M. Popovic

Accelerator design concept for future neutrino facilities

This document summarizes the findings of the Accelerator Working Group (AWG) of the International Scoping Study (ISS) of a Future Neutrino Factory and Superbeam Facility. The work of the group took place at three plenary meetings along with three workshops, and an oral summary report was presented at the NuFact06 workshop held at UC-Irvine in August, 2006. The goal was to reach consensus on a baseline design for a Neutrino Factory complex. One aspect of this endeavor was to examine critically the advantages and disadvantages of the various Neutrino Factory schemes that have been proposed in recent years.This document summarizes the findings of the Accelerator Working Group (AWG) of the International Scoping Study (ISS) of a Future Neutrino Factory and super-beam Facility. The work of the group took place at three plenary meetings along with three workshops, and an oral summary report was presented at the NuFact06 workshop held at UC-Irvine in August, 2006. The goal was to reach consensus on a baseline design for a Neutrino Factory complex. One aspect of this endeavor was to examine critically the advantages and disadvantages of the various Neutrino Factory schemes that have been proposed in recent years.

Gaseous Hydrogen and Muon Accelerators

Ionization cooling, a method for shrinking the size of a particle beam, is an essential technique for future particle accelerators that use muons. In this technique, muons lose energy in all three directions by passing through an absorber while only the longitudinal energy is regenerated by RF cavities. Thus the beam phase space area decreases down to the limit of multiple scattering in the energy absorber. Hydrogen is the material of choice for ionization cooling because of its long radiation length relative to its energy loss. In the application discussed here, dense gaseous hydrogen also suppresses RF breakdown by virtue of the Paschen effect, thereby allowing higher accelerating gradients and a shorter and less‐expensive cooling channel. As described in this paper, a channel of RF cavities pressurized with about 3 tons of cold hydrogen gas could provide transverse muon cooling for a Muon Collider or Neutrino Factory. The present status of this research effort and several issues related to the use of h...

nuSTORM - Neutrinos from STORed Muons: Letter of Intent to the Fermilab Physics Advisory Committee

The idea of using a muon storage ring to produce a high-energy ({approx_equal} 50 GeV) neutrino beam for experiments was first discussed by Koshkarev in 1974. A detailed description of a muon storage ring for neutrino oscillation experiments was first produced by Neuffer in 1980. In his paper, Neuffer studied muon decay rings with E{sub {mu}} of 8, 4.5 and 1.5 GeV. With his 4.5 GeV ring design, he achieved a figure of merit of {approx_equal} 6 x 10{sup 9} useful neutrinos per 3 x 10{sup 13} protons on target. The facility we describe here ({nu}STORM) is essentially the same facility proposed in 1980 and would utilize a 3-4 GeV/c muon storage ring to study eV-scale oscillation physics and, in addition, could add significantly to our understanding of {nu}{sub e} and {nu}{sub {mu}} cross sections. In particular the facility can: (1) address the large {Delta}m{sup 2} oscillation regime and make a major contribution to the study of sterile neutrinos, (2) make precision {nu}{sub e} and {bar {nu}}{sub e} cross-section measurements, (3) provide a technology ({mu} decay ring) test demonstration and {mu} beam diagnostics test bed, and (4) provide a precisely understood {nu} beam for detector studies. The facility is the simplest implementation of the Neutrino Factory concept. In our case, 60 GeV/c protons are used to produce pions off a conventional solid target. The pions are collected with a focusing device (horn or lithium lens) and are then transported to, and injected into, a storage ring. The pions that decay in the first straight of the ring can yield a muon that is captured in the ring. The circulating muons then subsequently decay into electrons and neutrinos. We are starting with a storage ring design that is optimized for 3.8 GeV/c muon momentum. This momentum was selected to maximize the physics reach for both oscillation and the cross section physics. See Fig. 1 for a schematic of the facility.

