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Experimental Demonstration of Energy-Chirp Control in Relativistic Electron Bunches Using a Corrugat
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Experimental Demonstration of Energy-Chirp Control in Relativistic Electron Bunches Using a Corrugated Pipe

The linear accelerator driving the modern x-ray free-electron-lasers (FELs) requires precise control of the electron bunch phase space, including compression to high peak currents and providing a constant energy along the bunch’s length. The magnetic compression employed in such FELs typically leaves an undesired time-energy correlation in the bunch (an energy chirp), which can broaden the FEL bandwidth. While the chirp can be removed in the following linac section by a combination of rf phasing and the wakefields of the accelerating structures, this solution can be costly or impractical in nextgeneration FELs. For such cases, it was recently proposed to insert a relatively short, dedicated structure into the accelerator driver, one that can intentionally generate strong longitudinal wakefields that “dechirp” the beam. Such a passive wakefield device - a “dechirper” - can result in considerable cost savings and/or improved FEL performance. Note that in addition to the longitudinal wake that can correct a chirp, a dechirper can induce strong transverse wakes which, if not properly controlled, will increase the beam projected emittance and lead to deterioration of the FEL performance. Measurement of these wakefields is highly desirable prior to inclusion of a dechirper into a practical FEL design. It is the purpose of this Letter to report the first experimental study of a corrugated-wall dechirper, in particular, one with rectangular cross-sections and adjustable jaws.

The experiment was conducted at the PAL-ITF, an electron test accelerator at the Pohang Accelerator Laboratory in Pohang, Republic of Korea. The PAL-ITF beam line layout is shown in Fig. 1 and consists of an S-band (2856 MHz) rf photocathode gun (beam charge Q = 200 pC), two 3-m long S-band accelerating structures (beam energy, E0, of 70 MeV), a 1-m long S-band vertical rf deflector (0.5 MV), a horizontal bend (30 deg) with an electron spectrometer line, three quadrupole focusing magnets, and various beam screens. For this experiment, a 1-m long corrugated rectangular vacuum chamber was added following the second accelerating structure (L0b). The chamber support includes two separate motors allowing independent vertical positioning of each corrugated jaw, providing remote control of the full vertical gap (g) and its vertical offset. In the experiment, an aluminum structure of 1 m in length was used. The gap g was varied from g = 5 mm to g = 28 mm.

The beam line arrangement is well suited for measurement of the transverse wakefields as well. With the rf deflector switched off and L0a (Fig. 1) the rf phase adjusted to about 10 degrees off its accelerating crest, a strong linear energy chirp can be induced, converting the axis of screen 6 to time. This configuration provides a time-resolved measure of the vertical kick of a vertically offaxis beam in the gap. Figure 2 reports measured (top row) and 150-pC simulated (bottom row) beam images on screen 6. The 6-mm gap is vertically centered on the beam in the left two images, but the gap is 1 mm offset in the right two images. Note that the kick at the tail of the bunch is quite significant (the bunch head is on the left).

Fig. 1: Layout of the PAL-ITF Wakefield Experiment.

Fig. 2: Measurement of the Transverse Wakefields at Screen 6 (Top-row: Measured, Bottom-row: Simulated, Q = 150 pC).

These time-resolved measurements confirm both the linearity and the approximate scale of the wake-induced chirp of the corrugated chamber, as predicted by the Wakefield Model. We obtain reasonably good agreement between the measurement and the model in both the longitudinal and transverse planes. Our results provide confidence in the practicality of including a corrugatedwall dechirper in the design of the next-generation FELs.


P. Emma, M. Venturini, K. L. F. Bane, G. Stupakov, H.-s. Kang, M. S. Chae, J. Hong, C.-k. Min, H. Yang, T. Ha, W.W. Lee, C. D. Park, S. J. Park, And I. S. Ko, Prl 112, 034801 (2014).

AAPPS Bulletin        ISSN: 2309-4710
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