ABSTRACT
Using a novel (to our knowledge) broadband Yb-doped Yb3+,Na+:CaF2 crystal cooled in a closed loop to 130 K we demonstrate a chirped pulse regenerative laser amplifier delivering the energy of up to 3 mJ at a repetition rate of 1 kHz and an average output power of 6 W at 20 kHz. The gain narrowing in the laser crystal is compensated by shaping the amplitude of the seed pulse spectrum. As the result, at the highest amplified pulse energy we obtain a 12 nm FWHM bandwidth supporting a 130 fs pulse duration, assuming ideal compression. Amplified pulses were recompressed from 250 ps to 195 fs with a 1700 lines/mm transmission grating compressor.
ABSTRACT
We report on an electron accelerator based on few-cycle (8 fs full width at half maximum) laser pulses, with only 40 mJ energy per pulse, which constitutes a previously unexplored parameter range in laser-driven electron acceleration. The produced electron spectra are monoenergetic in the tens-of-MeV range and virtually free of low-energy electrons with thermal spectrum. The electron beam has a typical divergence of 5-10 mrad. The accelerator is routinely operated at 10 Hz and constitutes a promising source for several applications. Scalability of the few-cycle driver in repetition rate and energy implies that the present work also represents a step towards user friendly laser-based accelerators.
ABSTRACT
We report on a passive cavity-enhanced Yb-fiber laser frequency comb generating 230 MW of peak power (3 kW of average power) at a 136 MHz pulse repetition rate. The intracativy peak intensity of 3 x 10(14) W/cm2 for the 95 fs pulse is sufficient to ionize noble gases, such as Xe, Kr, or Ar. The laser system is based on a mode-locked Yb-fiber similariton oscillator in conjunction with a cladding-pumped chirped-pulse fiber amplifier. After recompression, 75 fs duration pulses at a 13.1 W average power are obtained. These pulses are then coherently added inside a passive ring cavity by controlling the fiber oscillator's pulse repetition rate and carrier-envelope offset frequency. This system is well suited for studying high-field phenomena at very high pulse repetition rates.
ABSTRACT
The emission of fundamental and harmonic radiation from the rear side of thin foils in the thickness range 50-460 nm irradiated by intense frequency doubled Ti:sapphire laser pulses of the duration of 150 fs and intensities up to a few 10(18) W/cm(2) was investigated. Following up a previous study of the rear side harmonic emission [Teubner, Phys. Rev. Lett. 92, 185001 (2004)], we measured the emission efficiencies, polarization properties, and the spectral shapes of the fundamental frequency and the second harmonic. Rear side emission is only observed when the obliquely incident laser light is p -polarized. Particle-in-cell (PIC) simulations indicate that the foils remain strongly overdense during the interaction with the laser pulse and that the rear side emission is caused by energetic electron bunches which are generated at the front side by resonance absorption. They are accelerated into the foil and drive strong plasma oscillations at the fundamental and higher harmonic frequencies.
ABSTRACT
Late complications after operations on aorta and iliac arteries are analyzed in 763 patients with athesclerosis over 15-year period. Indications for recurrent reconstructive operations are presented, and the results of these operations are analyzed depending on surgical technique, incidence of recurrent thromboses and types of complications. Angiography, rheovasography, pathohistological and microscopic studies, raster electron microscopy, IR-spectroscopy were used in post-operative period. On the basis of the obtained data the following conclusions have been drawn. For more than a quarter of patients operated for aorto-iliac occlusions it was necessary to perform recurrent reconstructive operations in late post-operative period. The incidence of recurrent operations depends on many factors: stage of ischemia, nature of atherosclerotic lesions, technique of reconstruction. In primary operations profundoplasty was performed in comparatively rare cases.
Subject(s)
Aorta, Abdominal/surgery , Blood Vessel Prosthesis/adverse effects , Iliac Artery/surgery , Postoperative Complications/surgery , Angiography , Arteriosclerosis/etiology , Humans , Reoperation , Thrombosis/etiologyABSTRACT
A comparison of the clinical data and those of ECG under physical exercises with the results of coronary angiography is presented for 130 patients with the ischaemic heart disease, aged 28 to 68 years; 63 of them had survived myocardial infarction, 63 had angina of effort, 65--angina at rest and angina of effort, 2 presented no complaints. The comparison of the clinical and coronarographic data demonstrated a high correlation in cases of angina; in 85.3% of the patients with typical angina pectoris coronary angiography revealed anatomic changes in the coronaries. Having compared the data to the positive exercise test with the results of coronary angiography, the authors found that 87.9% of the examined patients have--with a positive exercise test--coronary lesions and the S--T segment depression during physicial exercises 1 mm below the iso-electric line, which permits to diagnose anatomic changes in the coronaries. According to the authors, an acute onset of the disease, a brief history (up to 1 year) and a prompt restoration of the ECG following physical exercises are typical rather for local lesions of one coronary branch.
