S.V. Milton

Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet

Researchers demonstrate the FERMI free-electron laser operating in the high-gain harmonic generation regime, allowing high stability, transverse and longitudinal coherence and polarization control.

Nonlinear harmonic generation in free-electron lasers

A three-dimensional nonlinear simulation code to treat multiple frequencies simultaneously is described and used to study nonlinear harmonic generation in free-electron lasers (FELs). Strong nonlinear harmonic gain is found where the gain length varies inversely with the harmonic number. Substantial power levels are found in the harmonics. The odd harmonics are favored with generally higher power levels since a planar wiggler geometry is employed; however, the second harmonic exhibits substantial power as well. The analysis is relevant to the emission expected from self-amplified spontaneous emission (SASE) free-electron laser schemes.

Neural Networks for Modeling and Control of Particle Accelerators

Particle accelerators are host to myriad nonlinear and complex physical phenomena. They often involve a multitude of interacting systems, are subject to tight performance demands, and should be able to run for extended periods of time with minimal interruptions. Often times, traditional control techniques cannot fully meet these requirements. One promising avenue is to introduce machine learning and sophisticated control techniques inspired by artificial intelligence, particularly in light of recent theoretical and practical advances in these fields. Within machine learning and artificial intelligence, neural networks are particularly well-suited to modeling, control, and diagnostic analysis of complex, nonlinear, and time-varying systems, as well as systems with large parameter spaces. Consequently, the use of neural network-based modeling and control techniques could be of significant benefit to particle accelerators. For the same reasons, particle accelerators are also ideal test-beds for these techniques. Many early attempts to apply neural networks to particle accelerators yielded mixed results due to the relative immaturity of the technology for such tasks. The purpose of this paper is to re-introduce neural networks to the particle accelerator community and report on some work in neural network control that is being conducted as part of a dedicated collaboration between Fermilab and Colorado State University (CSU). We describe some of the challenges of particle accelerator control, highlight recent advances in neural network techniques, discuss some promising avenues for incorporating neural networks into particle accelerator control systems, and describe a neural network-based control system that is being developed for resonance control of an RF electron gun at the Fermilab Accelerator Science and Technology (FAST) facility, including initial experimental results from a benchmark controller.

Electron Back-Bombardment and Mitigation in a Short Gap, Thermionic Cathode RF Gun

When an un-gated thermionic cathode is operated in a radio-frequency gun, some fraction of the emitted electrons will return to the cathode due to the change in sign of the electric field in the gun. This back-bombardment current causes heating of the cathode, and this reduces the ability of the cathode heater to control the bunch charge. In this paper, we investigate the fundamental factors in single-frequency back-bombardment for a short-gap electron gun. Simulations revealed that the back-bombardment power depends strongly on the operating frequency and the bunch charge. Additionally, the use of a two-frequency TM010/TM020 electron gun to mitigate this effect was investigated which revealed that the effectiveness of this technique depends strongly upon single-frequency back-bombardment power but with the optimal reduction in back-bombardment power (62% of the baseline) occurring in the low-frequency limit.

The effect of wiggler imperfections on nonlinear harmonic generation in free-electron lasers

The generation of harmonics through a nonlinear mechanism driven by bunching at the fundamental has sparked interest in using this process as a path toward an X-ray free-electron laser (FEL). An important issue in this regard is the sensitivity of the nonlinear harmonic generation to wiggler imperfections. Typically, linear instabilities in FELs are characterized by increasing sensitivity to both electron beam and wiggler quality with increasing harmonic number. However, since the nonlinear harmonic generation mechanism is driven by the growth of the fundamental, the sensitivity of the nonlinear harmonic mechanism is not severely greater than that of the fundamental. In this paper, we study the effects of wiggler imperfections on the nonlinear harmonics in a 1.5-/spl Aring/ FEL, and show that the decline in the third harmonic emission with increasing levels of wiggler imperfections roughly tracks that of the fundamental.

Temporal model for quasi-phase matching in high-order harmonic generation

We present a model for quasi-phase matching (QPM) in high-order harmonic generation (HHG). Using a one-dimensional description, we analyze the time-dependent, ultrafast wave-vector balance to calculate the on-axis harmonic output versus time, from which we obtain the output pulse energy. Considering, as an example, periodically patterned argon gas, as may be provided with a grid in a cluster jet, we calculate the harmonic output during different time intervals within the drive laser pulse duration. We find that identifying a suitable single spatial period is not straightforward due to the complex and ultrafast plasma dynamics that underlies HHG at increased intensities. The maximum on-axis harmonic pulse energy is obtained when choosing the QPM period to phase match HHG at the leading edge of the drive laser pulse.

Deep saturated Free Electron Laser oscillators and frozen spikes

We analyze the behavior of Free Electron Laser (FEL) oscillators operating in the deep saturated regime and point out the formation of sub-peaks of the optical pulse. These are very stable configurations and the sub-peaks are found to have a duration corresponding to the coherence length. We speculate on the physical mechanisms underlying their growth and attempt an identification with natural mode-locked structures in FEL oscillators. Their impact on the intra-cavity nonlinear harmonic generation is also discussed along with the possibility of exploiting them as cavity out-coupler

Pathway to a compact SASE FEL device

Abstract Newly developed high peak power lasers have opened the possibilities of driving coherent light sources operating with laser plasma accelerated beams and wave undulators. We speculate on the combination of these two concepts and show that the merging of the underlying technologies could lead to new and interesting possibilities to achieve truly compact, coherent radiator devices.

Advanced Controls for Accelerators

We present our efforts in developing adaptive, artificial intelligence-based tools specifically to address control challenges found in particle accelerator systems. We will discuss the opportunities and challenges, especially for future compact sources that will not necessarily be co-located with accelerator operators or experts. Finally, we will discuss our modeling and simulation of systems as well as the experiments we have performed thus far showing great promise.

Next-Generation Light Sources in 2010

As electron beam quality and control continue to improve, so do the capabilities of free-electron lasers (FELs). Significant advances have occurred in the field over the last few years and have greatly enabled the end users of the FELs, i.e., those doing cutting-edge research via use of the extremely high-brightness FEL light pulses. After a brief review of the basics of FELs, we describe the present frontiers of FEL research and development (R&D), along with several of the most recent demonstrated advances.