Micropulse Width Research Articles

Observations of Bright Pulses from Pulsar B0031–07 at 4.82 GHz

Abstract We report on the detection of 146 (0.96%) bright pulses with a peak flux density >0.86 Jy from PSR B0031–07 in 4 hr of continuous observations made at 4.82 GHz, with a bandwidth of 1 GHz, using the Shanghai Tian Ma 65 m radio telescope. We find that the peak flux density distribution follows a power law with index α = −3.6 ± 0.2, while the pulse energy appears to follow a logarithmic normal distribution. The bright pulses from PSR B0031–07 were observed to repeat in an irregular fashion. The intervals between bright pulses can be described with an exponentiated Weibull distribution. Furthermore, the sensitive observations at higher frequency with a short time resolution of 65.54 μs form a reliable basis for detailed microstructure studies. Close inspection of the detected bright pulses reveals that 38 pulses exhibit well-defined quasi-periodicities. No unanimous period for the quasi-periodicities is found, and the peak flux density is independent of the micropulse width. Finally, possible physical mechanisms are discussed.

Research of coherent edge radiation generated by electron beams oscillating free-electron lasers

We have studied far-infrared coherent radiation with S-band linacs at Laboratory for Electron Beam Re-search and Application (LEBRA) at Nihon University and at Kyoto University Free Electron Laser. We have already developed a couple of terahertz-wave sources based on coherent synchrotron radiation and coherent transition radiation, which have been applied to spectroscopic research. Moreover, we developed coherent edge radiation (CER) at the downstream bending magnets in the FEL sections. Because the edge radiation has an annular shape distribution characterized by the asymmetric first-order Laguerre-Gaussian mode, the CER can be extracted from an optical cavity of the FEL system without a diffraction loss of the FEL beam. The root-mean-squared bunch length of the electron beam was evaluated by measuring the CER spectra, which was about the same level as the FEL micropulse width. Although the infrared FELs at LEBRA had a long slippage length, the FEL energy became higher as the bunch length was shorter. The CER intensity can be a guidepost enhancing the FEL power because of the existence of their correlation.

Quasi-continuous-wave 589-nm radiation based on intracavity frequency-doubled Nd:GGG/BaWO4 Raman laser

Quasi-continuous-wave (QCW) 589-nm radiation was realized based on a frequency-doubled crystalline Raman laser. The fundamental wave with macro–micro-pulse trains was generated from an acousto-optically Q-switched QCW diode side-pumped Nd:GGG laser. Intracavity Raman conversion was accomplished by a BaWO4 crystal and the second harmonic generation was finished by a KTP crystal. Under a pumping power of 126.0W with a macro-pulse frequency of 300Hz and duration of 300µs, the maximum 589nm output power of 4.2W was obtained at a micro-pulse frequency of 33.3kHz. The micro-pulse width was 13.6ns.

Sub-pm linewidth nanosecond Nd:GYSGG laser at 1336.6 nm.

We demonstrate a sub-pm linewidth acousto-optic (AO) Q-switched nanosecond Nd:GYSGG ring laser at 1336.6 nm side-pumped by 808-nm quasi-continuous wave (QCW) diode lasers for the first time. With incident pulse energy of 4.23 J at 10 Hz, a maximum output macropulse energy of 36.7 mJ at 1336.6 nm with linewidth of less than 0.85 pm and a micropulse width of 300 ns was obtained at a repetition rate of 80 Hz, corresponding to an average micropulse peak power of 15.3 kW. The M² factors were measured to be 1.42 and 1.10 in x and y directions, respectively. It can be tuned from 1336.576 to 1336.652 nm with a tuning resolution of 1 pm. The 1336.632 nm can be converted to deep ultraviolet (DUV) laser at 167.079 nm through its eighth harmonics, which is very useful for the ²⁷Al⁺ optical frequency standard.

