Wu, Y. K.

Paper	Title	Page
MOPPH068	Controlling Wiggler Harmonic Radiation to Reduce Damage to FEL Cavity Mirrors	135
	S. Huang, G. Swift, Y. K. Wu FEL/Duke University, Durham, North Carolina
	Wigglers emit both fundamental and harmonic radiation. For oscillator FELs, UV-VUV harmonic radiation can cause serious damage to the downstream mirror of the FEL cavity. Compared to a planar wiggler, harmonic radiation from a helical wiggler is peaked off-axis, and using a helical wiggler can significantly improve the FEL mirror lifetime. With a helical wiggler. However, the off-axis harmonic radiation from helical wigglers can still cause serious damage to the downstream mirror or limit the maximum beam current for FEL operation due to UV-VUV power loading on the mirror. At Duke FEL lab, we have developed a mechanism to control the off-axis harmonic radiation from the helical wigglers by using a set of motorized, water-cooled, in-vacuum apertures. These apertures can reduce the harmonic power load on the downstream mirror by one order of magnitude. With these apertures, operation with high electron beam currents becomes feasible for stable, high power UV-VUV lasing. In this work, we report the effectiveness of the apertures in reducing the wiggler harmonic radiation under various operation conditions and for different configurations of the Duke FELs.
MOPPH069	A Pass-by-Pass Gain Measurement Technique for Oscillator FELs	139
	S. F. Mikhailov, S. Huang, J. Li, V. Popov, Y. K. Wu FEL/Duke University, Durham, North Carolina
	We present a new pass-by-pass gain measurement technique for a storage ring FEL Typically, the FEL oscillator gain is obtained by measuring the growth of the envelope of an optical macropulse using a slow photo-detector. While successfully used for low-gain FEL operation at Duke FEL laboratory for many years, this technique does not provide the information on the optical power growth from pass to pass. In addition, this method was not adequate for measuring higher gains of distributed optical klystron FELs. We have developed a new gain measurement technique based upon the direct measurement of the optical energy in micropulses. Using fast photo-detectors, the growth of an FEL macro-pulse can be recorded from pass to pass. This new gain measurement technique provides a powerful tool to study the details of the FEL gain process, including the onset of the FEL lasing. In the work, we describe this new gain measurement technique in detail and compare it with the old technique. Using fast photo-detectors with a sub-nanosecond time response, this new technique can be adopted for many oscillator FELs, including those driven by super-conducting linacs.
TUCAU04	Second Harmonic Lasing with Storage Ring Based FELs	459
	Y. K. Wu, J. Li, S. F. Mikhailov, V. Popov FEL/Duke University, Durham, North Carolina S. V. Benson, G. Neil JLAB, Newport News, Virginia
	The study of forbidden processes in many types of physical systems is critical for understanding the underlying symmetry breaking. The FEL second harmonic lasing of provides a unique opportunity to study the "forbidden" FEL gain mechanisms which are otherwise not allowed under normal operation conditions of an FEL. Because of its very low gain, the sole study of second harmonic lasing in the optical region was reported by the JLab using its high-gain IR FEL (PRL, 084801, 2001). This work reports the first second harmonic lasing results at Duke University with the storage ring based optical klystron and distributed optical klystron FELs. Several different mechanisms have been proposed for the second harmonic lasing, including relative misalignments between electron and optical beams, transverse field gradients, and longitudinal coupling (NIM A483, p. 527, 2002). Different gain mechanisms can also lead to preferred polarization states. In order to understand and distinguish various gain mechanisms, our work focuses on measurements of the gain and polarization of the second harmonic lasing under various optical and electron beam conditions and for a variety of FEL configurations.
	Slides