The Muonic Hydrogen Lamb Shift Experiment at PSI

A measurement of the 2S Lamb shift in muonic hydrogen (μ⁻p) is being prepared at the Paul Scherrer Institute (PSI). The goal of the experiment is to measure the energy difference ΔE(2⁵ P _3/2−2³ S _1/2) by laser spectroscopy (λ≈6μm) to a precision of 30 ppm and to deduce the root mean square (rms) proton charge radius with 10⁻³ relative accuracy, 20 times more precise than presently known. An important prerequisite to this experiment is the availability of long-lived μp_2S -atoms. A 2S-lifetime of ∼1 μs – sufficiently long to perform the laser experiment – at H₂ gas pressures of 1–2 hPa was deduced from recent measurements of the collisional 2S-quenching rate. A new low-energy negative muon beam yields an order of magnitude more muon stops in a small low-density gas volume than a conventional cloud muon beam. A stack of ultra-thin carbon foils is the key element of a fast detector for keV-muons. The development of a 2 keV X-ray detector and a 3-stage laser system providing 0.5 mJ laser pulses at 6 μm is on the way. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of sidelobes of focused flat-topped laser beams on vacuum electron acceleration

Using three-dimensional test particle simulations, we investigated electrons accelerated by a focused flat-top laser beam at different intensities and flatness levels of the beam profile before focusing in vacuum. The results show that the presence of sidelobes around the main focal spot of the focused flat-top laser beam influences the optimum (as far as electron acceleration is concerned) initial momentum (and incident angle) of electrons for acceleration. The difference of initial conditions between laser beams with and without sidelobes becomes evident when the laser field is strong enough (a₀>10, corresponding to intensities I>1×10²⁰ W/cm² for the laser wavelength λ=1 μm, where a₀ is a dimensionless parameter measuring laser intensity). The difference becomes more pronounced at increasing a₀. Because of the presence of sidelobes, there exist three typical CAS (capture and acceleration scenario) channels when a₀≥30 (corresponding to I>1×10²¹ W/cm² for λ=1 μm). The energy spread of the outgoing electrons is also discussed in detail. PACS 41.75.Jv; 42.60.Jf; 42.25.Fx     

Table-top kHz hard X-ray source with ultrashort pulse duration for time-resolved X-ray diffraction

The interaction of ultrashort laser pulses with solid state targets is studied concerning the production of short X-ray pulses with photon energies up to about 10 keV. The influence of various parameters such as pulse energy, repetition rate of the laser system, focusing conditions, the application of prepulses, and the chirp of the laser pulses on the efficiency of this highly nonlinear process is examined. In order to increase the X-ray flux, the laser pulse energy is increased by a 2nd multipass amplifier from 750 μJ to 5 mJ. By applying up to 4 mJ of the pulse energy a X-ray flux of 4×10¹⁰ Fe K _α photons/s or 2.75×10¹⁰ Cu K _α photons/s are generated. The energy conversion efficiency is therefore calculated to η _Fe≈1.4×10⁻⁵ and η _Cu≈1.0×10⁻⁵. The X-ray source size is determined to 15×25 μm². By focusing the produced X-rays using a toroidally bent crystal a quasi-monochromatic X-ray point source with a diameter of 56 μm×70μm is produced containing ≈10⁴ Fe K _α1 photons/s which permits the investigation of lattice dynamics on a picosecond or even sub-picosecond time scale. The lattice movement of a GaAs(111) crystal is shown as a typical application.     

Intracavity Raman laser generating a third stokes component at 1.5 μm

The lasing properties of an intracavity Nd:KGW Raman laser which converts the multimode radiation of an Nd:KGW laser operating on the ⁴ F _3/2–⁴ I _11/2 transition into the third Stokes component at a wavelength of 1.5 μm are studied. The energy in the third Stokes component is found to increase essentially linearly with the electrical energy delivered to the flashlamp. Lasing at the third Stokes component begins in the central portion of the Nd:KGW crystal and then propagates to its boundaries. Reducing the geometric aperture of the multimode pump beam in the Raman crystal lowers the divergence of the Stokes emission. For a source pump energy of 6 J, the intracavity Raman laser emits 14.7 mJ pulses of duration 3–4 nsec which are safe to the eyes. The divergence of the Raman laser beam at a level of 86% of the total energy is ≈ 9 mrad.     

