Radial focusing and energy compression of a laser-produced proton beam by a synchronous rf field

The dynamics of a MeV laser-produced proton beam affected by a radio frequency (rf) electric field has been studied. The proton beam was emitted normal to the rear surface of a thin polyimide target irradiated with an ultrashort pulsed laser with a power density of $4\times {10}^{18}\mathrm{W}/{\mathrm{cm}}^2$. The energy spread was compressed to less than 11% at the full width at half maximum (FWHM) by an rf field. Focusing and defocusing effects of the transverse direction were also observed. These effects were analyzed and reproduced by Monte Carlo simulations. The simulation results show that the transversely focused protons had a broad continuous spectrum, while the peaks in the proton spectrum were defocused. Based on this new information, we propose that elimination of the continuous energy component of laser-produced protons is possible by utilizing a focal length difference between the continuous spectral protons and the protons included in the spectral peak.

Figure 1

Schematic view of the experimental setup of phase rotation. The protons emitted normal to the rear surface of the tape target are injected into the rf cavity. The flight time of protons with and without phase rotation is measured by a TOF detector. The sweeping magnets are used to remove electrons so as to suppress the background in the scintillation counter of the TOF detector.

Figure 2

(a) TOF signal with and without the phase rotation. The horizontal axis is the arrival time of protons from the foil target to the TOF detector. Part (b) shows the TOF signal shot by shot. Although the number of protons is different shot by shot, the global structure of the distribution is reproduced precisely. (c) Energy spectra corresponding to the TOF spectra of (a).

Figure 3

Comparison of the transverse profile of the beam. (a)–(c) (upper pictures) are the profile detected in the experiment. (d)–(f) (lower pictures) are the profile obtained by Monte Carlo simulation with the same condition to the experiment. Parts (a), (b), (d), and (e) are distributions at 1.736 m downstream from the tape target the proton source. Parts (a) and (d) are the profile without phase rotation, (b) and (e) are with phase rotation. Parts (c) and (f) are measured at 2.365 m downstream from the tape target with phase rotation. In the experiment a mesh was inserted between the target and the rf cavity to measure the divergence. From (c), a diameter of the defocusing component is estimated to be about 40 mm at 2.365 m downstream from the target. The TOF detector is put at 2.383 m downstream from the target, and it has an aperture of 42 mm. Therefore, a significant part of the focused protons was detected by the TOF detector, even when the rf cavity was switched on in the present experiment.

Figure 4

(a) Simulated energy spectra of phase-rotated beam. The transverse profile is also simulated by the Monte Carlo method. Parts (b) and (c) show the simulated transverse profile of phase-rotated proton beam included in the valley of the energy spectrum (between 0.85 and 0.99 MeV) and that in the energy peak (between 0.99 and 1.11 MeV). Parts (b) and (c) are calculated at 2.365 m downstream of the tape target (the proton source) with the same condition as Fig. 3. Part (d) is a schematic illustration of the relation between the strength of the effects of energy compression and transverse focusing. The transverse axis is written with the velocity. (e) Setup of postprocessing used to remove protons with a continuous spectrum. If the phase-rotated beam passes through such a system, the energy spectrum is expected to change as shown in (a).