Synthesis of Polymerized C60 Bulk and Film in Three Dimensions by Irradiating Free-Electron Laser

Pressed C60 powder was polymerized with free-electron laser (FEL) irradiation. Brightened grains were synthesized in part at the sample surface, the Raman spectrum of which indicated the polymerization. It was noted that polymerization occurred as a result of irradiating a third harmonic FEL at 500 nm with an extremely low energy density of 2.6 µJ/(pulse·cm2). The polymerization is considered to be due to the micropulse width less than one ps forming the 20 µs macropulse of the FEL. Irradiating FEL during film growth, at 200 °C in particular, promoted the polymerization, as demonstrated by the Raman spectrum. The X-ray diffraction showed an oriented hcp structure with the relationship that the [0001] direction of the film was normal to the surface. However, Bragg reflection peaks did not shift in the film grown with FEL irradiation. These results indicate that the polymerized structure is a two-dimensional (2D) rhombohedron in the film plane. We expect that the (0001) surface of the oriented film will be suitable for a 2D polymerized rhombohedron and that migration energy will contribute to polymerization at an elevated temperature.

Second Harmonic Generation in Te Crystal using Free Electron Laser

The second harmonic generation signal converted from the fundamental wavelength of 22 µm of a free electron laser was observed for the first time using a birefringent Te crystal. The experimental conversion efficiency of Te crystal for second harmonic generation is 0.53%, which is equivalent to the theoretical value within a factor of 2. The Te crystal has been incorporated into an autocorrelator system to measure the micro-pulse width of infrared free electron laser successfully.

High-resolution single-pulse studies of the Vela pulsar

We present high-resolution multi-frequency single-pulse observations of the Vela pulsar, PSR B0833 − 45, aimed at studying micro-structure, phase-resolved intensity fluctuations and energy distributions at 1.41 and 2.30 GHz. We find micro-structure in about 80 per cent of all pulses independent of observing frequency. The width of a micro-pulse seems to depend on its peak flux density, whilst quasi-periodicities observed in micro-pulses remain constant. The width of the micro-pulses may decrease with increasing frequency, but confirmation is needed at higher frequencies. The fraction of pulses showing quasi-periodic micro-pulses may become smaller at higher frequencies. We show that the micro-pulse width in pulsars has a period dependence which suggests a model of sweeping micro-beams as the origin of the micro-pulses. Like individual pulses, the micro-pulses of Vela are highly elliptically polarized. There is a strong correlation between Stokes parameters V and I in the micro-structure. We do not observe any micro-pulses where the handedness of the circular intensity changes within a micro-pulse, although flips within a pulse are not uncommon. We show that the V/I distribution is Gaussian with a narrow width and that this width appears to be constant as a function of pulse phase. The phase-resolved intensity distributions of I are best fitted with log-normal statistics. Extra emission components, i.e. ‘bump’ and ‘giant micro-pulses’, discovered by Johnston et al. at 0.6 and 1.4 GHz are also present at the higher frequency of 2.3 GHz. The bump component seems to be an extra component superposed on the main pulse profile but does not appear periodically. The giant micro-pulses are time-resolved and have significant jitter in their arrival times. Their flux density distribution is best fitted by a power-law, indicating a link between these features and ‘classical’ giant pulses as observed for the Crab pulsar (PSR B0531 + 21), PSR B1937 + 21 and PSR B1821 − 24. We find that Vela contains a mixture of emission properties representing both ‘classical’ properties of radio pulsars (e.g. micro-structure, high degree of polarization, S-like position angle swing, orthogonal modes) and features which are most likely related to high-energy emission (e.g. extra profile components, giant micro-pulses). It hence represents an ideal test case to study the relationship between radio and high-energy emission in significant detail.

