[Paper Review] Multimegawatt DAE$\delta$ALUS Cyclotrons for Neutrino Physics

M. Abs, A. Calanna+19 more|arXiv (Cornell University)|Jul 22, 2012

Particle accelerators and beam dynamics46 references13 citations

TL;DR

This paper proposes a multimegawatt DAE𝛿ALUS cyclotron system using H₂⁺ ions to produce high-intensity, decay-at-rest neutrino beams for CP violation studies in the neutrino sector. By leveraging advanced beam dynamics simulations with 3D space charge and optimized stripping processes, the design achieves beam halo control within a 10⁻⁴ loss budget, demonstrating feasibility for 800 MeV, multimegawatt proton beams essential for next-generation neutrino experiments.

ABSTRACT

DAE$\\delta$ALUS (Decay-At-rest Experiment for $\\delta_{CP}$ studies At the Laboratory for Underground Science) provides a new approach to the search for CP violation in the neutrino sector. High-power continuous-wave proton cyclotrons efficiently provide the necessary proton beams with an energy of up to 800 MeV to create neutrinos from pion and muon decay-at-rest. The experiment searches for $\\bar{\ u}_{\\mu} \ ightarrow \\bar{\ u}_e$ at short baselines corresponding to the atmospheric $\\Delta m^2$ region. The $\\bar{\ u}_e$ will be detected via inverse beta decay. Thus, the cyclotrons will be employed at a future ultra-large gadolinium-doped water or scintillator detector. In this paper we address the most challenging questions regarding a cyclotron-based high-power proton driver in the megawatt range with a kinetic energy of 800 MeV. Aspects of important subsystems like the ion source and injection chain, the magnet design and radio frequency system will be addressed. Precise beam dynamics simulations, including space charge and the $\ ext{H}_2^+$ stripping process, are the base for the characterization and quantification of the beam halo -- one of the most limiting processes in high-power particle accelerators.

Motivation & Objective

• To design a high-power cyclotron system capable of delivering 0.8–4.8 MW proton beams at 800 MeV for decay-at-rest neutrino experiments.
• To address the challenge of beam loss and halo formation in multimegawatt cyclotrons, a key limitation in high-power accelerators.
• To identify and optimize H₂⁺ as the preferred ion species over protons due to superior beam dynamics and higher achievable current.
• To ensure feasibility of the accelerator complex by addressing critical subsystems: ion source, injection chain, magnet, RF system, and beam extraction.

Proposed method

• The design employs H₂⁺ ions to enable higher beam currents than protons, leveraging the reduced space charge effects at injection.
• 3D beam dynamics simulations including space charge and H₂⁺ stripping processes are used to model and quantify beam halo formation.
• The cyclotron uses a dual-injected configuration (DIC and DSRC) with a controlled beam loss budget of 10⁻⁴ to minimize activation and component damage.
• RF systems are based on enhanced versions of PSI’s existing cavities, with initial calculations confirming feasibility for 20% duty cycle operation.
• The beam extraction system is designed to match the stripped proton beam to the downstream transport channel with minimal emittance growth.
• A two-phase R&D program is proposed: first to demonstrate inflection and capture into the DIC, second to study and suppress vibrational states in H₂⁺ ions that increase losses.

Experimental results

Research questions

• RQ1 Can H₂⁺ ions be used effectively in a multimegawatt cyclotron to achieve the required beam current and power while maintaining beam loss below 10⁻⁴?
• RQ2 How can 3D space charge effects and the H₂⁺ stripping process be accurately modeled to predict and control beam halo formation?
• RQ3 What are the critical technical risks in maintaining beam quality at multimegawatt levels, and how can they be mitigated through subsystem design and R&D?
• RQ4 Is the proposed RF system, based on upgraded PSI cavities, capable of delivering the required power and pulse structure (20% duty cycle) for sustained operation?
• RQ5 Can the beam dynamics of H₂⁺ ions, particularly their vibrational state distribution, be controlled to reduce losses and improve transmission efficiency?

Key findings

• H₂⁺ ions are shown to enable significantly higher beam currents than protons due to favorable beam dynamics and reduced space charge effects at injection.
• 3D beam dynamics simulations confirm that beam halo losses can be controlled within the required 10⁻⁴ loss budget, even at multimegawatt levels.
• The stripping process for H₂⁺ to produce protons is quantified with initial vacuum and dissociation cross-section calculations within expected ranges, though further research is needed.
• The RF system design, based on enhanced PSI cavities, is found to be feasible for delivering the required 20% duty cycle and high RF power.
• The DSRC and DIC beam extraction system is shown to provide a well-matched, low-emittance proton beam suitable for downstream transport.
• A two-phase R&D program is established to optimize source performance and suppress weakly bound vibrational states in H₂⁺, directly addressing the most critical technical risk.