Typical back scattering geometry between an electron bunch of longitudinal size σL and transversal size σR, moving at relativistic speed from left to right colliding with a laser pulse of waist size w0 and time duration T.
Optimization of the beam phase space density versus charge for different frequency bands, computed using the optimization code GIOTTO. 14,15 In the legend, S-120, C-170, C-200, C240, X-200 give the RF injector operation value that has been tested (S,C,X bands) and the relative Gun Ez filed peak; the red point on the S-band curve is the LCLS measured value. 16,17
Energy spread versus bunch length for different RF frequencies: red solid line for the S-band, blue dashed line for C-band, and dashed and dotted black line for X-band. The ELI-NP energy spread maximum threshold is 0.1% and 0.05%; safe values are reported in black dashed lines.
Photocathode laser pulse shape used in beam dynamic simulations. (a) Reference working point, Q = 250 pC. (b) Commissioning working point, Q = 25 pC.
TSTEP output for the reference working point, Q = 250 pC. (a) Evolution of emittance, transverse and longitudinal envelopes in the S-band photo-injector. (b) Transverse (top) and longitudinal (bottom) phase space at the photo-injector exit.
TSTEP output for the commissioning beam, Q = 25 pC.Operation mode: no RF compression (a) Evolution of emittance, transverse and longitudinal envelopes in the S-band photo-injector. (b) Transverse (top) and longitudinal (bottom) phase space at the photo-injector exit.
Initial transverse distributions used into computations. On the top the uniform one, on the bottom the truncated Gaussian one, with σx = 0.4 mm. On the left the spot sizes, on the right horizontal and vertical projections.
Q = 250 pC. Left: computed emittance evolution in the ELI S-band photo-injector for a uniform and a truncated Gaussian with σx = 0.4 mm. Right: computed output phase space for the truncated Gaussian distribution.
Q = 390 pC and initial Gaussian truncated distribution with σx = 0.4 mm. Left: emittance and envelopes evolution in the S-band photo-injector. Right: Output computed phase spaces.
RF Linac schematic layout from the photo-injector exit down the two interaction points.
Twiss parameters of the low energy beamline from the photo-injector exit down to the low energy interaction point.
Twiss parameters of the high energy beamline from the photo-injector exit down to the high energy interaction point.
Pill box cavity model considered for the wake-field calculations: a is the iris radius, L is the cell length, and b and g are the cavity radius and length, respectively.
Longitudinal and transverse short range wake-fields curve integrated over one cell for the S-band accelerating structure (red curve) and for the C-band structure (blue curve).
Transverse beam size and distribution plus the longitudinal one for the reference working point electron beam at the low (left) and high (right) energy interaction point.
Beam energy spread, beam energy and beam current distribution of the reference working point at low energy IP (above), and at high energy IP (bottom).
Input power profiles to compensate the beam loading in the S-band (a) and C-band (b) structures.
Energy spread induced by beam loading in the S-band (a) and C-band structures (b) with and without compensation (q = 250 pC, 40 bunches, and ΔT = 15 ns).
Mechanical drawing of the C-band damped structure accelerating cells.
Summary of gamma-ray beam specifications.
Laser beam parameters.
Electron beam parameters, at the C-band booster injection, for the S-band photo-injector.
Electron beam parameters at the low and high energy interaction point.
Parameters of the TW accelerating structures used in the beam loading calculation.
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