Precision Control of the Electron Longitudinal Bunch Shape Using an Emittance-Exchange Beam Line

We report on the experimental generation of relativistic electron bunches with a tunable longitudinal bunch shape. A longitudinal bunch-shaping (LBS) beam line, consisting of a transverse mask followed by a transverse-to-longitudinal emittance exchange (EEX) beam line, is used to tailor the longitudinal bunch shape (or current profile) of the electron bunch. The mask shapes the bunch’s horizontal profile, and the EEX beam line converts it to a corresponding longitudinal profile. The Argonne wakefield accelerator rf photoinjector delivers electron bunches into a LBS beam line to generate a variety of longitudinal bunch shapes. The quality of the longitudinal bunch shape is limited by various perturbations in the exchange process. We develop a simple method, based on the incident slope of the bunch, to significantly suppress the perturbations.

Figure 1

Schematic diagram of the experimental beam. The acronyms Q, B, TDC, and SPE stand for the quadrupole magnet, dipole magnet, transverse deflecting cavity, and spectrometer, respectively. Dipole magnets and TDC1 bend or kick the beam horizontally, and TDC2 kicks the beam vertically.

Figure 2

Electron beam images at YAG1 with different masks (a)–(e) and corresponding beam images at YAG3 with TDC2 on. The $x$ axis of images (f)–(j) is rescaled based on the TDC2 calibration. The red traces are the corresponding projections.

Figure 3

Horizontal density profiles measured at YAG1 with the first EEX dipole off (blue curve) and longitudinal density profiles measured at YAG3 with TDC2 on (red curve). The base lines for horizontal and vertical slopes were zero, and the linac phase was $-15°$. Each setting was fixed to the base line while one of them was scanned. Horizontal slopes of the incoming electron beam were 0.0, 0.2, 0.4, and $0.6{\mathrm{m}}^{-1}$ for (a)–(d). Vertical slopes were $-0.6$, 0.0, 0.2, and $0.6{\mathrm{m}}^{-1}$ for (e)–(h). The linac phase which controls the longitudinal chirp was $-10°$, $-15°$, $-25°$, and $-40°$ for (i)–(l). Here the tail of the profile indicates the peak to the left end of the profile.