Spontaneous Emission Regime Research Articles

Three-dimensional simulations of harmonic radiation and harmonic lasing

Characteristics of the harmonic emission from free electron lasers (FELs) are examined in the spontaneous, coherent-spontaneous and stimulated emission regimes. The radiation at both odd and even harmonic frequencies is treated for electron beams with finite emittance and energy spread. In the spontaneous emission regime, the transverse radiation patterns including the transverse frequency dependences, are given. How this expression is modified to include energy spread and emittance is described. In the coherent-spontaneous emission and stimulated emission (lasing) regimes, the interaction of the radiation fields with the electrons must be treated self-consistently. Here, a single-frequency distributed transverse source function for each electron is used in the harmonic version (HELEX) of the 3D code FELEX to model the harmonic radiation. This code has recently been modified to simultaneously model the fundamental and harmonic interactions for multiple-pass oscillator simulations. These modifications facilitate the examination of FELs under various operating conditions. When the FEL is lasing at the fundamental, the evolution of the harmonic fields can be examined. This evolution is unique in the sense that the electron beam (which is bunched by the fundamental optical field) radiates at the harmonic frequencies in the presence of the harmonic radiation circulating in the cavity. As a result, enhancements of the harmonic emission can be observed. Finally, harmonic lasing can occur in cases where there is sufficient gain to overcome cavity losses and lasing at the fundamental can be suppressed. The characteristics and efficiency of these interactions are explored.

A universal classification of optimal undulator types and parameters for arbitrary storage ring environments

Prevailing insertion device technology utilizes transverse magnetostatic (TM) field configurations with fixed periods to induce periodic deviations in relativistic charged particle trajectories. Such devices are invariably tuned by varying the field strength in the region encompassing the trajectory. As a consequence, the highest first harmonic frequencies generated by such structures are limited by their period lengths, while the generation of lower first harmonics quickly leads to the production of excessive amounts of out-of-band radiation. In the spontaneous emission regime, this superfluous power varies directly with the length of the device. In view of this, recent access to straight sections in high energy storage rings exceeding lengths of 100 m has raised the undesirable prospect of unmanageable power levels limiting the utility of such facilities. In this article a general study of a set of major types and classes of transverse insertion devices operating on storage rings of arbitrary energy and current is made and the results are represented with a set of universal performance curves. The graphical tabulation enables easy identification of optimum undulator types and operating parameters for any storage ring environment; in particular, optimal devices which do not suffer from excessive power generation on high energy storage rings are defined.

Surface tension and instability of shock waves in gases

The pressure anisotropy within a shock layer results in an additional property of the shock wave: the surface tension. Its value amounts to 10 3 dyn/cm for Mach number 10. The surface tension term in the characteristic equation causes an absolute shock wave instability in the spontaneous sound emission regime.