Experimental detection of specular reflection of gamma quanta

Scattering of bremsstrahlung electron beam (E _e = 6.25 MeV) collimated with a vertical divergence δ_γ ≈ ±60 μrad and incident on the mercury surface at glancing angles α = −37, −83, and −140 μrad is studied on the 118-m-long path length. The specular reflection effect is detected in the spectral and angular distributions of γ quanta with energies up to 3 MeV. The reflection coefficients vary from 0 to ∼17% depending on the angles of incidence and the γ quanta energy.     

Anomalous burn-through of thin foils by high-intensity laser radiation

The paper presents results of experiments performed on the Pico facility in which foils were heated by laser radiation, and anomalously fast burn-through of foils by a structured laser beam was detected. Comparison with two-dimensional calculations has allowed us to suggest a tentative mechanism for the effect under investigation. The targets in the experiments were thin aluminum foils of thickness 3 to 40 μm. The flux density of laser radiation on the target surface varied between 10¹³ and 10¹⁴ W/cm². We detected a strong dependence of the transmitted energy on the foil thickness and the shortening of the transmitted laser pulse. Penetration of laser radiation through foils with thicknesses considerably larger than 3 μm has been observed, although it was stated in earlier publications [V. V. Ivanov, A. K. Knyazev, A. V. Kutsenko, et al., Kratk. Soobshch. Fiz. FIAN No. 7–8, 37 (1997)]; A. é. Bugrov, I. N. Burdonskii, V. V. Gol’tsov et al., Zh. éksp. Teor. Fiz. 111, 903 (1997) [JETP 84, 903 (1997)] that, at the laser radiation parameters used in our experiment, the evaporated layer of the foil could not be thicker than 2 μm. Two-dimensional calculations have allowed us to interpret this effect in terms of local “piercing” of the target at spots on the target surface where the radiation intensity has its peaks. The possibility of reducing these peaks by using a symmetrizing prepulse is discussed in the paper. Zh. éksp. Teor. Fiz. 116, 1287–1299 (October 1999)     

<Emphasis Type="Italic">K</Emphasis><Subscript>α</Subscript> radiation of foil under interaction with laser pulse of relativistic intensity

An analytical model of K _α radiation of thin laser targets has been developed. It has been shown that, for such targets, the motion of fast electrons is significant not only in the target itself but also in vacuum. The considered dependences for the free path length of a fast electron and for the absorption coefficient of laser radiation on the laser intensity with allowance for the electron motion in vacuum make it possible to match the results of the proposed model with the experimental data on generation of K _α radiation in wide ranges of laser intensities (10¹⁸–10²¹ W/cm²) and thicknesses (1–100 μm) of targets.     

Corrugated neat thin-film conjugated polymer distributed-feedback lasers

Wave-guided thin-film distributed-feedback (DFB) polymer lasers are fabricated by spin coating a PPV-derived semiconducting polymer, thianthrene-DOO-PPV, onto oxidised silicon wafers with corrugated second-order periodic gratings. The gratings are written by reactive ion beam etching. Laser action is achieved by transverse pumping with picosecond laser pulses (wavelength 347.15 nm, duration 35 ps). The DFB-laser surface emission and edge emission are analysed. Outside the grating region the polymer film is used for comparative wave-guided travelling wave laser (amplified spontaneous emission (ASE)) studies. The pump pulse threshold energy density for wave-guided DFB-laser action (4–9 μJ cm^-2) is found to be approximately a factor of two lower than the threshold for wave-guided travelling wave laser action. The spectral width of the DFB laser (down to Δλ_DFB≈0.07 nm) is considerably narrower than that of the travelling wave laser (Δλ_TWL≈14 nm). The DFB-laser emission is highly linearly polarised transverse to the grating axis (TE mode). Only at high pump pulse energy densities does an additional weak TM mode build up. The surface-emitted DFB-laser radiation has a low divergence along the grating direction. For both the DFB lasers and the travelling wave lasers, gain saturation occurs at high excitation energy densities. Received: 7 January 2002 / Revised version: 15 February 2002 / Published online: 14 March 2002     

