Generation and Characterization of Electron Bunches with Ramped Current Profiles in a Dual-Frequency Superconducting Linear Accelerator

We report on the successful experimental generation of electron bunches with ramped current profiles. The technique relies on impressing nonlinear correlations in the longitudinal phase space using a superconducing radio frequency linear accelerator operating at two frequencies and a current-enhancing dispersive section. The produced $\sim 700\mathrm{\text{−}}\mathrm{MeV}$ bunches have peak currents of the order of a kilo-Ampère. Data taken for various accelerator settings demonstrate the versatility of the method and, in particular, its ability to produce current profiles that have a quasilinear dependency on the longitudinal (temporal) coordinate. The measured bunch parameters are shown, via numerical simulations, to produce gigavolt-per-meter peak accelerating electric fields with transformer ratios larger than 2 in dielectric-lined waveguides.

Figure 1

Analytically computed current profiles for several values of $b_f$ for fixed $a_f=2.5$ (a) and for several values of $a_f$ with $b_f=0.7$ (b) The numbers in (a) [(b)] are the values of $b_f$ [$a_f$]; for all the cases ${\sigma }_u=0.05$.

Figure 2

Diagram of the FLASH facility. Only components affecting the longitudinal phase space beam (LPS) dynamics of the bunches are shown. The acronyms ACC, BC, and DBL stand, respectively, for accelerating modules, bunch compressors, and dogleg beam line (the blue rectangles represent dipole magnets). The transverse deflecting structure (TDS), spectrometer and Ce:YAG screen compose the LPS diagnostics.

Figure 3

Simulated LPS distribution [(a) and (b)] with associated current profile downstream of BC1 (solid blue trace) and DBL (dash red trace) [(c) and (d)]. The set of plots [(a),(c)] and [(b),(d)] correspond to different ($V_{1,3}$, ${\phi }_{1,3}$) settings.

Figure 4

Snapshots of the measured longitudinal phase spaces (left column) and associated current profiles (right column) for different settings of the ACC1 and ACC39 accelerating modules. The values ($V_1$, ${\phi }_1$, $V_3$, ${\phi }_3$) [in (MV,°; MV,°)] are: (150.5, 6.1; 20.7, 3.8), (156.7, 3.8; 20.8, 168.2), (155.6, 3.6; 20.6, 166.7), and (156.8, 4.3; 20.7, 167.7) for, respectively, case (a),(b),(c), and (d).

Figure 5

Cylindrical-symmetric dielectric-loaded waveguide considered (a) and axial wakefield produced by the current profile shown in Fig. 4c for $(a,b)=(20,60)\mu \mathrm{m}$.

Figure 6

Simulated transformer ratio versus peak accelerating field (a) for the four measured current profiles (displayed as different colors with label corresponding to cases shown in Fig. 4). Transformer ratio (false color map) as a function of the DLW inner radius $a$ and dielectric layer thickness $b-a$ with corresponding $|E^{+}|$ shown as isoclines with values quoted in $\mathrm{MV}/\mathrm{m}$ for case (c) of Fig. 4.