A Low Time-Dispersion Refractive Optical Transmission Line for Streak Camera Measurements

Streak camera measurements of the electron bunch length are limited in resolution by several factors. These include: (1) the light source itself (e.g. OTR, Cherenkov, Synchrotron radiation), (2) dispersion from the refractive optical transport line (OTL) between the light source and the streak camera, and (3) the streak camera itself. The limiting resolution for pulses length of a few psec is caused by the OTL due to the very broad bandwidth of the light pulse (hundreds of nm). While an all-reflective OTL can eliminate dispersion, the system is complicated and expensive. In this paper, we examine the time spread of the light pulse due to the dispersion in the lenses of the OTL. We present an analysis of the dispersion and benchmark it to measurements of the time dispersion for several different lens materials. Finally, we conclude with recommendations of the proper design of the refractive OTL to minimize the time dispersion while maximizing the signal. STREAK CAMERA MEASURMENTS OF THE ELECTRON BUNCH LENGTH Streak camera measurements of the electron (in general, charged particle) bunch length are well suited for the regime of greater than a few picoseconds. Our examples are based on a Hamamatsu [1] C1587 streak camera with a resolution of 2ps since this is the camera we own at the Argonne Wakefield Accelerator (AWA) Facility. Note that there exist streak cameras with significantly better resolution such as the Hamamatsu Fresca-200 with a resolution of 200 femtoseconds. In any case, the examples here can be extended to any particular case as long as one requires that the dispersion introduced by the optical transport line is less than one third of the minimum bunch length one desires to measure since this only introduces an error of less than ~5% (√ + .33 = . 5). A typical setup for the measurement of the electron bunch length is shown in Fig. 1. (1) The process begins when the electron bunch passes through the radiator (e.g. OTR) and generates a prompt, broadband, light pulse, of similar transverse and longitudinal profile to the electron bunch. (2) To avoid damage to the streak camera it is typically located far from the radiator and therefore the light pulse is often transported through a long, refractive, optical transport line (OTL) made up of a series of glass lenses [2]. Other solutions are possible, such as an all reflective OTL or to locate the streak camera near the radiator with adequate shielding, but we only consider the case of a long, refractive OTL in this paper. (For the remainder of the paper, we use the shorter phrase "OTL" to refer to "a long, refractive OTL".) The problem with the OTL is that light pulse length increases as it travels through the OTL due to the variation of the group velocity with frequency (a.k.a. group velocity dispersion) in the glass (pulse at bottom of Fig. 1). This means that by the time the light pulse arrives at the streak camera its length is significantly longer than the electron's bunch length. A bandwidth filter of ~10nm is typically used to reduce the lengthening to an acceptable level. (3) Finally, the light pulse arrives at the streak camera for measurement. Proper use of the streak camera is beyond the scope of the paper so we limit comments to bandwidth requirements of the streak camera since it has direct bearing on the subject of this paper. The bandwidth of the streak camera is determined by the input optics and the photocathode quantum efficiency. The AWA streak camera uses the N1643 streak tube with bandwidth of [200-850nm] and we used this for the examples in this paper. REFRACTIVE AND GROUP INDEX Dispersive media are characterized by a wavelength (or frequency) dependent susceptibility (), refractive index � � , and group index �� � [3]. The refractive index of Figure 1: Setup for a Steak Camera based bunch length measurement. ___________________________________________