Progress toward muon accelerator capabilities

Mark Palmer

Mark Palmer, director of the Muon Accelerator Program, wrote this column.

Last week, as part of the Muon Accelerator Program (MAP) Spring Collaboration Meeting at Fermilab, many of us were privileged to hear a seminar by Nobel laureate Carlo Rubbia. He described his vision for developing muon accelerator capabilities in support of a muon-based Higgs factory. A major thrust of his seminar was to emphasize the need to complete a demonstration of muon ionization cooling.

Muon ionization cooling is a method to reduce the transverse and longitudinal sizes of a muon beam to the dimensions required for future muon accelerators. For the past three years, the MAP collaboration has targeted the key demonstrations required to realize muon ionization cooling. This has included extensive design and simulation of cooling channel concepts capable of providing the performance required for neutrino factories and muon colliders; development of radio-frequency (RF) cavities that can reliably operate in the strong magnetic fields of a cooling channel; tests of those cavity designs in the MuCool Test Area (MTA) at Fermilab; and prototyping and construction of RF cavities and magnets for an initial demonstration of ionization cooling at the Muon Ionization Cooling Experiment (MICE).

Over the last several months, crucial progress has been made in our preparations for the MICE demonstration, which is based at the Rutherford Appleton Laboratory in the UK. This international effort will provide the first operational demonstration of the integrated beam optics and RF hardware required for an actual cooling channel suitable for a muon accelerator complex.

MICE will be ready to begin commissioning for the first of two major rounds of experimental studies early next month. Through mid-2016, the experiment will study the performance of absorber materials proposed for muon cooling channels. These studies will provide important data with which we can refine our detailed models of the cooling process. After approximately one year of experimental time in this configuration, the cooling channel will be extended to include a pair of RF modules (which comprise cavities and associated instrumentation) to reaccelerate the muons, as well as an additional focusing magnet to complete the cooling channel optics. Data in the final configuration will be obtained starting in mid-2017.

Here at Fermilab, a critical MICE milestone was passed earlier this month: a prototype RF module achieved stable operation in the magnetic field of the MTA test magnet with a nominal gradient of 11 megavolts per meter, exceeding the MICE specification of 10.3 MV/m. (The gradient is one measure of how effectively a cavity can transfer energy to a particle beam.) In the MICE cooling channel, each RF module sits adjacent to a focus coil magnet. For our test in the MTA, the cavity was placed adjacent to our test magnet, the original prototype for the focus coil, which is capable of producing a 5-Tesla field in its bore. On May 5, we informed DOE that the module acquired more than 3 million pulses in the tested configuration, with no breakdown events.

The excellent results are testament to the careful work that went into preparing the MICE module. The cavity was prepared with an electropolished surface, as is commonly used for superconducting RF cavities, at Lawrence Berkeley National Laboratory. The LBNL group also simulated key aspects of the cavity’s performance in magnetic field. In the MTA, a team from Berkeley Lab, Fermilab, the Illinois Institute of Technology, Rutherford Appleton Lab and the University of Strathclyde carried out the prototype test program.

The path is now clear to complete construction of the remaining MICE RF hardware and obtain the first cooling demonstration data by 2017.

In order to mark the start of the detailed study of ionization cooling with MICE, RAL will host a special public science event on June 25.

The prototype MICE 201-megahertz RF module, with the copper cavity mounted, is shown during assembly at Fermilab. A titanium nitride-coated beryllium window covers the cavity iris. The six tuner arms, attached to the cavity body, provide roughly 400 kilohertz of tuning range. Photo courtesy of Y. Torun, IIT