We present measurements of dark currents and x rays in a six cell 805 MHz cavity, taken as part of an rf development program for muon cooling, which requires high power, high stored energy, low frequency cavities operating in a strong magnetic field. We have done the first systematic study of the behavior of high power rf in a strong (2.5-4 T) magnetic field. Our measurements extend over a very large dynamic range in current and provide good fits to the Fowler-Nordheim field emission model assuming mechanical structures produce field enhancements at the surface. The locally enhanced field intensities we derive at the tips of these emitters are very large, (-10 GV/m), and should produce tensile stresses comparable to the tensile strength of the copper cavity walls and should be capable of causing breakdown events. We also compare our data with estimates of tensile stresses from a variety of accelerating structures. Preliminary studies of the internal surface of the cavity and window are presented, which show splashes of copper with many sharp cone shaped protrusions and wires which can explain the experimentally measured field enhancements. We discuss a 'cold copper' breakdown mechanism and briefly review alternatives. We also discuss amore » number of effects due to the 2.5 T solenoidal fields on the cavity such as altered field emission due to mechanical deformation of emitters, and dark current ring beams, which are produced from the irises by ExB drifts during the nonrelativistic part of the acceleration process.« less

https://link.aps.org/pdf/10.1103/PhysRevSTAB.6.072001

Dark current, breakdown, and magnetic field effects in a multicell, 805 MHz cavity.

The success of digital feedback with synchronous IQ sampling for cavity field control in recent accelerator projects make this LLRF control scheme a popular choice. This short-period synchronous sampling does not, however, average out well-known defects in modern ADC and DAC hardware. That limits the achievable control precision for digital IQ LLRF controllers, while demands for precision are increasing for future accelerators such as International Linear Collider. For this reason, a collaborative effort is developing a digital LLRF control evaluation platform to experiment using coherent sampling with much longer synchronous periods, on the order of the cavity closed-loop bandwidth. This exercise will develop and test the hardware and software needed to meet greater future RF control challenges.

/pdf/digital-low-level-rf-control-using-non-iq-sampling-2rqsbeyc72.pdf

Digital Low-Level RF Control Using Non-IQ Sampling

The focusing solenoids for MICE surround energy absorbers that are used to reduce the transverse momentum of the muon beam that is being cooled within MICE. The focusing solenoids will have a warm-bore diameter of 470 mm. Within this bore is a flask of liquid hydrogen or a room temperature beryllium absorber. The focusing solenoid consists of two coils wound with a copper matrix Nb-Ti conductor originally designed for MRI magnets. The two coils have separate leads, so that they may be operated at the same polarity or at opposite polarity. The focusing magnet is designed so that it can be cooled with a pair of 1.5 W (at 4.2 K) coolers. The MICE cooling channel has three focusing magnets with their absorbers. The three focusing magnets will be hooked together in series for a circuit stored-energy of about 9.0 MJ. Quench protection for the focusing magnets is discussed. This report presents the mechanical and thermal design parameters for this magnet, including the results of finite element calculations of mechanical forces and heat flow in the magnet cold mass.

/pdf/the-mechanical-and-thermal-design-for-the-mice-focusing-3fpdls9est.pdf

The mechanical and thermal design for the MICE focusing solenoid magnet system

LBNL-57396 The Mechanical and Thermal Design for the MICE Focusing Solenoid Magnet System* S Q Yang, M A Green, G Barr, U Bravar, J Cobb, W Lau, R S Senanayake, A E White, and H Witte Abstract— The focusing solenoids for MICE surround energy absorbers that are used to reduce the transverse momentum of the muon beam that is being cooled within MICE The focusing solenoids will have a warm-bore diameter of 470 mm Within this bore is a flask of liquid hydrogen or a room temperature beryllium absorber The focusing solenoid consists of two coils wound with a copper matrix Nb-Ti conductor originally designed for MRI magnets The two coils have separate leads, so that they may be operated at the same polarity or at opposite polarity The focusing magnet i s designed so that it can be cooled with a pair of 15 W (at 42 K) coolers The MICE cooling channel has three focusing magnets with their absorbers The three focusing magnets will be hooked together in series for a circuit stored-energy of about 90 MJ Quench protection for the focusing magnets i s discussed This report presents the mechanical and thermal design parameters for this magnet, including the results of finite element calculations of mechanical forces and heat flow in the magnet cold mass Index Terms—Superconducting Solenoids, and Muon Cooling I I NTRODUCTION he development of a muon collider or a neutrino factory requires that beams of low emittance muons be produced A key to the production of low emittance muons is muon cooling A demonstration of muon cooling is essential to the development of muon accelerators and storage rings [1], [2] The international Muon Ionization Cooling Experiment (MICE) will be a demonstration of muon cooling in a configuration of superconducting magnets [3] that may be useful for a neutrino factory Ionization cooling of muons means that muons have their momentum reduced in both the longitudinal direction and the transverse direction by passing them through a low Z absorber RF cavities are used to re-accelerate the muons to their original momentum If the scattering in the absorbing medium is not too large, the reaccelerated muon beam will have a lower emittance than the muon beam that entered the absorbers In order to reduce the multiple scattering of the muon beam in the absorber, the muon beam beta must be low in the absorber and the absorber must have a low Z The candidate absorbers include hydrogen (either as liquid or gas), lithium hydride, lithium metal or beryllium metal The absorber in MICE is located within the absorber focus coil module where the beam has low beta (is well focused) T Manuscript received 5 October 2004 S Q Yang, M A Green (email MAGree@lblgov), G Barr, U Bravar, J Cobb, W Lau, R S Senanayake, A E White, and H Witte are from the Oxford University Physics Department, Oxford OX1-3RH *This work was supported by the Oxford University Physics Department and the Particle Physics and Astronomy Research Council of the United Kingdom II T HE P ROPOSED MICE E XPERIMENT The proposed MICE experiment will test cooling on a low intensity muon beam from the ISIS ring at the Rutherford Appleton Laboratory in the United Kingdom The pions that decay into muons are produced by dipping a metal target into the ISIS proton beam The muons will be collected and will be carried to an experimental hall containing the MICE The beam enters the experiment by passing through a foil that will scatter the muons to produce a beam with the desired emittance (beam scattering or beam transverse momentum) The muon emittance will be measured by four planes of scintillating fibers that are in a uniform (to about a percent) solenoidal magnetic field from 28 to 4 T [4] Fig 1 shows a schematic cross-section view of MICE that shows the detectors as well as the cooling channel Once the emittance of the muon beam entering the cooling section has been measured, the beam passes into the first absorber focus coil module (AFC module) The absorber cools the muon beam (reduces both the transverse and longitudinal momentum) by ionization cooling The MICE absorbers may be liquid hydrogen, liquid helium, beryllium, or plastic The preferred cooling absorber material is hydrogen, because it has the lowest multiple scattering of any of the absorber materials for a given energy absorption and beam beta The absorber is surrounded by a two-coil superconducting solenoid, which is the topic of this report The magnet is designed to produce either a relatively uniform field or a cusp shaped field (a solenoidal quadruple field) that changes polarity as one goes along the axis A key feature of the focusing magnet is the fact that the magnet is designed to operate as a two coil uniform field solenoid or as a gradient solenoid When the focusing magnet operates in the gradient mode (with both coils at opposite polarity) the field is zero at the magnet center and the center of the absorber The muon beam longitudinal momentum is recovered by accelerating the beam with a four cell 20125 MHz RF cavity that are in the coupling coil 20 to 25T magnetic field [5] If the scattering within the absorber is low enough, the re- accelerated muon beam will be cooled After the muon beam has been re-accelerated, it passes through a second absorber focus coil module for more beam cooling The process of re- acceleration is repeated then the beam passes through the final AFC module At this point, the beam should be cooled enough, so that the emittance change can be clearly measured Once the beam has passed through the ionization cooling section, it enters the second detector section The second detector section is identical to the first detector, except for the time of flight detectors at the end of the experiment

The Mechanical and Thermal Design for the MICE Focusing Solenoid Magnet System

Electronic structure changes are at the origin of the making, breaking and transformation of bonds. These changes can be visualized by measuring the geometric structure in "real-time" during the course of a chemical reaction, a biological function or a physical process. Time-resolved X-ray Absorption Spectroscopy (XAS) delivers information about both the electronic (via XANES) and geometric (via EXAFS) transient structural changes, when interfaced with an ultrafast laser in a pump-probe scheme. Moreover, XAS offers unique flexibility, since it is both element-selective and it can be applied to any kind of disordered or ordered systems. In this thesis, we successfully investigated the excited state electronic and geometric structures of two different transition metal complexes. In both cases, for the first time, their excited state molecular geometries were characterized "on the fly", without any a priori assumptions about its excited state structure. More importantly, it has been shown that time-resolved XAS is the only method capable of delivering the transient molecular structures of their short-lived excited states. First, we investigated ruthenium(II)-tris(2,2'-bipyridine), [RuII(bpy)3]2+. This molecule has served as a prototype and a model system of intramolecular electron and energy transfer reactions, due to its unique excited state properties. Our studies focused on the energy and structural relaxation process of the short-lived excited states of this molecule. By using the combined ultrafast laser and x-ray spectroscopies, we have determined various relaxation pathways of its excited states down to 15 fs lifetimes. The geometrical distortion of its lowest-lying excited state (3MLCT state) has also been determined by picosecond XAS, delivering a Ru-N bond contraction of ∼ -0.04 A. Second, our study focused on iron(II)-tris(2,2'-bipyridine) [FeII(bpy)3]2+. This class of compounds is being extensively studied in relation to the phenomenon of spin crossover, where a spin transition takes place, involving the low-spin (LS) ground state and the high-spin (HS) excited state. Here, we have characterized its excited states by means of both ultrafast optical and x-ray spectroscopy. The optical studies have revealed several new aspects concerning the relaxation pathways of its charge transfer and ligand-field states, including their corresponding lifetimes. The structural analysis has determined the geometric distortions taking place in the lowest-lying excited HS state of [FeII(bpy)3]2+.The extracted Fe-N bond elongation of 0.2 A agrees well with previously predicted values and it is for the first time that the room-temperature solvated structure of the HS short-lived excited state of a ferrous transition metal is obtained.

Time-resolved x-ray absorption spectroscopy of transition metal complexes

We present an updated design for a proposed source of ultra-fast synchrotron radiation pulses based on a recirculating superconducting linac, in particular the incorporation of EUV and soft x-ray production. The project has been named LUX - Linac-based Ultrafast X-ray facility. The source produces intense x-ray pulses with duration of 10-100 fs at a 10 kHz repetition rate, with synchronization of 10's fs, optimized for the study of ultra-fast dynamics. The photon range covers the EUV to hard x-ray spectrum by use of seeded harmonic generation in undulators, and a specialized technique for ultra-short-pulse photon production in the 1-10 keV range. High-brightness rf photocathodes produce electron bunches which are optimized either for coherent emission in free-electron lasers, or to provide a large x/y emittance ration and small vertical emittance which allows for manipulation to produce short-pulse hard x-rays. An injector linac accelerates the beam to 120 MeV, and is followed by four passes through a 600-720 MeV recirculating linac. We outline the major technical components of the proposed facility.

/pdf/a-recirculating-linac-based-facility-for-ultrafast-x-ray-2w52bvfba0.pdf

A recirculating linac-based facility for ultrafast x-ray science

The first superconducting magnets to be installed in the union ionization cooling experiment (MICE) will be the tracker solenoids. The tracker solenoid module is a five coil superconducting solenoid with a 400 mm diameter warm bore that is used to provide a 4 T magnetic field for the experiment tracker module. Three of the coils are used to produce a uniform field (up to 4 T with better than 1 percent uniformity) in a region that is 300 mm in diameter and 1000 mm long. The other two coils are used to match the muon beam into the MICE cooling channel. Two 2.94-meter long superconducting tracker solenoid modules have been ordered for MICE. The tracker solenoid will be cooled using two-coolers that produce 1.5 W each at 4.2 K. The magnet system is described. The decisions that drive the magnet design will be discussed in this report.

/pdf/the-design-parameters-for-the-mice-tracker-solenoid-3mtyfu33fe.pdf

The Design Parameters for the MICE Tracker Solenoid

The superconducting magnets and absorbers for MICE will be cooled using PT415 pulse tube coolers. The cooler 2nd stage will be connected to magnets and the absorbers through a helium or hydrogen re-condensing system. It was proposed that the coolers be connected to the magnets in such a way that the cooler can be easily installed and removed, which permits the magnets to be shipped without the coolers. The drop-in mode requires that the cooler 1st stage be well connected to the magnet shields and leads through a low temperature drop demountable connection. The results of the PT415 drop-in cooler tests are presented.

/pdf/tests-of-four-pt-415-coolers-installed-in-the-drop-in-mode-bzh9ianyj7.pdf

Tests of Four PT-415 Coolers Installed in the Drop-in Mode

The 1.9 GeV Advance Light Source (ALS) at the Lawrence Berkeley National Laboratory (LBNL) produces photons with a critical energy of about 3.1 keV at each of its thirty-six 1.3 T gradient bending magnets. It is proposed that at three locations around the ring the conventional gradient bending magnets be replaced with superconducting bending magnets with a maximum field of 5.6 T. At the point where the photons are extracted, their critical energy will be about 12 keV. In the beam lines where the SuperBend superconducting magnets are installed, the X ray brightness at 20 keV will be increased over two orders of magnitude. This report describes three different refrigeration options for cooling the three SuperBend dipoles. The cooling options include: 1) liquid helium and liquid nitrogen cryogen cooling using stored liquids, 2) a central helium refrigerator (capacity 70 to 100 W) cooling all of the SuperBend magnets, 3) a Gifford McMahon (GM) cryocooler on each of the dipoles. This paper describes the technical and economic reasons for selecting a small GM cryocooler as the method for cooling the SuperBend dipoles on the LBNL Advanced Light Source.

/pdf/refrigeration-options-for-the-advanced-light-source-4ahrcxcps0.pdf

Refrigeration options for the Advanced Light Source Superbend Dipole Magnets

This report describes the MICE spectrometer solenoids as built. Each magnet consists of five superconducting coils. Two coils are used to tune the beam going from or to the MICE spectrometer from the rest of the MICE cooling channel. Three spectrometer coils (two end coils and a long center coil) are used to create a uniform 4 T field (to plusmn0.3 percent) over a length of 1.0 m within a diameter of 0.3 m. The three-coil spectrometer set is connected in series. The two end coils use small power supplies to tune the uniform field region where the scintillating fiber tracker is located. This paper will present the results of the preliminary testing of the first spectrometer solenoid.

/pdf/preliminary-test-results-for-the-mice-spectrometer-36l95wu3ls.pdf

S.T. Wang

Papers

A recirculating linac-based facility for ultrafast x-ray science

The Design Parameters for the MICE Tracker Solenoid

Tests of Four PT-415 Coolers Installed in the Drop-in Mode

Refrigeration options for the Advanced Light Source Superbend Dipole Magnets

Preliminary Test Results for the MICE Spectrometer Superconducting Solenoids