Draft:ProtonBeamIntro

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Introduction

Mu2e requires a high intensity, pulsed proton beam to produce large numbers muons. The experiment relies on Fermilab’s Booster, which is also used by other Fermilab experiments, such as g-2, to provide the protons. The beam for Mu2e can be delivered while other experiments are running simultaneously. While the beam of protons must be intense during these pulses in order to deliver enough muons to the stopping target for Mu2e to reach its design sensitivity, the beam of protons must also disappear between pulses. This allows for the muons captured by the experiment to decay over time, and these decay products to be measured. Protons delivered between pulses will contribute to the experiment’s backgrounds, and their rate will be measured with the extinction monitor.

Figure 1. Layout of the Mu2e facility (lower right) relative to the accelerator complex that provides proton beam to the detector. Protons are transported from the Booster through the MI-8 beamline to the Recycler Ring where they will circulate while they are re-bunched by a 2.5 MHz RF system. The reformatted bunches are kicked into the P1 line and transported to the Delivery Ring where they are slow extracted to the Mu2e detector through a new external beamline.

Proton Beam Timeline

Following will be a rough outline of path protons take from initial creation to the eventual "Production Target" where they'll be converted to pions which will eventually decay to muons.

Linac, Booster Ring

The fundamental timing of the whole experiment is predicated on the timing of the Booster Synchrotron. Protons from the Proton Source will be accelerated down the Linac and around the Booster Ring to an energy of 8 GeV every 67 msec (15 Hz). The 67 msec Booster cycle is commonly called a "Booster tick" or a "15 Hz tick", and the group of protons created and accelerated into the experiment every tick (every 67 msec) is called a "batch". Each batch will consist of 81 "bunches" of protons (corresponding to RF buckets in the ring). Mu2e will have access to the Booster during the times when slip-stacking operations are underway for NOνA.

The Booster will accelerate a total of two batches (162 bunches) into the Recycler Ring (therefore using two Booster ticks, or 133.33 msec). Each batch occupies 1/7 of the circumference of the Recycler, so a total of 2/7 of the circumference will be occupied.

Recycler Ring

Once the two Booster ticks worth of protons are in the Recycler Ring, these 162 bunches will occupy the Recycler Ring for six additional Booster ticks for a total of eight Booster ticks per 21-tick Main Injector cycle.

Immediately after the second and final batch is injected, RF manipulations of the Recycler Ring coalesce the 162 53-MHz bunches into eight larger, more intense 2.5 MHz bunches. These RF manipulations take a total of 90 msec. Once the beam is rebunched into eight 2.5 MHz bunches, each bunch is separately transferred to the Delivery Ring at a 48.1 msec interval.

Delivery Ring

2.5 MHz bunches arriving at the Delivery Ring are synchronously captured in the 2.36 MHz RF system of the Delivery Ring. Once one bunch arrives, resonant extraction of the bunch to the M4 beamline immediately takes place over 43.1 msec. After the full completion of the extraction there is a gap of 5 msec to allow the resonant extraction magnet circuits and Delivery Ring RF systems to reset in preparation for the arrival of the next 2.5 MHz bunch.

Resonant Extraction of Protons from Delivery Ring

A single 2.5 MHz bunch that is sent to the Delivery Ring and eventually to the M4 beamline contains 10¹² protons. Over 43.1 msec, a resonant extraction system will pull out 39 × 10⁶ protons per proton pulse every 1695 nsec for a total of 25,440 pulses. Each proton pulse will have a length of ±125 nsec (total width 250 nsec), with 95% of the pulse falling within a ± 106.0 ns window [3]. The proton beam will have a transverse radius of about 1 mm (rms).

To help control the spill rate uniformity during resonant extraction a technique known as RF knockout will be used. RF knockout will allow for fast transverse heating of the beam. It will also serve as a feedback tool for fine control of the spill rate.

The resonant extraction process will not completely remove the entire beam, so what remains must be disposed of in a controlled way. Therefore, a beam abort system will be required for the Delivery Ring to “clean up” beam that remains after resonant extraction is complete.

AC Dipole

An extinction system, in the form of a high frequency AC dipole, is required to suppress unwanted beam between successive pulses that can generate experimental backgrounds [4]. It will operate at a primary frequency of ~295 kHz (for a half-period of 1,695 nsec to match the Delivery Ring bunch rate, 3,390 nsec full period), and a 15th harmonic ~4.42 MHz wave will be overlaid on top of that (226 nsec period).

The power supplies for the AC Dipoles will be controlled by FPGA circuits to ensure the dipoles stay in-phase (phase-locked) with the incoming beams, and also to manage the required phase jumps after all the planned gaps in the beam.

Production Target, Production Solenoid

After transiting the extinction system the proton pulses are delivered to the production target located in the evacuated warm bore of a high-field superconducting solenoid (referred to as the production solenoid). The proton beam deflects in the magnetic field of the solenoid before striking the production target, complicating the final focus beamline optics and steering. The production target is a radiatively cooled tungsten rod about the size and shape of a pencil.

Beam Absorber, Extinction Monitor

Not all of the proton beam interacts in the production target. The unspent beam is absorbed in an air-cooled beam absorber downstream of the production target. The extinction monitor, located above the beam absorber, will measure scattered protons as a function of time to provide a statistical measure of the residual beam between pulses that traverses the extinction system. The proton delivery scheme is shown in Figure 1. The Mu2e proton beam requirements are described in [1].

General Requirements

Most of the infrastructure required to deliver proton beam to the Mu2e production target already exists or will exist before Mu2e needs it. The g-2 experiment, scheduled to take data before Mu2e, requires much of the same infrastructure. To satisfy the common needs of both projects a program to develop a Muon Campus through a series of Accelerator Improvement Plans (AIP) and General Plant Projects (GPP) has been initiated. The accelerator infrastructure required exclusively by Mu2e is part of the Mu2e Project and includes:

• Resonant Extraction System
• MHz Delivery Ring RF system
• Mu2e external beamline
• Extinction System
• Extinction Monitor System
• Production Target
• Radiation Safety and Shielding
• Beamline instrumentation and controls
• Diagnostic Beam Absorber
• Proton Target Beam Absorber.

These elements are described in detail in this Technical Design Report [2].

[1] R. Bernstein et al., “Mu2e Proton Beam Requirements,” Mu2e-doc-1105.
[2] Mu2e Mu2e Technical Design Report Mu2e-doc-4299.
[3] S. Werkema, "Mu2e Proton Beam Longitudinal Structure," Mu2e-doc-2771.
[4] E. Prebys and D. Still, "Mu2e AC Dipole and Dipole Power Supply Specifications," Mu2e-doc-12113