Design of a 30 GHz bragg reflector for a Raman FEL

A design of a Bragg reflector for a Raman FEL is described. It is shown that mode conversion occurs whenever the axial wavenumbers of the two modes fulfil the Bragg condition. With a constant ripple of the corrugation it is shown that the reflected radiation also contains higher order modes, assuming that the incident radiation consists only of a TE11 mode. The mode purity can be increased by increasing the length of the reflector at the expense of a smaller reflection bandwidth. A more flexible method is by applying a Hamming window to the corrugation of the reflector. Contributions of other modes to the reflected radiation can in that case be neglected. The reflector will be installed in a Raman laser to be able to compare the amplifier with the oscillator configuration. Therefore some preliminary results are also presented about the start-up of the Raman laser

Studies of a Terawatt X-Ray Free-Electron Laser

The possibility of constructing terawatt (TW) x-ray free-electron lasers (FELs) has been discussed using novel superconducting helical undulators [5]. In this paper, we consider the conditions necessary for achieving powers in excess of 1 TW in a 1.5 {\AA} FEL using simulations with the MINERVA simulation code [7]. Steady-state simulations have been conducted using a variety of undulator and focusing configurations. In particular, strong focusing using FODO lattices is compared with the natural, weak focusing inherent in helical undulators. It is found that the most important requirement to reach TW powers is extreme transverse compression of the electron beam in a strong FODO lattice. The importance of extreme focusing of the electron beam in the production of TW power levels means that the undulator is not the prime driver for a TW FEL, and simulations are also described using planar undulators that reach near-TW power levels. In addition, TW power levels can be reached using pure self-amplified spontaneous emission (SASE) or with novel self-seeding configurations when such extreme focusing of the electron beam is applied.Comment: 10 pages, 12 figure

An X-Ray Regenerative Amplifier Free-Electron Laser Using Diamond Pinhole MIrrors

Free-electron lasers (FELs) have been built ranging in wavelength from long-wavelength oscillators using partial wave guiding through ultraviolet through hard x-ray FELs that are either seeded or start from noise (SASE). Operation in the x-ray spectrum has relied on single-pass SASE due either to the lack of seed lasers or difficulties in the design of x-ray mirrors. However, recent developments in the production of diamond crystal Bragg reflectors point the way to the design of regenerative amplifiers (RAFELs) which are, essentially, low-Q x-ray free-electron laser oscillators (XFELOs) that out-couple a large fraction of the optical power on each pass. A RAFEL using a six-mirror resonator providing out-coupling of 90% or more through a pinhole in the first downstream mirror is proposed and analyzed using the MINERVA simulation code for the undulator interaction and the Optics Propagation Code (OPC) for the resonator. MINERVA/OPC has been used in the past to simulate infrared FEL oscillators. For the present purpose, OPC has been modified to treat Bragg reflection from diamond crystal mirrors. The six-mirror resonator design has been analyzed within the context of the LCLS-II beamline under construction at the Stanford Linear Accelerator Center and using the HXR undulator which is also to be installed on the LCLS-II beamline. Simulations have been run to optimize and characterize the properties of the RAFEL, and indicate that substantial powers are possible at the fundamental (3.05 keV) and third harmonic (9.15 keV).Comment: 9 pages, 14 figure

Comparison between a FEL amplifier and oscillator

Previous experiments with the Raman FEL, situated at the Twente University, showed that the output was influenced by the rather strong increase of the current density with time. The field emission diode has been modified to produce a more constant current pulse to simplify the analysis of the measurements. This resulted in a lower current density of the electron beam. With this new diode two set-ups are studied. In the first set-up the laser is still configured as an amplifier whereas in the second set-up the laser configuration is changed into an oscillator using a Bragg reflector with a space-variable corrugation height. For both set-ups we measured the frequency spectrum for specific values of undulator and guide magnetic fields. The relative performance of the amplifier and the oscillator configuration will be presented

The effect of a Bragg reflector on the spectral stability of the Twente Raman free electron laser

The spectral distribution of the Twente Raman FEL has been studied as a function of the interaction length for an amplifier configuration. A stable spectrum was found for the minimum required interaction length necessary for the RF signal to be detected. Large variations in total emitted energy are observed however. With increasing interaction length the spectral distribution not only evolves but deviations are also observed, i.e. for some shots distributions are found which do not conform to the average distribution. For even longer interaction lengths the spectra can be grouped in a few different patterns. The influence of the feedback on the spectral distribution has been studied by changing the configuration to an oscillator using a Bragg reflector. For all settings investigated, the oscillator showed a more stable spectrum, i.e., less spread in total emitted energy as well as less spread in spectral distribution. For some settings operation on a single frequency in the Ka band was observed

