Time structure of the OK-4/Duke storage ring FEL

doi:10.1016/S0168-9002(01)01569-8

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 475, Issues 1–3, 21 December 2001, Pages 240-246

https://doi.org/10.1016/S0168-9002(01)01569-8 Get rights and content

Abstract

In this paper, we present results of experimental and theoretical studies of macro- and micro-temporal of dynamics of the OK-4/Duke storage ring FEL (SR FEL) and electron beams. The experimental part of these studies utilized the Hamamatsu C5680 dual-sweep streak-camera with 1.2 ps resolution. We use both numerical and analytical tools for theoretical analysis of the FEL and e-beam distributions without any pre-imposed limitations. Our experimental results are in good agreement with the theoretical predictions.

Introduction

The macro- and micro-temporal structure of SR FEL beam is well studied experimentally [1], [2], [3], [4], [5]. In this paper, we review the experimental results obtained with the OK-4/Duke storage ring FEL and compare them with our theoretical models and predictions. We use the self-consistent, 3D model of the interaction between the electron and the optical beams in the FEL for the description of the evolution of the system. The set of two 3D codes #felTD and #vuvFEL, based on the analytical 3D SR FEL theory [6], [7], was developed in 1994 [8]. This set turned out to be a universal tool for an accurate description of SR FEL dynamics in the linear regime, which is typical for SR FELs. We used it for simulation of the gain [9], of the giant pulse mode [10], as well as of the average lasing power in the OK-4/Duke storage ring [11]. The codes are very efficient, allowing 30,000 macro-particles and runs for hundreds and thousands of turns [8], [10].

In addition, this set of codes reveals complete dynamics of the FEL pulse and the phase-space distribution of the e-beam [9]. The laser light is represented by a wave-packet with slowly varying amplitude and phase (to be exact, the real and imaginary parts of the amplitude) evolving from turn to turn with all known effects taken into account. The incomplete list of effects on the FEL pulse includes the local complex gain with the slippage, the local interaction, the spontaneous radiation and the effect of the optical cavity. It is important to note that the codes are of multi-frequency with the bandwidth as large as 1.5% [9]. This set of programs provides both time and spectral information on the FEL pulse. In the present state, this set of codes cannot handle nonlinear effects in FEL, such as harmonics generation.

In this paper, we focus on the temporal structure of the FEL pulses operated in so-called “general lasing mode” (GLM), while information on the other modes of operation is published separately [10], [12]. First, we discuss the macro-temporal structure of the OK-4/Duke storage ring FEL and effects of the de-tuning. Second, we focus on details of the GLM micro-temporal structure and its specific features.

Section snippets

GLM

The de-tuning from exact synchronism, δ=cT₀–2L, is defined by the beam revolution frequency f₀=1/T₀ and the length of the optical cavity L. We will use a dimensionless de-tuning parameter Y defined as [7], [12] $Y=1+ δ Δg_{0}$ where Δ=(N_w+N_d)λ, λ is the FEL wavelength, N_w is the number of wiggler periods and N_d is dimensionless dispersion of its buncher [12], and g₀ is the peak FEL gain. The de-tuning from the synchronism strongly affects practically all parameters of an SR FEL from its power to its

Details of the temporal structure in general lasing mode

Fig. 4 shows three characteristic dual-scans of the temporal structures in GLM, similar to those observed at UVSOR [16], [17]. The slow, horizontal scans allow to see the FEL bunch microstructure evolving from turn to turn. The characteristic “stripes”, i.e. short wave-packets (SWP), are typical for dual-scan images for GLM SR FELs, where the SWP “slips” through the e-bunch, as described in the previous chapter. The lifetime of an SWP depends on the de-tuning, and can be estimated as $τ=T_{0} υ$ ,

Conclusions

The experimentally observed temporal structures in the OK-4/Duke storage ring FEL are in good agreement with existing theory and with the simulations. The set of codes, we have developed, provides all essential information on the temporal behavior of the OK-4 FEL beam. The additional information on the FEL and beam structures available from these codes complements our experimental results. We were pleased that the predictions of our codes are in excellent agreement with the real OK-4 FEL

Acknowledgements

The authors are grateful to the Office of Naval Research MFEL program for financial support. The authors would like to thank the staff, the engineers of the Duke FEL laboratory, and especially the accelerator operation group led by Dr. Ping Wang, for their help in these experiments.

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^☆: This work is supported by ONR Contract N00014-94-1-0818.

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Time structure of the OK-4/Duke storage ring FEL☆

Abstract

Introduction

Section snippets

GLM

Details of the temporal structure in general lasing mode

Conclusions

Acknowledgements

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Nucl. Instr. and Meth. A

Nucl. Instr. and Meth. A

Nucl. Instr. and Meth. A

Nucl. Instr. and Meth. A

Nuovo Cimento B