Masaki Fujimoto

Correlation between maximum temperature increase and peak SAR with different average schemes and masses

This paper investigates the correlation between maximum temperature increases and peak spatial-average specific absorption rates (SARs), calculated by different average schemes and masses. For evaluating the effect of mass on the correlation properly, a three-dimensional Greens function is presented. From our computational investigation, no best average mass for peak spatial-average SAR exist from the aspect of the correlation with maximum temperature increase. This is attributed to the frequency dependent penetration depth of EM waves. Maximum temperature increase in the head including the pinna is reasonably correlated with peak spatial-average SARs for most average schemes and masses considered in this paper. Maximum temperature increase in the head only (excluding the pinna) is reasonably correlated with peak 10-g SARs for the average schemes considered in this paper. The rationale for this result is explained using the Greens function. The point to be stressed here is that the slope correlating them is largely dependent on the average scheme and mass. Additionally, good agreement is observed in the slopes obtained by using two head models, which have been developed at Osaka University and Nagoya Institute of Technology. However, weak correlation is observed for the brain, which is caused by the difference of the positions where peak SAR and maximum temperature increase appear. The 95th percentile values of the slope correlating maximum temperature increases in the head or brain and peak spatial-average SAR are quantified for different average schemes and masses

FDTD-derived correlation of maximum temperature increase and peak SAR in child and adult head models due to dipole antenna

This paper investigates the correlation between the peak specific absorption rate (SAR) and the maximum temperature increase in head models of adults and children due to a dipole antenna. Much attention is paid to the effect of variation of electrical and thermal constants on the correlation for the child models, since these constants of child tissues are different from those of adult tissues. For investigating these correlations thoroughly, a total of 1400 situations are considered for the following six models: 3-year-old child, 7-year-old child, and adult models developed at the Nagoya Institute of Technology and the Osaka the University. The numerical results are analyzed on the basis of statistics. We find that the maximum temperature increases in the head can be estimated linearly in terms of peak SAR averaged over 1- or 10-g of tissue. In particular, no clear difference is observed between the adult and child models in terms of the slopes correlating the maximum temperature increase with the peak SAR. Also, the effect of electrical and thermal constants of tissue on these correlation is found to be marginal. Further, we discuss possible maximum temperature increases in the head and brain for SAR limits prescribed in safety guidelines. For the adult model developed at the Osaka Univ., these are found to be 0.26degC and 0.10degC at the SAR value of 1.6 W/kg for 1-g cubic tissue and 0.59degC and 0.21degC at the SAR value of 2.0 W/kg for 10-g cubic tissue. Similarly, for the 3-year-old child model at Osaka Univ., these are 0.23degC and 0.11degC for the value of 1-g SAR and 0.53degC and 0.20degC for the value of 10-g SAR

Proposal for the edge-focusing wiggler for SASE

Abstract We are developing a new type of wiggler with a strong focusing force, named the edge-focusing wiggler. It is a Halbach-type wiggler made with permanent magnet blocks with an edge angle φ. An application is given for SASE in the shorter wavelength region. The characteristics of the wiggler are studied in detail using a three-dimensional magnetic field calculation program with the magnetic charge method.

Comparison of maximum temperature increase in the infant and adult head models due to dipole antenna

We investigated the correlation between the peak SAR and the maximum temperature increase in the models of infants and adults for exposure to a dipole antenna. Numerical results show that the maximum temperature increases in the head and brain were reasonably proportional to the peak SAR in the corresponding regions. No clear difference in the correlation between the peak SAR and the maximum temperature increase was observed for infant and adult models. Additionally, the effect of the material constants on the correlation was at most 10%.

Maximum temperature increases in the head and brain for SAR averaging schemes prescribed in safety guidelines

This paper presents maximum temperature increase in the head and brain with peak SAR calculated by different averaging schemes. For our computational investigations, it is found that maximum temperature increases in the head are well correlated with peak spatial-average SARs. However, the slope correlating them is largely dependent on the averaging scheme. Additionally, no clear difference is observed in the slope obtained using two head models. Possible maximum temperature increase in the brain for the SAR values is also quantified.

Characteristic measurements of higher harmonics generated in the SASE-FEL process

The nonlinear harmonic generation of self-amplified spontaneous emission (SASE) has been studied in the far-infrared region. Angular distributions of the second harmonic and the third harmonic as well as the fundamental of SASE have been measured. The third harmonic radiation is emitted on the electron beam axis, while the second harmonic radiation is emitted slightly off-axis in the horizontal direction. It seems that there is competition in development of radiation between the fundamental and the other higher harmonics.

Suppression of Longitudinal Coupled Bunch Instability by Harmonic Cavity in UVSOR Electron Storage Ring

In the UVSOR electron storage ring [1], which is dedicated for a VUV synchrotron radiation light source, a longitudinal coupled bunch instability (LCBI) is observed in multi-bunch operation. To suppress the LCBI, we routinely operate a third harmonic cavity (HCV) in a passive mode. By properly tuning the HCV, the instability is almost completely suppressed. Because of the lower beam energy (750 MeV) and brilliant beam emittance (17.5 nm-rad), the Touschek effect becomes severe in the UVSOR. To guarantee enough beam lifetime, we also apply the HCV for lengthening the bunch. The suppression of the instability and increasing the beam lifetime are crucial benefits by the HCV for the UVSOR. However, not only the origin of the LCBI but also the Landau damping effect by the HCV has not been understood systematically yet. We have noticed that one of the HOMs at the HCV itself could cause the LCBI and observed the behavior of the instability, which strongly depends on the beam current. From the experiment we have discussed the cause of the instability with the HOM theory. We have also tried to observe synchrotron tune spread and discussed a competition between the Landau damping and the instability growth.

Generation of vector beam with tandem helical undulators

We propose a scheme to produce structured light in synchrotron light sources. In this scheme, light beams from two undulators are superposed by using a technique akin to the “cross undulator.” We demonstrate that a vector beam, in which the polarization direction varies with the azimuthal angle about the beam axis, is produced by superposing harmonic radiation from two helical undulators in tandem. Although this scheme is demonstrated in the ultraviolet range at the low-energy synchrotron UVSOR-III, it can be applied to high-energy synchrotrons to produce vector x-ray beams, which would open a new field in the application of synchrotron radiation.We propose a scheme to produce structured light in synchrotron light sources. In this scheme, light beams from two undulators are superposed by using a technique akin to the “cross undulator.” We demonstrate that a vector beam, in which the polarization direction varies with the azimuthal angle about the beam axis, is produced by superposing harmonic radiation from two helical undulators in tandem. Although this scheme is demonstrated in the ultraviolet range at the low-energy synchrotron UVSOR-III, it can be applied to high-energy synchrotrons to produce vector x-ray beams, which would open a new field in the application of synchrotron radiation.

Time constant of temperature increase in the human head models due to dipole antenna at microwave frequencies

The paper investigates the time constant of temperature increase in human head models due to a dipole antenna. The frequency band considered is from 900 MHz to 2.5 GHz. In order to discuss this problem thoroughly, hundreds of cases are considered: five frequency bands; two polarizations; two head models, with pressed and impressed ears; ten antenna feeding points. The results provide useful information in this field.