Deep Saturated Free Electron Laser Oscillators and Frozen Spikes

We analyze the behavior of Free Electron Laser (FEL) oscillators operating in the deep saturated regime and point out the formation of sub-peaks of the optical pulse. They are very stable configurations, having a width corresponding to a coherence length. We speculate on the physical mechanisms underlying their growth and attempt an identification with FEL mode locked structures associated with Super Modes. Their impact on the intra-cavity nonlinear harmonic generation is also discussed along with the possibility of exploiting them as cavity out-coupler.Comment: 28 page

Wakefields in superconducting rf cavities and the impact on vacuum ultraviolet free-electron laser oscillator performance

The Fermilab Accelerator Science and Technology facility is currently in operation with its linac based on TESLA-type superconducting rf cavities. Using a 3-MHz micropulse repetition rate with a long macropulse composed of up to 3000 micropulses, and with beam energies demonstrated at 300 MeV and projected to reach 800 MeV with two additional cryomodules, the feasibilities for a vacuum ultraviolet (VUV) and an extreme ultraviolet (EUV) free-electron laser oscillator (FELO) with the two energies are evaluated. We have used both the ginger code with an oscillator module and the minerva/opc code to assess FELO saturation prospects at 120 nm with a 5.0-cm-period undulator of 4.5-m length and the minerva/opc code to assess the FELO at 13.4 nm with adjusted parameters. The simulation results support saturation at both of these wavelengths which are much shorter than the demonstrated shortest wavelength record of 168.6 nm from a storage-ring-based FELO. This indicates superconducting rf linac-driven FELOs can be extended into this VUV-EUV wavelength regime previously only reached with single-pass FEL configurations. In addition, emittance-dilution effects due to wakefields in the cavities and the resulting submacropulse centroid slew effects on FELO performance are addressed using minerva/opc simulations for the first time.</p

Mapping individual electromagnetic field components inside a photonic crystal

We present a method to map the absolute electromagnetic field strength inside photonic crystals. We apply the method to map the electric field component Ez of a two-dimensional photonic crystal slab at microwave frequencies. The slab is placed between two mirrors to select Bloch standing waves and a subwavelength spherical scatterer is scanned inside the resulting resonator. The resonant Bloch frequencies shift depending on the electric field at the position of the scatterer. To map the electric field component Ez we measure the frequency shift in the reflection and transmission spectrum of the slab versus the scatterer position. Very good agreement is found between measurements and calculations without any adjustable parameters.Comment: 12 pages, 7 figure

End-fire versus side-fire:a randomized controlled study of transrectal ultrasound guided biopsies for prostate cancer detection

Objectives: To compare prostate cancer detection rates between end-fire and side-fire ultrasound guided prostate biopsy techniques. Methods: A prospective randomized controlled trial was performed in patients who underwent prostate biopsy between 2009 and 2014. Patients were randomly assigned to the end-fire or side fire biopsy groups and underwent transrectal ultrasound guided prostate biopsy. The overall prostate cancer detection rate was compared between the two probe configurations. Trial was registered at Clinical Trials.gov with identifier: NCT00851292. Results: A total of 730 patients were included and randomized, 371 patients underwent prostate biopsy with side-fire probe and 359 patients with the end-fire probe. Prostate cancer detection rates were 52.4% in the end fire group and 45.6% in the side fire group (p = .066). Conclusions: No significant difference was found in detection rate of prostate cancer between the end-fire and side-fire probe in transrectal ultrasound guided prostate biopsy, neither for detection rate of prostate cancer in the apex

Cluster size dependence of high-order harmonic generation

We investigate high-order harmonic generation (HHG) from noble gas clusters in a supersonic gas jet. To identify the contribution of harmonic generation from clusters versus that from gas monomers, we measure the high-order harmonic output over a broad range of the total atomic number density in the jet (from 3*10^16 cm^{-3} to 3x10^18 cm{-3}) at two different reservoir temperatures (303 K and 363 K). For the firrst time in the evaluation of the harmonic yield in such measurements, the variation of the liquid mass fraction, g, versus pressure and temperature is taken into consideration, which we determine, reliably and consistently, to be below 20% within our range of experimental parameters. By comparing the measured harmonic yield from a thin jet with the calculated corresponding yield from monomers alone, we find an increased emission of the harmonics when the average cluster size is less than 3000. Using g, under the assumption that the emission from monomers and clusters add up coherently, we calculate the ratio of the average single-atom response of an atom within a cluster to that of a monomer and find an enhancement of around 10 for very small average cluster size (~200). We do not find any dependence of the cut-off frequency on the composition of the cluster jet. This implies that HHG in clusters is based on electrons that return to their parent ions and not to neighbouring ions in the cluster. To fully employ the enhanced average single-atom response found for small average cluster sizes (~200), the nozzle producing the cluster jet must provide a large liquid mass fraction at these small cluster sizes for increasing the harmonic yield. Moreover, cluster jets may allow for quasi-phase matching, as the higher mass of clusters allows for a higher density contrast in spatially structuring the nonlinear medium.Comment: 16 pages, 6 figure