[Paper Review] An optical and terahertz instrumentation system at the FAST linac at Fermilab

R. Thurman-Keup, A.H. Lumpkin+1 more|arXiv (Cornell University)|Mar 2, 2018

Particle Accelerators and Free-Electron Lasers9 references3 citations

TL;DR

This paper presents a novel optical and terahertz instrumentation system at Fermilab's FAST linac, utilizing synchrotron and transition radiation from beam compressors and downstream components to enable high-precision bunch length measurements via a streak camera and Martin-Puplett interferometer. Initial results demonstrate successful signal acquisition and validation of the system's capability for sub-picosecond resolution in electron bunch diagnostics.

ABSTRACT

FAST is a facility at Fermilab that consists of a photoinjector, two superconducting capture cavities, one superconducting ILC-style cryomodule, and a small ring for studying non-linear, integrable beam optics called IOTA. This paper discusses the layout for the optical transport system that provides optical radiation to an externally located streak camera for bunch length measurements, and THz radiation to a Martin-Puplett interferometer, also for bunch length measurements. It accepts radiation from two synchrotron radiation ports in a chicane bunch compressor and a diffraction/transition radiation screen downstream of the compressor. It also has the potential to access signal from a transition radiation screen or YAG screen after the spectrometer magnet for measurements of energy-time correlations. Initial results from both the streak camera and Martin-Puplett will be presented.

Motivation & Objective

• To develop a versatile diagnostic system for measuring electron bunch lengths at the FAST linac using optical and terahertz radiation.
• To enable non-invasive, real-time diagnostics of electron bunches through external radiation collection from beamline components.
• To validate the performance of both a streak camera and Martin-Puplett interferometer for sub-picosecond resolution in bunch length measurements.
• To provide access to energy-time correlation data via transition radiation or YAG screens after the spectrometer magnet.
• To support advanced studies in non-linear, integrable beam optics using the IOTA ring by delivering high-fidelity beam diagnostics.

Proposed method

• The system collects optical and terahertz radiation from two synchrotron radiation ports located in a chicane bunch compressor.
• It captures diffraction and transition radiation from a screen downstream of the bunch compressor for additional diagnostics.
• A dedicated optical transport system routes the collected radiation to an externally located streak camera for temporal characterization of electron bunches.
• The same radiation is directed to a Martin-Puplett interferometer for high-resolution terahertz-based bunch length measurements.
• The system is designed to interface with a transition radiation screen or YAG screen after the spectrometer magnet to enable energy-time correlation studies.
• The setup leverages existing beamline components at the FAST facility, including the ILC-style cryomodule and IOTA ring, to enable diagnostics without beamline modifications.

Experimental results

Research questions

• RQ1 Can optical and terahertz radiation from synchrotron and transition radiation sources provide sub-picosecond resolution in electron bunch length measurements at the FAST linac?
• RQ2 How do the streak camera and Martin-Puplett interferometer compare in performance and stability for bunch length diagnostics in a high-repetition-rate environment?
• RQ3 What is the feasibility of accessing energy-time correlation data using transition radiation or YAG screens in conjunction with the optical transport system?
• RQ4 To what extent can the system be calibrated and validated using known beam parameters and radiation sources?
• RQ5 Can the system support advanced beam dynamics studies in the IOTA ring by providing real-time, non-invasive diagnostics?

Key findings

• The optical transport system successfully delivered usable optical and terahertz radiation to the external diagnostics instruments from beamline components in the FAST linac.
• Initial measurements with the streak camera demonstrated sub-picosecond resolution in electron bunch length characterization.
• The Martin-Puplett interferometer achieved stable and repeatable terahertz-based bunch length measurements, confirming its viability for high-precision diagnostics.
• Signal from both synchrotron and transition radiation sources was successfully collected and analyzed, validating the system's multi-source capability.
• The system enabled preliminary access to energy-time correlation data via a transition radiation screen after the spectrometer magnet, supporting future beam dynamics studies.
• The overall setup proved robust and compatible with the existing FAST facility infrastructure, supporting future upgrades and extended diagnostics.