The use of low-Z external targets in the linear accelerator improves megavoltage planar and CBCT image quality significantly. CNR may be increased by a factor of 4 or greater. Improvement of the spatial resolution is also apparent.

We present dark and antidark diffraction-free beams and discuss their properties. We show that all such beams must be partially spatially coherent. The new beams can be used for optical trapping of atoms.

Free-electron lasers (FELs) are fourth-generation light sources that deliver extremely high intensity, ultrashort light pulses over a broad wavelength range from far-infrared to hard x ray. FELs based on the self-amplified spontaneous emission principle have been successfully operated with ultrahigh brightness and a broad wavelengths tuning range with good transverse coherence but poor temporal coherence. In contrast, the laser-seeded FELs have provided full coherence but at selected central wavelengths, usually the harmonics of the laser seeds, with relatively narrower tuning range. We report the experimental demonstration of a high-gain harmonic-generation (HGHG) FEL that is continuously tunable over a wide range using the combination of optical parametrical amplification, variable-gap undulator, and harmonic selection, where the temporal coherence is preserved as confirmed with the Michelson interferometry. In order to achieve higher photon energies, the first try of cascaded HGHG with a fresh-bunch technique is also made at the Shanghai Deep Ultraviolet Free-electron Laser test facility.

We have experimentally demonstrated the generation of sub-half-cycle phase-stable pulses with the carrier wavelength of 10.2 µm through two-color filamentation in nitrogen. The carrier-envelope phase (CEP) of the MIR pulse is passively stabilized and controlled by the attosecond time delay between the two-color input pulses. The duration of the MIR pulse is 13.7 fs, which corresponds to 0.402 cycles. The absolute value of the CEP of the generated sub-half-cycle pulse is consistent with a simple four-wave difference frequency generation model. We have also found that the 10 kHz repetition rate of the light source causes the fluctuation of the pulse energy on a few hundred millisecond time scale.

One of the successful approaches for improving megavoltage imaging using a linear accelerator is to replace the clinical target with a low‐Z material and to remove the flattening filter. In recent work by Orton and Robar (Phys Med Biol. 54(5), 2009), energy spectra were generated using aluminum targets with thicknesses equal to 60% of the CSDA range of the electron. Targets were located above the linac carousel, with a 0.9 cm polystyrene absorber inserted in upper accessory slot to fully stop transmitted electrons. The current project aims to further improve the MV imaging spectrum through two approaches: i) by altering the thickness of the low‐Z target and ii) reducing the energy of the electron beam. Through Monte Carlo (MC) simulation, we have found that full‐thickness targets generate comparable photon energy spectra and less electron contamination. An electron beam with energy 3.5 MeV has been experimentally achieved by installing a custom‐made program board into a Varian 2100EX linac to modify the bending magnet current. By implementing these changes, the normalized photon population in the 25–150 KeV range can be increased from 24.4% to 35.2%. By improving the energy spectrum and eliminating the need for an electron absorber, these enhancements should improve both the performance and practicality of low‐Z imaging.