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Future high-power (10 - 20 MW) hadron accelerators offer many unique opportunities for fundamental science and nuclear applications. With the completion of Proton Power Upgrade (PPU) at 2.8 MW [1] and European Spallation Source (ESS) at 5 MW [2], the attention will be focused on the next high-power machine in hadron accelerator community. Charge exchange injection into a synchrotron using diamond stripper foils offers great advantage over direct proton acceleration. However, there are clear limitations associated with foil sublimation above critical beam power density and beam loss due to scattering (~10 rem/hour at 10 MW for SNS)., white paper

A new laser stripping scheme for charge exchange injection of H⁻ beam is considered. The sequential scheme for the planned demonstration experiment includes two step excitation that requires much smaller laser power compared to the traditional 1-step excitation. The new scheme can be applied to a wider range of H⁻ beam energies and provides more flexibility on the choice of laser frequency. In this paper we discuss the two-step excitation method and estimate laser stripping parameters and stripping efficiency for the SNS accelerator and its future H⁻ energy upgrade to 1.3 GeV., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

Resonant laser excitation of the electron in a hydrogen atom is essential to achieve high-efficiency laser stripping of hydrogen ion (H⁻) beam. In the laser stripping experiments recently carried out at SNS, an ultra-violet (UV) laser was used to excite the electrons in 1-GeV hydrogen atoms from the n=1 state to the n=3 state. In this talk, we propose a sequential resonant excitation scheme by using two laser beams to excite electrons in a sequence of two steps: from the n=1 state to the n=2 state and from the n=2 state to any higher state. The advantages of the sequential resonant excitation scheme include (1) lower laser power requirement due to higher transition probability in the first excitation step and (2) possibility of shifting the stripping laser wavelength from UV regime to longer wavelengths. An application of the sequential resonant excitation scheme in combination with the double-resonance optical cavity technology to the laser stripping of 1.3-GeV H⁻ beam (envisioned in the SNS proton power upgrade project) will be described., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Lasers have recently been used in many applications to H⁻ beams, including laser charge exchange, laser wire scanners, and laser temporal pulse patterning. The H⁻ beam in these applications has wide variation ofμpulse length width dependence on focusing of the RF cavities, energy spread of the beam, and space charge forces. Achieving the required laser pulse length for complete overlap with the H⁻ can be challenging in some scenarios when available laser power constrained. The scheme proposed here utilizes a crab-crossing concept between the laser and the ion beam to achieve overlap of a short laser pulse with an arbitrarily long H⁻ beam pulse. An experiment to test the hypothesis in the context of H⁻ charge exchange is described., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

In recent years lasers have been playing a vital role in many H- beam measurements and experiments. This talk will review current state of development of various applications using lasers for manipulating H- ion beams in accelerators. A wide range of applications will be reviewed such as beam diagnostics, laser-assisted charge-exchange injection, generation of arbitrary H⁰ pulse patterns and others. An overview of ongoing developments and prospects for other laser H- beam interactions will also be given., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

The SNS uses charge exchange injection to minimize losses during the accumulation of the accelerated beam in the ring. A stripper foil implements this by removing the electrons from the high intensity H⁻ beam coming from the linac. At a beam power of 1.2 MW, the foil lasts for many weeks, sometimes months. However, given the upgrade to 2.8 MW, it is important to know the current temperature of stripper foil in order to estimate its lifetime for the new beam power and beam size. In this paper, we discuss several methods to measure the temperature of stripper foil exposed to current operating conditions of the SNS accelerator. Given the high radiation in the vicinity of the foil, the uncertainty in the foil's emissivity, and available resources, we chose a two-wavelength pyrometer that is located 40 m from the foil. The pyrometer is composed of two mirrors, a refracting telescope, and two photodiodes. We present the calibration data and the temporally resolved measurements made with this pyrometer., Proceedings of the 61\textsuperscript{st} ICFA ABDW on High-Intensity and High-Brightness Hadron Beams, HB2018, Daejeon, Korea

