Coherent phenomena in the interaction of pulsed particle beams and radiation

Abstract

In this thesis, an analytical study is performed of phenomena occurring in the interaction of bunches of charged particles with electromagnetic radiation. The work concentrates on bunches smaller than the wavelength of the radiation, for which coherent effects become significant. Novel physical phenomena are identified and the feasibility of experimental observation and technological exploitation is considered. The studied system of a subwavelength body of charge emerges in a variety of contexts in physics. The oldest one is probably that of classical electromagnetic mod- els of charged particles. Derivations of the electromagnetic self-force of rigid charged objects, as have been developed in such models, are reviewed in this thesis. The mathematical equivalence of the various dissimilar self-force expressions is demonstrated explicitly. The position of the presented self-force calculations in the wider context of classical electrodynamic descriptions of charged particles is discussed, as well as their relevance to the description of macroscopic bunches of charged particles. In modern high-power laser physics, phenomena associated with the electromagnetic self-force are referred to as radiation reaction effects. In this work, the coherent enhancement of such effects is considered and its influence on the motion of subwavelength electron bunches in interaction with intense laser pulses is analyzed. It is shown that the radiation reaction force behaves as a radiation pressure in the laser beam direction, combined with a damping force in the perpendicular direction. Due to Coulomb expansion of the electron bunch, coherent radiation reaction takes effect only in the initial stage of the laser-bunch interaction while the bunch is still smaller than the wavelength. However, this initial stage can have observable effects on the trajectory of the bunch. By scaling the system to larger bunch charges, the radiation reaction effects are strongly increased. On the basis of the usual equation of motion, this increase is shown to be such that radiation reaction may suppress the radial instability normally found in ponderomotive acceleration schemes, thereby enabling the full potential of laser-vacuum electron bunch acceleration to GeV energies. However, the applicability of the used equation of motion still needs to be validated experimentally, which becomes possible using the presented experimental scheme. In order to obtain an accurate description of electron bunch trajectories in a laser pulse, it proves to be essential to take into account the so-called ponderomotive force. This is the time-averaged Lorentz force experienced by a charged particle in an inhomogeneous, harmonically oscillating electromagnetic field. In this thesis, this force is studied in more detail for the special case of a relativistic charged particle entering an electromagnetic standing wave with a general three-dimensional field distribution and a nonrelativistic intensity. It is demonstrated that the standard ponderomotive force expression is not valid in this case, and the correct force is derived using a perturbation expansion method. The modified expression is still of simple gradient form, but contains additional polarization-dependent terms. These terms arise because the relativistic translational velocity induces a quiver motion in the direction of the magnetic force, which is the direction of large field gradients. Consistent perturbation expansion of the equation of motion leads to an effective doubling of this magnetic contribution. The derived ponderomotive force generalizes the polarization-dependent electron motion in a standing wave obtained earlier. Comparison with simulations in the case of a realistic, non-idealized, three-dimensional field configuration confirms the general validity of the analytical results. Motivated by the usually rapid Coulomb expansion of electron bunches, and the correspondingly temporary nature of coherent effects, subwavelength quasi-neutral plasmas are considered in this thesis as alternatives in which the repulsive Coulomb force is absent. However, plasmas expand as well, although the expansion is driven by the thermal pressure. Therefore, several mechanisms by which an external electromagnetic field influences the temperature of a plasma are studied and specialized to the system of an ultracold plasma driven by a uniform radio frequency field. Heating through collisional absorption is reviewed and applied to ultracold plasmas. It is shown that the rf field modifies the three body recombination process by ionizing electrons from intermediate high-lying Rydberg states and upshifting the continuum threshold, resulting in a suppression of three body recombination. Heating through collisionless absorption associated with the finite plasma size is analyzed, revealing a temperature threshold below which collisionless absorption is ineffective. In addition, also the electromagnetic aspect of the interaction of radiation with cold subwavelength plasmas is studied, and the ponderomotive forces induced in the plasma by the radiation are evaluated. To this end, the plasma is modeled as a sphere with a radially varying permittivity, and the internal electric fields are calculated by solving the macroscopic Maxwell equations using an expansion in Debye potentials. It is found that the ponderomotive force is directed opposite to the plasma density gradient, similarly to large-scale plasmas. In case of a uniform density profile, a residual spherically symmetric compressive ponderomotive force is found, suggesting possibilities for contactless ponderomotive manipulation of homogeneous subwavelength objects. The presence of a surface ponderomotive force on discontinuous plasma boundaries is derived. This force is essential for a microscopic description of the radiation-plasma interaction consistent with momentum conservation. It is shown that the ponderomotive force integrated over the plasma is equivalent to the radiation pressure exerted on the plasma by the incident wave. The concept of radiative acceleration of subwavelength plasmas, proposed earlier, is applied to ultracold plasmas. It is estimated that these plasmas may be accelerated to keV ion energies, resulting in a neutralized beam with a brightness comparable to that of current high-performance ion sources. Finally, in this thesis a system is studied in which subwavelength electron bunches act as a radiation source, rather than a passive receiver of applied radiation. A novel method is proposed to generate electromagnetic surface waves of terahertz bandwidth on a metal wire, by launching electron bunches onto a tapered end of the wire. To show the potential of this method, Maxwell’s equations are solved for the appropriate boundary conditions. The metal wire tip is modeled as a perfectly conducting semi-infinite cone. It is shown that the surface waves can be recovered from the idealized fields by well-known perturbation techniques. The emitted radiation is strongly con-centrated into a narrow solid angle near the cone boundary for cones with a small opening angle. It is found that sub-picosecond surface waves with peak electric fields of the order of MV/cm on a 1 mm diameter wire can be obtained using currently available technology, which has been confirmed experimentally.