Characteristics of the NIJI-IV UV-VUV FEL system—toward lasing down to 150 nm using a compact storage ring

Improvement of the NIJI-IV free electron laser (FEL) system is in progress. Recent gain enhancement made the lasing more stable in the wavelength range from 216 to 211 nm. The temporal lasing characteristics were studied with a dual sweep streak camera and a relatively stable cw lasing with a short micro-pulse width of ∼14 ps (FWHM) was observed at 214 nm. Numerical calculation for the bunch lengthening and the laser gain revealed that full operation of our new RF cavity and installation of low impedance ring chambers will drastically increase the laser gain up to 20% at 200 nm and 12% at 150 nm. We can expect FEL lasing below 200 nm and even at wavelengths down to 150 nm using the compact storage ring NIJI-IV.

Ultrafast interband-light modulation by intersubband light (free-electron laser) in quantum wells

Ultrafast interband-resonant light modulation induced by an intersubband transition in GaAs/AlGaAs multiple quantum wells is observed by using a free-electron laser (FEL) for intersubband excitation. The FEL operates with a picosecond pulse width and a wide wavelength tunability. The dependence of the modulation depth on the wavelength of the intersubband-resonant light shows good agreement with the intersubband absorption spectrum of the multiple quantum wells measured by Fourier transform infrared (FTIR) spectroscopy. We also investigated the relaxation time of the intersubband transition by a pump–probe experiment and found that it was less than the FEL micropulse width (∼3 ps).

Simulation of radiation build-up in an FEL oscillator: effect of electron energy spread and micro pulse width

The characteristics of radiation build-up in an FEL oscillator and the effect of electron beam energy spread on the radiation build-up including its effect on electron micropulse width, radiation wavelength and slippage distance is investigated. By the numerical simulation, it is clarified that the effect of electron beam energy spread on radiation build-up for the case using the coherent start up oscillator scheme is smaller than that for the case of the normal FEL (long electron micropulse case), while the saturation power of radiation for the normal FEL is larger than that for the coherent start up FEL. So in order to make the best use of these characteristics, at first the short electron micropulses are driven to the wiggler to attain the fast radiation build-up in the oscillator, then long electron micropulse is driven causing bunching after a few interactions, then reaching saturation. This oscillation scheme may be effective in the millimeter to sub-millimeter regime of radiation wavelength.

First lasing of the compact infrared free-electron laser

On 15 May 1996, lasing was achieved at the Compact Infrared Free-Electron Laser (CIRFEL) at 14 μm. The electron beam energy was 11.2 MeV and the micropulse charge was 1 nC. The width of the detuning curve was approximately 50 μm. Since achieving this milestone, we have lased repeatedly from 12.5 to 20 μm with between 0.25 and 1.5 nC of charge. Saturation is typically reached in less than 1.5 μs. The measured FEL spectrum is Gaussian in shape with a line width of 56 nm at 13 μm. Assuming a transform limited pulse, this corresponds to an FEL light pulse of 4.4 ps. The CIRFEL is a photocathode based free-electron laser with a micropulse width of 4–10 ps and a micropulse separation of 7 ns. The macropulse length is between 4 and 6 μs and the repetition rate is between 1 and 5 Hz. We present details of the CIRFEL machine as well as experimental measurements of the FEL radiation.

Current status of the superconducting RF linac driver for the JAERI free electron laser

Abstract A prototype for a quasi-cw, and 1 kW high-average power free electron laser (FEL) driven by a 15 MeV superconducting rf linac has been developed, and constructed at Tokai, JAERI. Cryogenic stand-by loss ( 7 MV/m and Q ∼ 2 × 10+9) of four JAERI superconducting accelerator modules have been successfully demonstrated. Beam test and commissioning of the JAERI superconducting rf linac have been successfully performed to get electron beams ranging from 10 to 23 MeV and the fully transmitted of ∼ 16 A peak current around 17 MeV. The measured energy resolution of a pre-accelerated beam is about 3% of FWHM, and that of a fully-accelerated beam about 0.8% or less. The measured micropulse width is adjustable in the range from 10 to 60 ps. FEL opticals and beam transport elements around the undulator are now under commissioning. Spontaneous emissions in the wavelength of 20 to 80 μm or longer have been observed by using the Ge(Cu) and other detectors, a home-made fast current amplifier, and a 24 m long light transporting system.