Recent success of laser-assisted charge exchange for 10 μs duration Hydrogen ion beams at SNS motivates laser development necessary for efficient stripping of 1.0 ms duration beam at full duty cycle. To overcome the laser power challenge, the interaction point was chosen inside an optical cavity. A doubly-resonant enhancement cavity and a novel locking technique have been developed, and a coherent enhancement of 402.5 MHz, 50 ps, 1.05 MW peak power ultraviolet (355 nm) laser pulses operating at 10-μs/10-Hz burst mode has been demonstrated. This will enable 1.0 ms duration laser macropulses at 60 Hz to be stored inside such a cavity to achieve efficient stripping at SNS., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

The experimental development of H⁻ laser assisted charge exchange, a.k.a. laser stripping, has been ongoing at the SNS accelerator since 2006 in a three-phase approach. The first two phases associated with proof-of-principle and proof-of-practicality experiments have been successfully completed and demonstrated >95% H⁻ stripping efficiency for up to 10 us. The final phase is a proof-of-scalability stage to demonstrate that the method can be deployed for realistic beam duty factors. The experimental component of this effort is centered on achieving high efficiency stripping through the use of a laser power amplification scheme to recycle the macropulse laser light at the interaction point of the H⁻ stripping. Such a recycling cavity will be necessary for any future operational laser stripping system with at least millisecond duration H⁻ pulses. A second component of the proof-of-scalability phase is to develop a conceptual design for a realistic laser stripping scheme. The status of these efforts and challenges associated with deploying the recycling cavity into the laser stripping experiment will be described in this talk., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

Laser-assisted stripping is a novel approach to H- charge exchange that overcomes long-standing limitations associated with the traditional, foil-based method of producing high-intensity, time-structured beams of protons. This paper reports on the first successful demonstration of the laser stripping technique for microsecond duration beams. The experiment represents a factor of 1000 increase in the stripped pulse duration compared with the previous proof-of-principle demonstration. The central theme of the experiment is the implementation of methods to reduce the required average laser power such that high efficiency stripping can be accomplished for microsecond duration beams using conventional laser technology. The experiment was performed on the Spallation Neutron Source 1 GeV H- beam using a 1 MW peak power UV laser and resulted in ~95% stripping efficiency., Comment: 17 pp

Recently, a high-efficiency laser assisted hydrogen ion (H⁻) beam stripping was successfully carried out in the Spallation Neutron Source (SNS) accelerator. The experiment was not only an important step toward foil-less H⁻ stripping for charge exchange injection, it also served as a first example of using megawatt ultraviolet (UV) laser in an operational high power proton accelerator facility. This talk reports the design, implementation, and commissioning results of the macropulse laser system, laser transport line, and laser operation for the laser stripping experiment. The macropulse laser consists of a mode-locked picosecond pulsed seed laser and a burst-mode Nd:YAG laser amplifier. The general design concept can be adapted to any temporal beam structures in most accelerators. We have achieved UV pulses with the pulse widths varying between 34 to 54 ps and a maximum peak power over 3.5 MW. A laser transport line is installed to deliver the UV beam to the laser stripping chamber at a transmission efficiency of 70%. Laser operation including remote control and monitor of laser parameters will be described., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

The laser assisted H⁻ charge exchange concept is under development at the Spallation Neutron Source (SNS) as on option for replacing traditional carbon-based foil technology in future accelerators. A laser based stripping system has the potential to alleviate limiting issues with foil technology, paving the way for accumulation of much higher density proton beams. This paper discusses the advantages and limitations of a laser-based stripping system compared with traditional foil-based charge exchange systems for various beam accumulation scenarios, scaling from SNS experience with high power beam injection and calculations of laser stripping parameters. In addition, preparations for an experimental demonstration of laser assisted stripping for microsecond long 1 GeV, H⁻ beams are described., Proceedings of the Workshop on Beam Cooling and Related Topics, COOL2015, Newport News, VA, USA

