Radiation instability of a relativistic electron beam moving in a split resonator

In this paper, we considered the radiation instability in a split asymmetric resonator for the relativistic case assuming the space charge of the beam. In the small-signal approximation, expressions for the energy loss by a particle passing through the resonator and for the beam current modulation are obtained. Based on analytical and numerical calculations, it is shown that the symmetric configuration provides the highest growth rate of instability. It is found that with the increase of the initial electron energy, the modulation of the beam current as well as the efficiency of the energy transfer from particles to the electromagnetic field decrease. The increase of the beam density has a positive effect on the radiation instability. The results obtained have to be taken into account when developing generators of electromagnetic radiation or a system for modulating the beam current based on a split resonator.

Timing with PMTs

The timing capability of photomultipliers (PMTs) can be inferred from the basic laws of electron motion. The relationships between time dispersion and field strength, initial electron energy, angle of emission, and electrode spacing follow from these laws. For conventional PMTs, the major contribution to dispersion arises from the cathode-to-first-dynode region. The field gradient at the cathode primarily determines the timing. This is verified by examining the electron motion in non-uniform electric fields. The contribution from interdynode transitions is small for linear focussed PMTs. Monte Carlo simulations of output waveforms from scintillators agree with measurements. The performance of threshold, zero crossing, and constant fraction (CF) discriminators is examined, revealing the superiority of the CF types. Two organizations have made detailed timing measurements, some of which show sub-nanosecond jitter. Proximity focussed PMTs from Hamamatsu confirm time dispersion measured in picoseconds.

FR II radio galaxies at low frequencies – II. Spectral ageing and source dynamics

In this paper, the second in a series investigating Fanaroff–Riley type II (FR II) radio galaxies at low frequencies, we use LOw Frequency ARray (LOFAR) and Very Large Array (VLA) observations between 117 and 456 MHz, in addition to archival data, to determine the dynamics and energetics of two radio galaxies, 3C 452 and 3C 223, by fitting spectral ageing models on small spatial scales. We provide improved measurements for the physical extent of the two sources, including a previously unknown low surface brightness extension to the northern lobe of 3C 223, and revised energetics based on these values. We find spectral ages of $77.05^{+9.22}_{-8.74}$ and $84.96^{+15.02}_{-13.83}$ Myr for 3C 452 and 3C 223, respectively, suggesting a characteristic advance speed for the lobes of around 1 per cent of the speed of light. For 3C 452, we show that, even for a magnetic field strength not assumed to be in equipartition, a disparity of a factor of approximately 2 exists between the spectral age and that determined from a dynamical standpoint. We confirm that the injection index of both sources (as derived from the lobe emission) remains steeper than classically assumed values, even when considered on well-resolved scales at low frequencies. However, we find an unexpected sharp discontinuity between the spectrum of the hotspots and the surrounding lobe emission. We suggest that this discrepancy is a result of the absorption of hotspot emission and/or non-homogeneous and additional acceleration mechanisms; as such, hotspots should not be used in the determination of the underlying initial electron energy distribution.

The electron-longitudinal optical phonon scattering rate in GaInAsP/InP stepped quantum well

Within the framework of effective mass approximation, the scattering rate via longitudinal optical (LO) phonon emission for an electron and the mean scattering rate via LO phonons emission for electrons initially in the first excited sub-band and finally in the ground sub-band in [Formula: see text] stepped quantum well (QW) is calculated adopting the shooting method and Fermi’s golden rule. The results show that the scattering rate and the mean scattering rate are highly dependent on alloy compositions, well width, initial electron energy, electron temperature and sub-band separation [Formula: see text] between the ground sub-band and the first excited sub-band. When [Formula: see text] is larger than the LO phonon energy, the scattering rate and the mean scattering rate increases with increasing Ga composition, decreasing As composition and increasing well width. However, when [Formula: see text] is smaller than the LO phonon energy, its change tendency is contrary. The scattering rate increases with decreasing initial electron energy if the separation between the initial electron energy and the ground state energy [Formula: see text] is not smaller than the LO phonon energy. The scattering rate and the mean scattering rate increases with rising electron temperature. The mean scattering rate reaches the maximum value when [Formula: see text] is equal to the LO phonon energy. The interruption in the scattering rate happens when the separation between the initial electron energy and [Formula: see text] is smaller than the LO phonon energy. The rapid decrease of the mean scattering rate happens when [Formula: see text] is smaller than the LO phonon energy if [Formula: see text] continues decreasing. In addition, both the scattering rate and the mean scattering rate show little change with different stepped layer widths.

Electron energy enhancement by a circularly polarized laser pulse in vacuum

Energy enhancement by a circularly polarized laser pulse during acceleration of the electrons by a Gaussian laser pulse has been investigated. The electrons close to the temporal peak of the laser pulse show strong initial phase dependence for a linearly polarized laser pulse. The energy gained by the electrons close to the rising edge of the pulse does not show initial phase dependence for either linearly- or circularly-polarized laser pulse. The maximum energy of the electrons gets enhanced for a circularly polarized in comparison to a linearly polarized laser pulse due to axial symmetry of the circularly polarized pulse. The variation of electron energy with laser spot size, laser intensity, initial electron energy, and initial phase has been studied.

Resonant enhancement of electron energy by frequency chirp during laser acceleration in an azimuthal magnetic field in a plasma

Electron acceleration by a chirped laser pulse in an azimuthal magnetic field in a plasma has been studied. The betatron resonance saturates and the electrons start losing energy beyond a specific point of time without a frequency chirp. The resonance can be maintained for a longer duration and the energy of the electrons can be enhanced if a suitable frequency chirp is introduced. The duration of interaction increases for a lower plasma density or a lower initial electron energy which causes increase in the electron energy gain. The value of magnetic field required for resonance increases with an increase in plasma density and with a decrease in initial electron energy.