Experiments on two-step heating of a dense plasma in the GOL-3 facility

This paper presents the results of experiments on two-stage heating of a dense plasma by a relativistic electron beam in the GOL-3 facility. A dense plasma with a length of about a meter and a hydrogen density up to 10¹⁷ cm⁻³ was created in the main plasma, whose density was 10¹⁵ cm⁻³. In the process of interacting with the plasma, the electron beam (1 MeV, 40 kA, 4 μs) imparts its energy to the electrons of the main plasma through collective effects. The heated electrons, as they disperse along the magnetic field lines, in turn reach the region of dense plasma and impart their energy to it by pairwise collisions. Estimates based on experimental data are given for the parameters of the flux of hot plasma electrons, the energy released in the dense plasma, and the energy balance of the beam-plasma system. The paper discusses the dynamics of the plasma, which is inhomogeneous in density and temperature, including the appearance of pressure waves. Zh. éksp. Teor. Fiz. 113, 897–917 (March 1998)     

Experimental and computational study of the passage of an electron beam through the curvilinear element of the Drakon stellarator system in a tenuous plasma

Results are presented from the first stage of studies on the passage of an electron beam with energy 100–500 eV in a magnetic field of 300–700 Oe through the curvilinear solenoid of the KRéL unit, the latter being a prototype of the closing segment of the Drakon stellarator system, in the plasma-beam discharge regime. The ion density at the end of the curvilinear part of the chamber, n _i ≈8×10⁸–10¹⁰ cm⁻³, the electron temperature T _e ≈4–15 eV, and the positions at which the beam hits the target for different distances from it to the electron source are determined experimentally. The motion of the electron beam is computationally modeled with allowance for the space charge created by the beam and the secondary plasma. From a comparison of the experimentally measured trajectories and trajectories calculated for different values of the space charge, we have obtained an estimate for the unneutralized ion density of the order of 5×10⁷ cm⁻³. Zh. Tekh. Fiz. 69, 22–26 (February 1999)     

Peculiarities of filamentation of sharply focused ultrashort laser pulses in air

Peculiarities of the self-focusing and filamentation of high-power femtosecond laser pulses in air have been experimentally and theoretically studied under conditions of broad variation of the beam focusing parameter. The influence of the numerical aperture (NA) of the initial radiation focusing on the main characteristics of laser-induced plasma columns (characteristic transverse size, length, and concentration of free electrons) is considered. It is established that, for a rigid (NA > 0.05) initial laser beam focusing, the transverse size of the plasma channel ceases to decrease at a level of R _pl ≈ 2–4 μm as a result of strong refraction of radiation on the plasma formed at the focal waist, which prevents further contraction of the laser beam due to its focusing and self-focusing.     

Coherent Thomson backscattering from laser-driven relativistic ultra-thin electron layers

The generation of laser-driven dense relativistic electron layers from ultra-thin foils and their use for coherent Thomson backscattering is discussed, applying analytic theory and one-dimensional particle-in-cell simulation. The blow-out regime is explored in which all foil electrons are separated from ions by direct laser action. The electrons follow the light wave close to its leading front. Single electron solutions are applied to initial acceleration, phase switching, and second-stage boosting. Coherently reflected light shows Doppler-shifted spectra, chirped over several octaves. The Doppler shift is found ∝ γ_x ²=1/(1-β_x ²), where β_x is the electron velocity component in normal direction of the electron layer which is also the direction of the driving laser pulse. Due to transverse electron momentum p_y, the Doppler shift by 4γ_x ²=4γ²/(1+(p_y/mc)²)≈2γ is significantly smaller than full shift of 4γ². Methods to turn p_y→0 and to recover the full Doppler shift are proposed and verified by 1D-PIC simulation. These methods open new ways to design intense single attosecond pulses.     

XUV generation from plasma produced by a XeCl excimer laser on a Cu target

Summary We present a detailed study of XUV and soft X-ray emission from Cu plasma produced by an excimer laser at intensitiesI _L≦8·10¹¹ W/cm². The XeCl excimer laser (ψ≈308 nm) delivers pulses with energyE _L≈2.3 J, temporal durationt _L≈100 ns and brightnessB≧10¹⁴ W/cm² sr. We recorded a spectral conversion efficiency η=0.5% eV⁻¹ forI _L=4·10¹¹W/cm² in the aluminium window at 73eV with a harder X-ray tail around ≈400eV. We also measured the dependence of X-ray signal on laser intensity and viewing angle. Experimental results have been compared with some analytical laser-plasma interaction models.     

Spatial characteristics of Kα radiation from weakly relativistic laser plasmas

The spatial dependence of K _α emission generated from laser-produced hot electrons is investigated experimentally and theoretically. In addition, the conversion efficiency of K _α production as a function of laser intensity is measured and compared with modeling results. We use the terawatt Ti:sapphire laser at MPQ and vary the peak intensity from 10¹⁵ to 10¹⁸ W/cm² with a pulse duration of 200 fs. A solid Cu target is placed at various positions in the laser focus, which allows one to vary the intensity but keep the total energy on the target constant. When the target is near best focus, the FWHM of the K _α emission, measured using a knife-edge, is considerably larger than the FWHM of the laser intensity. In measuring the efficiency of K _α production using the fundamental wavelength of the laser, a clear maximum of K _α emission is observed at a position away from best focus, where the peak intensity is down by more than an order of magnitude from the value at best focus. When the second harmonic of the laser is used, the K _α emission is peaked near best focus. The K _α emission from layer targets is used to obtain an estimate of the temperature of the hot electrons. Modeling of K _α production, using a Monte Carlo electron/photon transport code, shows the relationship between incident electron energy and the emitted K _α emission. Efficient K _α generation from the low-intensity wings of the laser pulse contributes to the large spot size of the K _α emission. The lower electron temperatures that are expected for the second harmonic explain the differences in the location of maximum K _α emission for the two wavelengths. We discuss the use of K _α emission in photoionizing inner-shell electrons with the goal of achieving X-ray lasing at short wavelengths. Received: 6 April 1999 / Revised version: 31 May 1999 / Published online: 11 August 1999     

High-intensity and high-brightness source of moderated positrons using a brilliant γ beam

Presently, large efforts are conducted toward the development of highly brilliant γ beams via Compton back scattering of photons from a high-brilliance electron beam, either on the basis of a normal-conducting electron linac or a (super-conducting) Energy Recovery Linac (ERL). Particularly, ERLs provide an extremely brilliant electron beam, thus enabling the generation of highest-quality γ beams. A 2.5 MeV γ beam with an envisaged intensity of 10¹⁵ photons s⁻¹, as ultimately envisaged for an ERL-based γ-beam facility, narrow band width (10⁻³), and extremely low emittance (10⁻⁴ mm² mrad²) offers the possibility to produce a high-intensity bright polarized positron beam. Pair production in a face-on irradiated W converter foil (200 μm thick, 10 mm long) would lead to the emission of 2×10¹³ (fast) positrons per second, which is four orders of magnitude higher compared to strong radioactive ²²Na sources conventionally used in the laboratory. Using a stack of converter foils and subsequent positron moderation, a high-intensity low-energy beam of moderated positrons can be produced. Two different source setups are presented: a high-brightness positron beam with a diameter as low as 0.2 mm, and a high-intensity beam of 3×10¹¹ moderated positrons per second. Hence, profiting from an improved moderation efficiency, the envisaged positron intensity would exceed that of present high-intensity positron sources by a factor of 100.     

ESR studies of sepiolites

The electron spin resonance (ESR) spectra for each beige and brown sepiolites from Polatli-Ankara, Turkey, a clay mineral of Mg₈Si₁₂O₃₀(OH)₄(OH₂)₄(H₂O)₈, at 77 K have their own single characteristic peak atg ≈ 1.99, which begins to anneal from 437 and 536°C, respectively. The sextet lines of Mn²⁺ were also observed in addition to two peaks (g = 4.4 and 5.0) associated with Fe³⁺. A high-quality sepiolite called “meerschaum” from Sepetci-Eskisehir, Turkey, has two characteristic peaks atg ≈ 1.99, which begins to anneal from 419°C, and atg ≈ 2.03. ESR intensities of the signal atg ≈ 1.99 for beige, brown sepiolites and meerschaum are enhanced by γ-ray irradiation to give equivalent dosesD _E = 11.8 ± 3.4, 4.7 ± 2.3 and 4.6 ± 1.2 kGy, respectively. The ESR ages obtained by assessing the annual dose rate from the content of²³⁸U,²³²Th and⁴⁰K determined by γ-ray spectroscopy are 1.8 ± 0.8, 0.7 ± 0.5 and 0.9 ± 0.2 Ma, respectively, falling into the Pliocene Epoch in geological time scale in agreement with the stratigraphy.     