FEL center user diagnostics and control

In the past year, the Stanford Picosecond FEL Center has produced more than two thousand hours of beam time dedicated to user experiments. To assure reliable beam delivery and to maximize productivity of our users we have developed a sophisticated system of diagnostics and control. An integrated display is now available in all experimental areas which provides continuously updated measurements of beam spectrum, micropulse duration, power, position, and pointing — all of which may be saved to document beam conditions during an experiment. The beam is actively wavelength and amplitude stabilized to better than 0.01% and 2%, respectively. Direct wavelength control is available to users in every experimental area, allowing changes of wavelength as large as a few percent. Larger wavelength shifts, and adjustments in macropulse or micropulse width or timing, are readily available with operator assistance.

Simulation of radiation build-up in an FEL oscillator: the effect of the electron beam micropulse shape

Using a one-dimensional multi-frequency simulation code, we have investigated how the radiation build-up time in an FEL oscillator depends upon the electron beam micropulse width and micropulse shape. From the simulation, it is found that in the Gaussian electron beam case when the electron beam micropulse width is shorter than the radiation wavelength, the radiation in the cavity builds up faster than in longer cases (because of the coherent spontaneous emission). But in the square electron beam pulse case, the spontaneous emission at the edge is much stronger than in the case of a Gaussian electron beam pulse, so the effect of coherent spontaneous emission is reduced. The radiation spectral intensity and temporal profiles have been evaluated for super Gaussian pulse shapes of a bunched electron beam.

Numerical analysis of radiation build-up in FEL oscillator

Using a one-dimensional multi-frequency simulation code, it is investigated how the radiation build-up time in a FEL oscillator depends upon the electron beam micropulse width. By the simulation, it is found that in the case where the electron beam micropulse width is shorter than the radiation wavelength, the radiation in the cavity builds up faster than for the opposite case. (This is because of the coherent spontaneous emission.) The effect of cavity length detuning on radiation build-up is also investigated. The output radiation power is found to be very sensitive to the detuning. When the cavity Q-value is given, the full detuning width for the half maxima is ΔL = λ s 4N pass for a radiation wavelength λ s and pass number N pass.

Observations of the Super-ACO FEL micropulse with a streak camera

The detailed measurement of the free electron laser micropulse with a streak camera is carried out on the Super-ACO storage ring for the first time. In this measurement, we reveal the temporal form of the laser micropulse in the cw and the Q-switched laser operation modes. In some operating conditions, the temporal distribution of the laser micropulse is different from a Gaussian. We have also measured the laser micropulse width ( σ RMS ≈ 26 ps). This value is compared to the previously obtained results and it shows a good agreement.

AM mode-locking of a free-electron laser oscillator

The authors present experimental and theoretical studies of a mode-locked free-electron laser (FEL) oscillator. In the experiment the FEL uses a continuous electron beam and operates in the microwave regime. AM mode-locking is performed by modulating the attenuation of the FEL ring cavity by a PIN diode modulator. The modulation period is tuned to match the RF roundtrip time in the ring cavity. The experimental results show the evolution of a single radiation macropulse, consisting of narrow micropulses in synchrony with the sinusoidal locking signal. The micropulse period ( approximately 37 ns) equals the roundtrip time and the modulation period. The micropulse width ( approximately 5 ns) is limited by the FEL slippage time and by the dispersion in the waveguide ring cavity. The effect of the mode locking consisting in suppressing asynchronous oscillations is clearly observed in the experiment. A theoretical model of the AM mode-locked FEL oscillator operating in the small signal regime is presented. This model includes the slow time variation of the e-beam energy and waveguide dispersion. The theoretical analysis agrees well with the experimental results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>