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Measuring the FEL gain with trajectory distortion Z. Huang, SLAC January 20, 2004

Measuring the FEL gain with trajectory distortion Z. Huang, SLAC January 20, 2004. Introduction. Stop the FEL amplification in the undulator and measure the radiation properties at the end (TTF results, PRL, 2001). Single Kick Error (SKE). Two effects of SKE (Tanaka et al., FEL2003):

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Measuring the FEL gain with trajectory distortion Z. Huang, SLAC January 20, 2004

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  1. Measuring the FEL gain with trajectory distortionZ. Huang, SLACJanuary 20, 2004

  2. Introduction • Stop the FEL amplification in the undulator and measure the radiation properties at the end (TTF results, PRL, 2001)

  3. Single Kick Error (SKE) • Two effects of SKE (Tanaka et al., FEL2003): reduction in coupling smearing of microbunching • Critical angle to destroy FEL interaction

  4. Trajectory distortion in LCLS • Offset the quad by Qx amount. If the focal length of the quad is F, then the kick angle is qQ = -Qx/F • Set qQ = qc, since F≈10m, Qx ≈ 60mm, so that the maximum betatron amplitude bxqQ≈ 2Qx

  5. GENESIS simulation results • Total power still grows due to interaction with higher-order modes & spontaneous radiation (div. angle up to 100 mrad) • On-axis power is dominated by the FEL fundamental mode only (~1 mrad) and stops growing when Qx~60 mm.

  6. Z-dependence • Initial stage: no microbunching to destroy, still some weak interaction + a lot of spontaneous • Exponential growth region, works reasonably well Qx = 60 mm Vary Qx

  7. Summary • It worked (as in TTF,…) • Trajectory distortion can stop further growth of FEL fundamental mode, but other higher-order modes (including spontaneous) increase the background • Collimation helps to pick up on-axis signal, or can we find the hot spot by data analysis? • Measurement results should be cross-checked with other methods, if possible

  8. Spontaneous v.s. FEL gain • Spontaneous fundamental radiation within 3% bandwidth reaches 60 MW, FEL signal can dominate after 40m Spontaneous fundamental in 3% BW (Z. Huang, K.-J. Kim FEL2002)

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