Effect of gas pressure on amplitude and duration of electron beam current in a gas-filled diode

The parameters of an electron beam generated in helium in the pressure range p = 10⁻⁴−12 atm are studied. Nanosecond high-voltage pulses are applied to a gap between a tubular cathode and planar anode, which is made of 45-μm-thick AlBe foil. Behind the anode, an electron beam is detected at a helium pressure of 12 atm. The pressure dependence of the beam current amplitude shows three peaks at p ≈ 0.01, ≈ 0.07, and ≈ 3 atm. The beam-induced glow of a luminescent film placed behind the foil and the discharge glow at different helium pressures in the gas-filled diode are photographed.     

Laser induced breakdown spectroscopy of germane plasma induced by IR CO2 pulsed laser

Laser-induced breakdown spectroscopy (LIBS) in germane (GeH₄), initially at room temperature and pressures ranging from 2 to 10 kPa, was studied using a high-power transverse excitation atmospheric (TEA) CO₂ laser (λ=10.653 μm, τ _FWHM=64 ns and power densities ranging from 0.28 to 5.52 GW cm⁻²). The strong emission spectrum of the generated plasma is mainly due to electronic relaxation of excited Ge, H and ionic fragments Ge⁺, Ge²⁺ and Ge³⁺. The weak emission is due to molecular bands of H₂. Excitation temperatures of 8100±300 K and 23,500±2500 K were estimated by Ge atomic and Ge⁺ singly ionized lines, respectively. Electron number densities of the order of (0.7–6.2)×10¹⁷ cm⁻³ were deduced from the Stark broadening of several atomic Ge lines. The characteristics of the spectral emission intensities from different species have been investigated as functions of the germane pressure and laser irradiance. Optical breakdown threshold intensities in germane at 10.653 μm have been determined. The mechanism of initiation of the laser-induced plasma in germane has been analyzed.     

Rapid crystallization of silicon films using Joule heating of metal films

50-nm thick amorphous silicon films formed on glass substrates were crystallized by rapid Joule heating induced by an electrical current flowing in 100-nm-thick Cr strips formed adjacently to 200-nm-thick SiO₂ intermediate layers. 3-μs-pulsed voltages were applied to the Cr strips. Melting of the Cr strips caused a high Joule heating intensity of about 1×10⁶ W/cm². Raman scattering measurements revealed complete crystallization of the silicon films at a Joule heating energy of 1.9 J/cm² via the SiO₂ intermediate layer. Transmission electron microscopy measurements confirmed a crystalline grain size of 50–100 nm. 1-μm-long crystalline grain growth was also observed just beneath the edge of the Cr strips. The electrical conductivity increased from 10^-5 S/cm to 0.3 S/cm for 7×10¹⁷-cm^-3-phosphorus-doped silicon films because of activation of the phosphorus atoms because of crystallization. The numerical analysis showed a density of localized defect states at the mid gap of 8.0×10¹⁷ cm^-3. Oxygen plasma treatment at 250 °C and 100 W for 5 min reduced the density of the defect states to 2.7×10¹⁷ cm^-3. Received: 3 April 2001 / Accepted: 9 April 2001 / Published online: 25 July 2001     

Stimulated Brillouin scattering of CO2 radiation in compressed xenon

Stimulated Brillouin scattering (STBS) and phase conjugation of CO₂ laser radiation have been demonstrated experimentally for the first time in compressed xenon (59 atm at 21°C) located inside the low-Q cavity of this laser. The nonlinear medium was exposed to the action of counterpropagating focused multimode radiation beams. The difference between the frequencies of the longitudinal cavity modes was set at the frequency of the acoustic wave (v _s=32.2±0.3 MHz) excited as a result of STBS by 9.584 μm radiation. The duration of the radiation pulse τ _L was close to the acoustic phonon lifetime (τ _L<τ _ph≈3× 10⁻⁶s). The excitation of STBS was manifested experimentally as the locking of longitudinal modes, an increase in power and energy, and also an increase in the duration of the lasing pulse and a reduction in the divergence to the diffraction limit. Zh. éksp. Teor. Fiz. 116, 1941–1946 (December 1999)