Search Results (4 total)

Thin carbon foils are used as strippers for charge exchange injection into high intensity proton rings. However, the stripping foils become radioactive and produce uncontrolled beam loss, which is one of the main factors limiting beam power in high intensity proton rings. Recently, we presented a scheme for laser stripping an H^- beam for the Spallation Neutron Source (SNS) ring. First, H^- atoms are converted to H⁰ by a magnetic field, then H⁰ atoms are excited from the ground state to the upper levels by a laser, and the excited states are converted to protons by a magnetic field. In this paper we report on the proof-of-principle demonstration of this scheme to give high efficiency (around 90%) conversion of H^- beam into protons at SNS in Oak Ridge. The experimental setup is described, and comparison of the experimental data with simulations is presented.

This paper presents a scheme for three-step laser-based stripping of an H^- beam for charge exchange injection into a high-intensity proton ring. First, H^- atoms are converted to H⁰ 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.

Three-wave nonlinear coupling of spatial Fourier modes is measured in the MST reversed field pinch by applying bispectral analysis to magnetic fluctuations measured at the plasma edge at 64 toroidal locations and 16 poloidal locations, permitting observation of coupling over 8 polodial modes and 32 toroidal modes. Comparison to bispectra predicted by resistive MHD computation indicates reasonably good agreement. However, during the crash phase of the sawtooth oscillation the nonlinear coupling is strongly enhanced, concomitant with a broadened (presumably nonlinearly generated) k spectrum.

Electrostatic fluctuations have been measured in a large reversed-field pinch, and are large (ñ_e/n∼20%–40%, T̃_e/T∼10%–25%). Frequency and wave-number spectra are broad (Δn∼70–150, Δm∼3–6), and differ from measured magnetic fluctuation spectra. The transport inferred from correlation measurements indicates that electrostatic fluctuations can account for significant particle losses, but contribute <15% to energy loss.