Nanoradian angular stabilization of x-ray optical components

An x-ray free electron laser oscillator (XFELO) has been recently proposed [K. Kim, Y. Shvyd'ko, and S. Reiche, Phys. Rev. Lett. 100, 244802 (2008)]. Angular orientation and position in space of Bragg mirrors of the XFELO optical cavity must be continuously adjusted to compensate instabilities and maximize the output intensity. An angular stability of about 10 nrad (rms) is required [K. Kim and Y. Shvyd'ko Phys. Rev. STAB 12, 030703 (2009)]. To approach this goal, a feedback loop based on a null-detection principle was designed and used for stabilization of a high energy resolution x-ray monochromator ($\Delta E/E \simeq 4 \times 10^{-8}$, $E$ = 23.7 keV) and a high heat load monochromator. Angular stability of about 13 nrad (rms) has been demonstrated for x-ray optical elements of the monochromators.Comment: 8 figure

Can a free electron laser operate with a broad momentum spread?

The gain in a usual free electron laser (FEL) vanishes when the energy spread of the electron beam is too broad because electrons with positive detuning parameter {ital v} are bunched around a certain phase contributing to a positive gain, while those with negative {ital v} are bunched around another phase contributing to a negative gain. The net gain therefore vanishes for a beam of electrons distributed over a broad range of positive and negative values of {ital v}. To avoid this problem, it was proposed recently to use two undulators and to insert a device between them which separates the electrons of negative v and to displace them in phase so that they contribute to an overall positive gain after passing through the second undulator. The device will be referred to as the ``redistributor``. The scheme, was an effort to extend the idea of the inversionless atomic laser to the FEL, and, if true, would have an important consequence for future FEL development. The purpose of this paper is to show that the scheme does not work as proposed. The reason is simple: consider an initial electron distribution which is uniform extending to a large positive and negative detuning where the FEL interaction vanishes. The distribution, because of Liouville`s theorem, will remain the same after the first undulator. The redistributor would have no effect on this distribution, and the gain of the total system vanishes. The reasoning above is wrong because we did not take into account the contributions of {ital all electrons}. The first group contributes to a positive net gain. However, this is canceled by the negative net gain due to the second group, irrespective of how the phase of electrons in the region of negative detuning is displaced relative to those in the positive detuning

Transverse Gradient Undulator in a Storage Ring X-ray Free Electron Laser Oscillator

Modern electron storage rings produce bright X-rays via spontaneous synchrotron emission, which is useful for a variety of scientific applications. The X-ray free-electron laser oscillator (XFELO) has the potential to amplify this output, both in terms of peak power and photon coherence. However, even current 4th generation storage rings (4GSRs) lack the requisite electron beam brightness to drive the XFELO due to its electron energy spread. The transverse gradient undulator (TGU) can overcome this issue, thus providing a practical means to couple the 4GSR to the XFELO. In this study, we first examine the theoretical basis of the TGU interaction by deriving the 3D TGU gain formula in the low gain approximation. Then, we perform an optimization study of the gain formula in order to determine optimal beam and machine parameters. Finally, we construct a hypothetical storage ring TGU-XFELO based on near-optimal parameters and report on its projected performance using multi-stage numerical simulation. We also discuss potential implementation challenges associated with the ring-FEL coupling.Comment: 17 pages, 11 figure

A Review of X-ray Free-Electron Laser Theory

High-gain free-electron lasers (FELs) are being developed as extremely bright sources for a next-generation x-ray facility. In this paper, we review the basic theory of the startup, the exponential growth, and the saturation of the high-gain process, emphasizing the self-amplified spontaneous emission (SASE). The radiation characteristics of an x-ray FEL, including its transverse coherence, temporal characteristics, and harmonic content, are discussed. FEL performance in the presence of machine errors and undulator wakefields is examined. Various enhancement schemes through seeding and beam manipulations are summarized

Performance of hole coupling resonator in the presence of asymmetric modes and FEL gain

We continue the study of the hole coupling resonator for free electron laser (FEL) application. The previous resonator code is further developed to include the effects of the azimutally asymmetric modes and the FEL gain. The implication of the additional higher order modes is that there are more degeneracies to be avoided in tuning the FEL wavelengths. The FEL interaction is modeled by constructing a transfer map in the small signal regime and incorporating it into the resonator code. The FEL gain is found to be very effective in selecting a dominant mode from the azimuthally symmetric class of modes. Schemes for broad wavelength tuning based on passive mode control via adjustable apertures are discussed. 12 refs., 7 figs., 1 tab

Psychometric properties of a short self-reported measure of medication adherence among patients with hypertension treated in a busy clinical setting in Korea.

BackgroundWe examined the psychometric properties of the Korean version of the 8-item Morisky Medication Adherence Scale (MMAS-8) among adults with hypertension.MethodsA total of 373 adults with hypertension were given face-to-face interviews in 2 cardiology clinics at 2 large teaching hospitals in Seoul, South Korea. Blood pressure was measured twice, and medical records were reviewed. About one-third of the participants (n = 109) were randomly selected for a 2-week test-retest evaluation of reliability via telephone interview.ResultsInternal consistency reliability was moderate (Cronbach α = 0.56), and test-retest reliability was excellent (intraclass correlation = 0.91; P < 0.001), although a ceiling effect was detected. The correlation of MMAS-8 scores with scores for the original 4-item scale indicated that convergent validity was good (r = 0.92; P < 0.01). A low MMAS-8 score was significantly associated with poor blood pressure control (χ(2) = 29.86; P < 0.001; adjusted odds ratio = 5.08; 95% CI, 2.56-10.08). Using a cut-off point of 6, sensitivity and specificity were 64.3% and 72.9%, respectively. Exploratory factor analysis identified 3 dimensions of the scale, with poor fit for the 1-dimensional construct using confirmatory factory analysis.ConclusionsThe MMAS-8 had satisfactory reliability and validity and thus might be suitable for assessment and counseling regarding medication adherence among adults with hypertension in a busy clinical setting in Korea