Three-step ${\mathrm{H}}^{-}$ charge exchange injection with a narrow-band laser

This paper presents a scheme for three-step laser-based stripping of an ${\mathrm{H}}^{-}$ beam for charge exchange injection into a high-intensity proton ring. First, ${\mathrm{H}}^{-}$ atoms are converted to ${\mathrm{H}}^0$ by Lorentz stripping in a strong magnetic field, then neutral hydrogen atoms are excited from the ground state to upper levels by a laser, and the remaining electron, now more weakly bound, is stripped in a strong magnetic field. The energy spread of the beam particles gives rise to a Doppler broadened absorption linewidth, which makes for an inefficient population of the upper state by a narrow-band laser. We propose to overcome this limitation with a “frequency sweeping” arrangement, which populates the upper state with almost 100% efficiency. We present estimates of peak laser power and describe a method to reduce the power by tailoring the dispersion function at the laser-particle beam interaction point. We present a scheme for reducing the average power requirements by using an optical ring resonator. Finally, we discuss an experimental setup to demonstrate this approach in a proof-of-principle experiment.

Figure 1

Angular spread vs field for 1 GeV (solid lines), 1.3 GeV (dashed lines), and 1.5 GeV (dash-dotted lines) ${\mathrm{H}}^{-}$ beam energies for the magnet gap 2 cm (blue lines) and 4 cm (green lines).

Figure 2

Experimental setup for laser excitation of the hydrogen beam (top view). Total length of the region is about 60 cm.

Figure 3

Interaction region setup for the elimination of the Doppler broadening. Total length of the region is about 60 cm.

Figure 4

Schematic of ring resonator for trapping of optical pulse.

Figure 5

Horizontal and vertical rms beam sizes in the linac dump line. The proposed interaction point is indicated.

Figure 6

Layout of accumulator ring injection region incorporating laser stripping. IC1–IC4 show injection chicane magnets.

Figure 7

Coordinate notation for the hydrogen atom in the field of the Gaussian beam.

Figure 8

Probability of the $n=3$ excitation versus time for reference energy particle (black line), the particle with the relative energy deviation 0.000 25 (blue line), and 0.000 75 (green line).