The Spallation Neutron Source (SNS) accelerator complex utilizes charge-exchange injection to stack a high-intensity proton beam in the accumulator ring for short-pulse neutron production. A foil-less charge exchange injection method was researched at SNS by using a laser assisted H⁻ beam stripping scheme. Following a proof-of-principle experiment using a Q-switched laser, a new experiment is being prepared to demonstrate laser stripping over a 10-us macropulse. In this talk, we will report the design and measurement results of the laser system for the next stage laser stripping experiment. The laser system adopts a master oscillator power amplifier (MOPA) configuration and contains an actively mode-locked fiber seeder, macropulse generator, multiple-stage Nd:YAG amplifiers, harmonic converters, and control electronics. The laser system generates 50 ps/402.5 MHz pulses (at a macropulse mode) with multiple megawatt peak power at a wavelength of 355 nm. The measurement results of laser pulse width, spectrum, spatial/temporal beam quality and their parameter dependence will be described., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

We studied simultaneously the 4He(e,e'p), 4He(e,e'pp), and 4He(e,e'pn) reactions at Q^2=2 [GeV/c]2 and x_B>1, for a (e,e'p) missing-momentum range of 400 to 830 MeV/c. The knocked-out proton was detected in coincidence with a proton or neutron recoiling almost back to back to the missing momentum, leaving the residual A=2 system at low excitation energy. These data were used to identify two-nucleon short-range correlated pairs and to deduce their isospin structure as a function of missing momentum in a region where the nucleon-nucleon force is expected to change from predominantly tensor to repulsive. Neutron-proton pairs dominate the high-momentum tail of the nucleon momentum distributions, but their abundance is reduced as the nucleon momentum increases beyond ~500 MeV/c. The extracted fraction of proton-proton pairs is small and almost independent of the missing momentum in the range we studied. Our data are compared with ab-initio calculations of two-nucleon momentum distributions in 4He., 6 pages, 2 figures

The concept of laser-assisted H⁻ stripping, originated over three decades ago, was successfully demonstrated for a 6 ns, 900 MeV H⁻ beam in 2006. Plans are underway to build on this foundation by performing laser-assisted H⁻ stripping of a 10 μs, 1 GeV H⁻ beam at the Spallation Neutron Source facility; this constitutes a three orders of magnitude improvement over the initial proof of principle demonstration. The central theme of the experiment is the reduction of the required laser power through ion beam manipulations and laser-ion beam temporal matching. This paper discusses the configuration of the experiment, the current and anticipated challenges, and the schedule., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

A number of laser-particle interaction experiments such as the laser assisted hydrogen ion beam stripping or X-/γ-ray generations via inverse-Compton scattering involve light sources operating in a burst mode to match the temporal structure of the particle beam. As the small cross-section in the laser-particle interaction process results in negligible laser power loss, it is desirable to make the interaction inside an optical cavity to recycle the laser power. In many cases, conventional cavity locking techniques will not work since the burst normally has very small duty factor and low repetition rate and it is impossible to generate an effective control signal. In this talk, we report on the development of a doubly-resonant optical cavity scheme and its locking technique that enables a simultaneous resonance of two laser beams with different spectra and/or temporal structures. We demonstrate that such a cavity can be used to recycle burst-mode ultra-violet laser pulses with arbitrary burst lengths and repetition rates. System implementation, technical challenges, experimental results and applications will be described., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

Measurement of stresses and strains in the mercury target vessel of the Spallation Neutron Source (SNS) is important to understand the structural dynamics of the target. Owing to their compactness, easy system integration, and invulnerability to the electromagnetic interference, fiber optic strain sensors have been installed into the SNS target module starting from the fall of 2015. In this talk, we report on the development of radiation-resistant fiber optic strain sensors for subsequent generations of SNS target instrumentation. The sensors are extrinsic Fabry-Perot interferometers (EFPIs) made from fluorine-doped single-mode fibers. The radiation induced loss of the fiber has been measured in the SNS target 13 at the energy-on-target level exceeding 500 MWhr which results in peak doses on fiber of more than 10⁹ Gy. A superluminescent diode laser at 1300 nm is used as the light source and the strain is measured in real-time using quadrature phase shifted signals generated from a local interferometer. We have demonstrated successful measurements of strains from 1 to 1000 με at a kHz frequency range on a test plate using the developed interrogation optical system., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea