Holger Witte

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

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.

The mechanical and thermal design for the MICE coupling solenoid magnet

The MICE coupling solenoids surround the RF cavities that are used to increase the longitudinal momentum of the muon beam that is being cooled within MICE. The coupling solenoids will have a warm-bore diameter of 1394 mm. This is the warm bore that is around the 200 MHz RF cavities. The coupling solenoid is a single superconducting coil fabricated from a copper matrix Nb-Ti conductor originally designed for MRI magnets. A single coupling magnet is designed so that it can be cooled with a single 1.5 W (at 4.2 K) cooler. The MICE cooling channel has two of these solenoids, which will be hooked together in series, for a magnet circuit with a total stored-energy of the order of 12.8 MJ. Quench protection for the coupling coils is discussed. This report also 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.

The Design Parameters for the MICE Tracker Solenoid

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.

The Advantages and Challenges of Helical Coils for Small Accelerators—A Case Study

Most of todays particle accelerators are used in industry or for medical applications, for example, in radioisotope production and cancer therapy. One important factor for these applications is the size of the accelerator, which ideally should be as small as possible. In this respect, fixed-field alternating-gradient accelerators (FFAGs) can be an attractive alternative, which combine the best features of conventional synchrotrons and cyclotrons: FFAGs deliver better performance than synchrotrons while retaining flexibility. Of particular interest are accelerators for protons of moderate energy (0.25-1 GeV) and light ions such as carbon (up to 400 MeV per nucleon), for example, for proton/carbon-ion charged particle therapy or potential future applications such as accelerator-driven subcritical reactors. Due to high magnetic rigidity, a compact machine can be only achieved by using high field superconducting magnets. A disadvantage of FFAGs is that the magnetic elements can be very challenging. Quite often, complicated multipole fields are required, in combination with stringent geometric constraints. In this paper, we demonstrate the advantages of helical coil technology by means of an accelerator for proton therapy.

Pulsed Magnets—Advances in Coil Design Using Finite Element Analysis

We discuss further developments in the application of Finite Element Analysis (FEA) to the design of pulsed magnets on the basis of our latest coil design. Two areas are addressed. Firstly, we present results concerning the current distribution in the conductor during the discharge. Eddy current heating leads to thermal gradients across the layers and in the conductor; the results are compared with well established analytical codes. Significantly higher stresses result due to temperature gradients and nonuniform current distribution, which can be partly offset by pre-stressing reinforcement fibers. Secondly, we report progress in designs that facilitate much more rapid cool-downs for pulsed magnets, which can sometimes take in the order of hours between pulses. We show that cool-down times of 20 minutes or less are achievable using copper discs (designed using FEA) that will not support eddy currents

Progress on the Coupling Coil for the Mice Channel

This report describes the progress on the coupling magnet for the international Muon Ionization Cooling Experiment (MICE). MICE consists of two cells of a SFOFO cooling channel that is similar to that studied in the level 2 study of a neutrino factory. The MICE RF coupling coil module (RFCC module) consists of a 1.56 m diameter superconducting solenoid, mounted around four cells of conventional 201.25 MHz closed RF cavities. This report discusses the progress that has been made on the superconducting coupling coil that is around the center of the RF coupling module. This report describes the process by which one would cool the coupling coil using a single small 4 K cooler. In addition, the coupling magnet power system and quench protection system are also described.

Rapid Cooling Methods for Pulsed Magnets

Pulsed magnets are generally evaluated and compared in terms of the magnetic field they can achieve in combination with a bore size. However, in practice another criterion is equally important: the waiting time for a researcher in between two consecutive shots. The cooling time of pulsed magnets can range from a few minutes up to several hours, depending on coil size and desired field. Using simulations and measurements several options to reduce the cool down time are compared in this paper. One of the discussed methods is now routinely in use at the Laboratoire National des champs magnetiques pulses (LNCMP) in Toulouse.

The Effect of Magnetic Field on the Position of HTS Leads and the Cooler in the Services Tower of the MICE Focusing Magnet

The MICE focusing solenoids have three 4 K coolers (two for the superconducting magnet and one for the liquid absorber) and four HTS leads that feed the current to the focusing coils. The focusing solenoids produce large radial external fields when they operate with the polarity of the two coils in opposition (the gradient or flip mode). When the MICE focusing coils operate at the same polarity (the solenoid or non-flip mode), the fields are much smaller and parallel to the axis of the solenoid. The worst-case magnetic field affects the selection of the cooler and the HTS leads. This magnetic field can also determine the height of the service towers that house the three coolers and the four HTS leads. This paper shows the criteria used for Cooler selection, HTS lead selection, and the position of both the cooler and leads with respect to the solenoid axis of rotation.

Pulsed synchrotrons for very rapid acceleration

When rapid acceleration is important, synchrotrons with very short pulse times can be used to accelerate particle beams. We will describe rapidly pulsed synchrotrons and their distinction from ordinary synchrotrons. We will introduce a hybrid synchrotron which interleaves pulsed magnets with superconducting dipoles to allow rapid acceleration while still maintaining a high average bending field. We will describe particular characteristics of the lattice design for these machines. We will describe how to design magnets to limit power consumption while still maintaining high fields. We will discuss the impact of the choice and properties of magnetic materials on the magnet performance. We show a magnet design that limits losses in the core while giving a high field by using multiple materials: 6.5% silicon steel for the back yoke due to its low losses at high frequencies, and 3% silicon steel in the pole for its high saturation field. The magnet has a unique coil configuration that minimizes eddy current lo...

Quench Protection and Magnet Supply Requirements for the MICEFocusingand Coupling Magnets

Oxford Physics Engineering Report 15 LBNL-57580 Quench Protection and Magnet Power Supply Requirements For the MICE Focusing and Coupling Magnets Michael A. Green Lawrence Berkeley National Laboratory Berkeley CA 94720, USA Holger Witte Oxford University Physics Department Oxford, OX1-3RH, UK 8 June 2005* Abstract This report discusses the quench protection and power supply requirements of the MICE superconducting magnets. A section of the report discusses the quench process and how to calculate the peak voltages and hotspot temperature that result from a magnet quench. A section of the report discusses conventional quench protection methods. Thermal quench back from the magnet mandrel is also discussed. Selected quench protection methods that result in safe quenching of the MICE focusing and coupling magnets are discussed. The coupling of the MICE magnets with the other magnets in the MICE is described. The consequences of this coupling on magnet charging and quenching are discussed. Calculations of the quenching of a magnet due quench back from circulating currents induced in the magnet mandrel due to quenching of an adjacent magnet are discussed. The conclusion of this report describes how the MICE magnet channel will react when one or magnets in that channel are quenched. TABLE OF CONTENTS Abstract Introduction Quench Propagation Velocity and the Hot Spot Temperature Active Quench Protection using a Resistor Quench Propagation in the Magnet Coils Coil Quench Back from the Aluminum Mandrel Passive Quenches in the Focusing and Coupling Magnets Magnetic Coupling between Various Magnets in MICE Magnet Power Supply Design Parameters Magnet Quench Back due to Inductive Coupling to the Mandrels Concluding Comments Acknowledgements References * Last revision 17 June 2005 Page