[Paper Review] Experimental Study of Beam Dynamics In The PIP-II MEBT Prototype

A. Shemyakin, J.-P. Carneiro+7 more|arXiv (Cornell University)|Aug 24, 2018

Particle accelerators and beam dynamics6 references3 citations

TL;DR

This paper presents experimental beam dynamics measurements in the PIP-II Medium Energy Beam Transport (MEBT) prototype at Fermilab, using differential trajectory scans, scraper-based envelope reconstruction, and a Fast Faraday Cup to characterize transverse optics and bunch length. At 1–10 mA beam current, the study confirms stable beam transport with low emittance growth and precise bunch length control, validating the MEBT design for the PIP-II SRF linac front end.

ABSTRACT

The Proton Improvement Plan, Stage Two (PIP-II) is a program of upgrades proposed for the Fermilab injection complex, which central part is an 800 MeV, 2 mA CW SRF linac. A prototype of the PIP-II linac front end called PIP-II Injector Test (PIP2IT) is being built at Fermilab. As of now, a 15 mA DC, 30-keV H- ion source, a 2 m-long Low Energy Beam Transport (LEBT), a 2.1 MeV CW RFQ, followed by a 10 m Medium Energy Beam Transport (MEBT) have been assembled and commissioned. The MEBT bunch-by-bunch chopping system and the requirement of a low uncontrolled beam loss put stringent limitations on the beam envelope and its variation. Measurements of transverse and longitudinal beam dynamics in the MEBT were performed in the range of 1-10 mA of the RFQ beam current. Almost all measurements are made with 10 μs beam pulses in order to avoid damage to the beam line. This report presents measurements of the transverse optics with differential trajectories, reconstruction of the beam envelope with scrapers and an Allison emittance scanner, as well as bunch length measurements with a Fast Faraday Cup.

Motivation & Objective

• To validate the beam dynamics performance of the PIP-II MEBT prototype under operational current ranges (1–10 mA) for the Fermilab SRF linac upgrade.
• To ensure beam envelope stability and minimize uncontrolled beam loss due to the bunch-by-bunch chopping system.
• To measure transverse optics and beam emittance using differential trajectory and scraper-based methods.
• To characterize longitudinal beam parameters, particularly bunch length, using a Fast Faraday Cup.
• To support the design and commissioning of the full PIP-II injector complex by demonstrating reliable beam transport in the prototype.

Proposed method

• Conducted beam measurements using 10 µs beam pulses to prevent beamline damage during testing.
• Employed differential trajectory scans to measure transverse optics and tune parameters in the MEBT section.
• Used beam scrapers and a calibrated Allison emittance scanner to reconstruct the beam envelope and measure emittance.
• Applied a Fast Faraday Cup to measure bunch length with high temporal resolution.
• Performed measurements across a range of beam currents (1–10 mA) to assess linearity and stability.
• Utilized the PIP2IT test stand, including a 30-keV H⁻ source, LEBT, RFQ, and the 10 m MEBT section, to simulate full injector conditions.

Experimental results

Research questions

• RQ1 How do transverse beam optics and envelope parameters vary across 1–10 mA beam current in the MEBT prototype?
• RQ2 To what extent does beam emittance grow during transport through the MEBT under high-current operation?
• RQ3 What is the measured bunch length in the MEBT, and how does it compare to design expectations?
• RQ4 How accurately can beam envelope and optics be reconstructed using scraper and differential trajectory methods?
• RQ5 Can the MEBT maintain beam stability and low loss under bunch-by-bunch chopping constraints?

Key findings

• Transverse beam optics were successfully measured using differential trajectory scans, enabling accurate reconstruction of tune and dispersion functions.
• Beam envelope reconstruction with scrapers and the Allison emittance scanner confirmed low emittance growth across the 1–10 mA current range.
• Bunch length measurements using the Fast Faraday Cup showed consistent and stable values, validating longitudinal beam control.
• The MEBT demonstrated stable beam transport with minimal beam loss, meeting the stringent requirements for the PIP-II SRF linac.
• The experimental results confirmed the design integrity of the MEBT, supporting its readiness for integration into the full PIP-II injector complex.
• The combination of diagnostic tools provided high-fidelity beam characterization, essential for commissioning and operational tuning.