Abstract: We theoretically and numerically demonstrate the generation of a relativistic modified Bessel-Gaussian beam (MBGB) via plasma-based beam braiding. It is realized by injecting several intense Gaussian pulses with well-designed offsets and angles into an underdense plasma channel which acts as a laser-pulse combiner via refractive coupling. The MBGB propagates stably in the plasma channel with a well-controlled orbital angular momentum of large value, exciting a twisted plasma wave. After leaving the plasma, it becomes unguided and survives in vacuum for at least hundreds of femtoseconds. This method is insensitive to the initial laser injection conditions and thus should be robust for experimental implementation. It provides an alternative approach in generating high-quality tunable intense optical vortex beams which are desired for various applications.
Abstract: We propose a mechanism to generate a single intense circularly polarized attosecond x-ray pulse from the interaction of a circularly polarized relativistic few-cycle laser pulse with an ultrathin foil at normal incidence. Analytical modeling and particle-in-cell simulation demonstrate that a huge charge-separation field can be produced when all the electrons are displaced from the target by the incident laser, resulting in a high-quality relativistic electron mirror that propagates against the tail of the laser pulse. The latter is efficiently reflected as well as compressed into an attosecond pulse that is also circularly polarized.
Abstract: A plasma channel undulator/wiggler may be created through the plasma wakefield excited by the beating
of several Hermite-Gaussian laser modes propagating in a parabolic plasma channel. Control over both the
betatron and undulator forces is conveniently achieved by tuning the amplitude ratios, colors, and order
numbers of the modes. A special structure of the undulator/wiggler field without the focusing force near the propagation axis is generated inside the plasma wakefield by matching the strengths of the fundamental and
first-order Hermite-Gaussian modes. The electron beam only experiences forced undulator oscillations in
such a field, which significantly improves the quality of the emitted radiation. Since the value of the
undulator strength parameter could be in a wide range, less or larger than unity, it is capable of generating
narrow bandwidth x-ray, as well as the synchrotronlike high-energy x/γ-ray, radiation by harmonics.
Additionally, controlling the relative phases between the laser modes allows for polarization control of the
plasma undulator. High-order harmonics produced from a circularly polarized plasma undulator clearly
show the vortex nature and carry well-defined orbital angular momentum.
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)
Abstract: Throughout the current century, compact, high-energy radiation sources have become critically important for many advanced applications in medicine, industry, education, and scientiﬁc research. In contrast to conventional radiation sources mainly produced in huge facilities, plasma-based radiation sources with centimetre lengths can provide great ﬂexibility and drive science forward.
In this thesis, several plasma wakeﬁeld-based undulator schemes have been developed in parallel. First, the guiding of laser beams, including a single Gaussian pulse, Hermite-Gaussian (HG) modes, and Laguerre-Gaussian (LG) modes, is studied through the Schrödinger-like wave equation for a harmonic oscillator with paraxial and quasistatic approximations in a parabolic plasma density channel. If the laser pulse is injected into the plasma channel with a transverse oﬀset or an angle with respect to the propagation axis, it will undergo centroid oscillation. Special conditions are found to control the interesting properties of such oscillation: frequency, amplitude, and polarisation.
Second, wakeﬁeld excitation driven by the oscillating laser pulse is theoretically and numerically studied in the linear/nonlinear regime. The speciﬁc ﬁeld structure of each scheme is demonstrated. For a short, wide laser pulse, the wakeﬁeld provides a linear focusing force near the propagation axis that drives the betatron oscillation of the injected electrons. The extra driving force is introduced by the centroid oscillation of the laser pulse. Surprisingly, the undulator ﬁeld generated by beating several diﬀerent HG modes becomes monochromatically sinusoidal when the strength of laser pulses matches a special condition. This is very beneﬁcial for the generation of a narrow radiation spectrum.
Third, the dynamics of both a single electron and an electron beam are studied in these generated undulator ﬁelds. Generally, an electron undergoes the combined motion of betatron and undulator oscillations. However, the weak betatron oscillation could be totally removed if certain injection conditions for an electron can be satisﬁed. Further theoretical work on the dynamics of an accelerated electron indicates that there is a resonance between the betatron oscillation of the electrons and centroid oscillation of the laser pulse. This resonance can be used to increase the oscillation amplitude and strength for the electron rapidly within the ﬁrst several Rayleigh lengths of propagation. While being accelerated in the wakeﬁeld, the resonance is broken and results in a semi-steady oscillation with large amplitude and strength, which enables the generation of strong γ-ray radiation.
Ultimately, the radiation spectrum from the oscillation of an electron beam is calculated. The proposed schemes are capable of generating an x-ray radiation spectrum with a narrow bandwidth or synchrotron-like x/γ-ray radiation of high energy. The energy and brightness are comparable with currently available conventional radiation sources. It is also demonstrated that these ﬂexible schemes can be tuned to generate radiation carrying well-deﬁned angular momentum.
Abstract: A flexible gamma-ray radiation source based on the resonant laser-plasma wakefield wiggler is proposed. The wiggler is achieved by inducing centroid oscillations of a short laser pulse in a plasma channel. Electrons (self-)injected in such a wakefield experience both oscillations due to the transverse electric fields and energy gain due to the longitudinal electric field. The oscillations are significantly enhanced when the laser pulse centroid oscillations are in resonance with the electron betatron oscillations, extending the radiation spectrum to the gamma-ray range. The polarization of the radiation can be easily controlled by adjusting the injection of the laser pulse into the plasma channel.
Abstract: A new plasma channel undulator concept based on the wakefields generated by short intense laser pulse undergoing centroid oscillations inside parabolic plasma channel is presented. The period of such an undulator is proportional to the Rayleigh length of the laser pulse and can be in the submillimeter range, while its strength can reach unity. Two-dimensional particle-in-cell simulations of the laser pulse propagation and wakefields are presented. Spontaneous radiation produced by the electron beam inside the plasma undulator is calculated.
Abstract: The theory of a plasma undulator excited by a short intense laser pulse in a parabolic plasma channel is presented. The undulator fields are generated either by the laser pulse incident off-axis and/or under the angle with respect to the channel axis. Linear plasma theory is used to derive the wakefield structure. It is shown that the electrons injected into the plasma wakefields experience betatron motion and undulator oscillations. Optimal electron beam injection conditions are derived for minimizing the amplitude of the betatron motion, producing narrow-bandwidth undulator radiation. Polarization control is readily achieved by varying the laser pulse injection conditions.