativistic electrons passing above a grating of finite length. The first calculation uses the finite-difference time-domain approach with the total-field/scattered-field procedure for fields incident on the grating. This calculation allows good physical insight into the radiation process and also allows arbitrary geometries to be treated. The second calculation uses an electric-field integral equation method. Good agreement is obtained between these two calculations. The results of these theoretical calculations are then compared with a theoretical formalism based on an infinite-length grating. The latter formalism allows periodic boundary conditions to be rigorously applied. For gratings with less than approximately 50 periods, a significant error in the strength of the radiated field is introduced by the infinite-grating approximation. It is shown that this error disappears asymptotically as the number of periods increases. The Wood-Rayleigh anomalies, predicted in the infinite-grating approximation, were not seen in our finite-grating calculations. The SPR resonance condition is the same in all three formalisms. Numerical examples are presented for an approximately 18 MeV, 50 nC/m, 200 microm bunch traveling 0.6 mm above a ten-period echelle grating having a 2.-mm periodicity.

Exposure of human peripheral blood lymphocytes to electromagnetic fields associated with cellular phones leads to chromosomal instability.

Whether exposure to radiation emitted from cellular phones poses a health hazard is at the focus of current debate. We have examined whether in vitro exposure of human peripheral blood lymphocytes (PBL) to continuous 830 MHz electromagnetic fields causes losses and gains of chromosomes (aneuploidy), a major "somatic mutation" leading to genomic instability and thereby to cancer. PBL were irradiated at different average absorption rates (SAR) in the range of 1.6-8.8 W/kg for 72 hr in an exposure system based on a parallel plate resonator at temperatures ranging from 34.5-37.5 degrees C. The averaged SAR and its distribution in the exposed tissue culture flask were determined by combining measurements and numerical analysis based on a finite element simulation code. A linear increase in chromosome 17 aneuploidy was observed as a function of the SAR value, demonstrating that this radiation has a genotoxic effect. The SAR dependent aneuploidy was accompanied by an abnormal mode of replication of the chromosome 17 region engaged in segregation (repetitive DNA arrays associated with the centromere), suggesting that epigenetic alterations are involved in the SAR dependent genetic toxicity. Control experiments (i.e., without any RF radiation) carried out in the temperature range of 34.5-38.5 degrees C showed that elevated temperature is not associated with either the genetic or epigenetic alterations observed following RF radiation-the increased levels of aneuploidy and the modification in replication of the centromeric DNA arrays. These findings indicate that the genotoxic effect of the electromagnetic radiation is elicited via a non-thermal pathway. Moreover, the fact that aneuploidy is a phenomenon known to increase the risk for cancer, should be taken into consideration in future evaluation of exposure guidelines.

Phase and gain measurements in a distributed-loss cyclotron-resonance maser amplifier.

The control of gain and phase delay in a cyclotron-resonance maser (CRM) amplifier is essential for a variety of applications. In this experiment, the gain and phase-delay variations are measured with respect to controlling parameters; the electron-beam current and the axial magnetic field. Following Chu et al. [Phys. Rev. Lett. 74, 1103 (1995)], the CRM amplifier comprises of a distributed-loss waveguide to enable high gain without oscillations. Our experiment yields an amplification up to 26 dB, and a phase-delay control range of 360 degrees. In order to keep a fixed gain with the varying phase delay, the two controlling parameters (i.e., the solenoid field and the beam current) are operated together in a compensating mode. The experiment is conducted in a frequency of 7.3 GHz, with an electron beam of 18-kV voltage and 0.25-0.4-A current. The experimental results are compared with a theoretical model. Practical implementations of gain and phase control in CRM devices are discussed.