Efficiency enhancement of nonlinear odd harmonics in thermal free electron laser

Abstract

The effect of axial energy spread on the radiation of third harmonic is studied in the free electron laser with planar wiggler and ion-channel guiding. Spread in the longitudinal momentum and so in the initial energy of electron beam, without any spread in the transverse velocity, is assumed in the form of Gaussian distribution function. The technique that is employed is a one-dimensional and steady-state simulation. A set of self consistent nonlinear differential equations that describes the system is solved numerically by Runge-Kutta method. Due to the sensitivity of harmonics to thermal effects, gain improvement of third harmonic radiation is achieved by using ion-channel guiding technique and efficiency enhancement is applied by tapering the magnetic field of wiggler to optimize radiation. The bunching parameter of the electron beam is also studied. It is found that the growth of the magnitude of the bunching parameter that is caused by the ponderomotive wave stops before the saturation point of the radiation. This means that ponderomotive wave saturates at a shorter distance compared to the radiation.

Nonlinear third harmonic radiation in thermal free electron laser with modified wiggler

“…is defined over an ensemble of electrons injected into the wiggler within one wave period. For this particle averages, a choice of 61 particles in Simpson technique was found to be adequate in the case of cold beam, which is increased to 16 × 16 = 256 particles in Gaussian quadrature technique for nonzero energy spread (Bazouband and Maraghechi 2012;2013).…”

“…For this reason, these FEL devices have been subject of much attention because of their many realistic applications in medicine and industry (Kong 1997;Sung et al 2006;Doria et al 2009). Although harmonic gain in FEL is much smaller than that of the fundamental, several gain and efficiency enhancement techniques have been suggested (Freund and Gold 1984;Freund and Pasour 2003;Wang 2007;Rouhani and Maraghechi 2009;Jafari Bahman and Maraghechi 2012;Zahedian et al 2012;Bazouband and Maraghechi 2013). Adding the harmonic wiggler field to the fundamental with the appropriate ratio and phase can increase the gain of harmonic (Asakawa et al 1992) that is considered in this paper and this nonconventional wiggler is known as modified wiggler.…”