R.P. Johnson

A high field hts solenoid for muon cooling

The ability of high temperature superconducting (HTS) conductor to carry high currents at low temperatures makes feasible the development of very high field magnets for uses in accelerators and beam-lines. A specific application of a very high field solenoid is to provide a very small beta region for the final cooling stages for a muon collider. Since ionization cooling in a solenoid acts simultaneously on both transverse planes, any improvement in maximum field has a quadratic consequence in the reduction of the 6-dimensional (6D) beam emittance. This paper describes a conceptual design of a 45 Tesla solenoid based on Bi-2223 HTS tape, where the magnet will be operated at 4.2 K to take advantage of the high current carrying capacity at that temperature. In this design, an outer Nb3Sn shell surrounds the HTS solenoid. This paper describes the technical issues associated with building this magnet. In particular it addresses how to mitigate the large Lorentz stresses associated with the high field magnet and how to design the magnet to reduce the compressive end forces. Also this paper discusses the important issue of how to protect this magnet if a quench should occur.

Superconducting Magnet System for Muon Beam Cooling

A helical cooling channel has been proposed to quickly reduce the six-dimensional phase space of muon beams for muon colliders, neutrino factories, and intense muon sources. A novel superconducting magnet system for a muon beam cooling experiment is being designed at Fermilab. The inner volume of the cooling channel is filled with liquid helium where passing muon beam can be decelerated and cooled in a process of ionization energy loss. The magnet parameters are optimized to match the momentum of the beam as it slows down. The results of 3D magnetic analysis for two designs of magnet system, mechanical and quench protection considerations are discussed.

Magnets for the MANX 6-D muon cooling demonstration experiment

MANX is a 6-dimensional muon ionization-cooling experiment that has been proposed to Fermilab to demonstrate the use of a Helical Cooling Channel (HCC) for future muon colliders and neutrino factories. The HCC for MANX has solenoidal, helical dipole, and helical quadrupole magnetic components which diminish as the beam loses energy as it slow down in a liquid helium absorber inside the magnets. Additional magnets that provide emittance matching between the HCC and upstream and downstream spectrometers are also described as are the results of G4Beamline simulations of the beam cooling behavior of the complete magnet and absorber system.

Magnet system for helical muon cooling channels

A helical cooling channel (HCC) consisting of a pressurized gas absorber imbedded in a magnetic channel that provides superimposed solenoidal, helical dipole, and helical quadrupole fields has shown considerable promise in providing six-dimensional cooling of muon beams. The analysis of this muon cooling technique with both analytic and simulation studies has shown significant reduction of muon phase space emittance. A particular channel that has been simulated is divided into four segments each with progressively smaller apertures and stronger fields to reduce the equilibrium emittance so that more cooling can occur. The fields in the helical channel are sufficiently large that the conductor for segments 1 and 2 can be Nb3Sn and the conductor for segments 3 and 4 may need to be high temperature superconductor. This paper will describe the magnetic specifications for the channel and two conceptual designs on how to implement the magnetic channel.

High temperature superconductors for high field superconducting magnets

Ionization cooling, a method for shrinking the size of a muon beam, requires a low Z energy absorber, high field magnets, and high gradient radio frequency cavities. The use of high temperature superconductors (HTS) for the high field superconducting magnets is being considered to realize a helical muon cooling channel using hydrogen refrigeration. A test stand was set up at Fermilab to perform critical current (Ic) measurements of HTS wires under externally applied perpendicular and parallel fields at various temperatures. A description of the test setup and results on (Bi,Pb)2Sr2Ca2Cu3Ox (BSCCO‐2223) tapes and Bi2Sr2CaCu2O8 (BSCCO‐2212) round wires are presented. Finally, the engineering critical current density, JE, of HTS and Nb3Sn were compared in the application’s field and temperature range of interest.

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...

Longitudinal Motion of the Beam in the Fermilab Booster

When the Fermilab Booster Accelerator is operated at or above 1.5 × 1012 protons per pulse (extraction current about 155 mA) large amplitude coupled bunch longitudinal dipole oscillations occur between transition and extraction times. The oscillations do not contribute to beam loss in the booster but, because the beam is transferred synchronously into preexisting buckets in the Main Ring, the oscillations contribute to a deterioration of beam quality in the main ring. Two mode numbers have been established for the instabilities and the primary source frequencies have been isolated, although the offending objects have not. Operation of one of the eighteen accelerating cavities at a harmonic number one unit lower than the operating value (83 instead of 84) effectively damps the motion by the introduction bunch to bunch synchrotron tune spread.

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.

Optimization of Brittle Superconducting

Finite element simulations and experimental measurements of deformed strand cross sections were performed to study their structural behavior during cabling. A variety of strand designs were modeled to identify and optimize design parameters like sub-element shape, number of sub-elements, and their spacing. The model results were correlated to the experimental results. This led to a numerical-experimental approach that is effective in predicting fracture, merging, and deformation of the sub-elements. Strains were calculated as a function of strand deformation for strands with 54, 120, and 210 sub-elements and a local Cu-to-non-Cu ratio of 0.165. Strains as a function of strand deformation were also calculated for 120/127 strands with a local Cu-to-non-Cu ratio of 0.11, 50% increased spacing, and 100% increased spacing between sub-elements. Results showed that increasing the spacing by 100% reduces the maximum strain-x, maximum strain-y, and maximum strain-xy by 14%, 13%, and 29% respectively at a 30% strand deformation level. Also, results revealed that the maximum strain components are always located in the sub-elements close to the center of the strands, which agrees with the experimental findings.

{\rm Nb}_{3}{\rm Sn}

MANX is an experiment to prove that effective six- dimensional (6D) muon beam cooling can be achieved in a Helical Cooling Channel (HCC) using ionization- cooling with helical and solenoidal magnets in a novel configuration. The aim is to demonstrate that 6D muon beam cooling is understood well enough to plan intense neutrino factories and high-luminosity muon colliders. The experiment consists of the HCC magnet that envelops a liquid helium energy absorber, upstream and downstream instrumentation to measure the beam parameters before and after cooling, and emittance matching sections between the detectors and the HCC.