STUDIES OF BREAKDOWN IN A PRESSURIZED RF CAVITY

Microscopic images of the surfaces of metallic electrodes used in high-pressure gas-filled 805 MHz RF cavity experiments1 have been used to investigate the mechanism of RF breakdown.2 The images show evidence for melting and boiling in small regions of ~10 micron diameter on tungsten, molybdenum, and beryllium electrode surfaces. In these experiments, the dense hydrogen gas in the cavity prevents electrons or ions from being accelerated to high enough energy to participate in the breakdown process so that the only important variables are the fields and the metallic surfaces. The distributions of breakdown remnants on the electrode surfaces are compared to the maximum surface gradient E predicted by an ANSYS model of the cavity. The local surface density of spark remnants, proportional to the probability of breakdown, shows a strong exponential dependence on the maximum gradient, which is reminiscent of Fowler-Nordheim behavior of electron emission from a cold cathode. New simulation results have shown good agreement with the breakdown behavior of the hydrogen gas in the Paschen region and have suggested improved behavior with the addition of trace dopants such as SF6.3 Present efforts are to extend the computer model to include electrode breakdown phenomena and to use scanning tunneling microscopy to search for work function differences between the conditioned and unconditioned parts of the electrodes.

Progress in absorber R & D for muon cooling

A stored-muon-beam neutrino factory may require transverse ionization cooling of the muon beam. We describe recent progress in research and development on energy absorbers for muon-beam cooling carried out by a collaboration of university and laboratory groups.

Recent Innovations in Muon Beam Cooling

Eight new ideas are being developed under SBIR/STTR grants to cool muon beams for colliders, neutrino factories, and muon experiments. Analytical and simulation studies have confirmed that a six-dimensional (6D) cooling channel based on helical magnets surrounding RF cavities filled with dense hydrogen gas can provide effective beam cooling. This helical cooling channel (HCC) has solenoidal, helical dipole, helical quadrupole, and helical sextupole magnetic fields to generate emittance exchange and achieve 6D emittance reduction of over 3 orders of magnitude in a 100 m segment. Four such sequential HCC segments, where the RF frequencies are increased and transverse physical dimensions reduced as the beams become cooler, implies a 6D emittance reduction of almost five orders of magnitude. Two new cooling ideas, Parametric-resonance Ionization Cooling and Reverse Emittance Exchange, then can be employed to reduce transverse emittances to a few mm-mr, which allows high luminosity with fewer muons than previously imagined. We describe these new ideas as well as a new precooling idea based on a HCC with z dependent fields that can be used as MANX, an exceptional 6D cooling demonstration experiment.

Convection-type LH2 absorber R&D for muon ionization cooling

Abstract A feasibility study on liquid hydrogen (LH 2 ) absorbers for muon ionization cooling is reported. In muon ionization cooling, an LH 2 absorber is required to have a high cooling power greater than 100 W to cool heat deposited by muons passing through. That heat in LH 2 can be removed at either external or internal heat exchangers, which are cooled by cold helium gas. As one of the internal heat exchanger types, a convection-type absorber is proposed. In the convection-type absorber, heat is taken away by the convection of LH 2 in the absorber. The heat exchanger efficiency for the convection-type absorber is calculated. A possible design is presented.

Current LH2-absorber R&D in MuCool

The MuCool hydrogen-absorber R&D program is summarized. Prototype absorbers featuring thin aluminum windows and flow-through or convection cooling are under development for eventual power-handling tests in a proton beam and a cooling demonstration in a muon beam. Testing these prototypes and their components involves application of novel techniques.

The radiation environment in and near high gradient rf cavities

Abstract Measurements of the X-ray flux, electron flux and spectrum are presented with the aim of evaluating the usefulness of ionization sensitive single particle detectors in the immediate environment of an rf cavity. The measured fluxes are found to be roughly consistent with field emission of electrons followed by one generation of bremsstrahlung photon production, with absorption and scattering of the final photons and electrons in the cavity and vacuum chamber structure.

Project X and a Muon Facility at Fermilab

An integrated program is described, starting with muon experiments in the Booster era, continuing with a 2 MW target station, a 4 GeV Neutrino Factory and a 3 TeV Muon Collider, all driven by Project X. This idea provides an integrated approach to the Intensity and Energy Frontiers at Fermilab.