[Paper Review] Measurement of the six Dimensional Phase Space at the New GSI High Current Linac

P. Forck, F. Heymach+3 more|ArXiv.org|Aug 17, 2000

Particle accelerators and beam dynamics11 citations

TL;DR

This paper presents a novel, non-destructive diagnostic system for measuring the six-dimensional phase space of high-current ion beams at the GSI High Current Linac. Using a time-of-flight technique with a CVD diamond detector and a coincidence method with secondary electrons, the system achieves 25 ps timing resolution to map longitudinal bunch structure and emittance, revealing significant current-dependent filamentation and energy spread broadening due to space charge effects.

ABSTRACT

For the characterization of the 10 mA ion beam delivered by the new High Current Linac at GSI, sophisticated, mainly non-intersepting diagnostic devices were developed. Besides the general set-up of a versatile test bench, we discuss in particular bunch shape and emittance measurements. A direct time-of-flight technique with a diamond particle detector is used to observe the micro-bunch distribution with a resolution of $\sim 25 $ ps equals 0.3$^{o}$ in phase. For the determination of the energy spread a coincidence technique is applied, using secondary electrons emitted by the ion passing through an aluminum foil 80 cm upstream of the diamond detector. The transverse emittance is measured within one macro pulse with a pepper-pot system equipped with a high performance CCD camera.

Motivation & Objective

• To develop non-destructive diagnostics for characterizing the six-dimensional phase space of high-current ion beams at the GSI High Current Linac.
• To measure longitudinal bunch structure with sub-ns resolution to validate beam dynamics simulations and optimize Linac performance.
• To determine longitudinal emittance using a coincidence technique between ion arrival at a diamond detector and secondary electron emission from an aluminum foil.
• To measure transverse emittance within a single macro-pulse using a high-speed pepper-pot system with a CCD camera, enabling real-time diagnostics under high beam power.
• To investigate space charge effects on beam quality by comparing low- and high-current beam measurements.

Proposed method

• A time-of-flight system with a CVD diamond detector and double-threshold discriminator measures ion arrival times with 25 ps resolution relative to the 36 MHz RF cycle.
• Ion bunch shape is reconstructed by scattering 1+ ions at 2.5° in a 210 μg/cm² tantalum foil, attenuating beam current by 10⁻⁸ to avoid detector saturation.
• Longitudinal emittance is measured via a coincidence technique: ion arrival at the diamond detector is correlated with secondary electron emission from a 15 μg/cm² Al foil 80 cm upstream.
• Transverse emittance is measured using a pepper-pot system with a 45×45 mm² copper plate containing 15×15 holes (0.1 mm diameter), followed by beamlet imaging on a sapphire screen via a HeNe laser.
• Beam profiles are captured by a 12-bit CCD camera (PCO SensiCam) and processed with background subtraction and calibration to correct for mechanical uncertainties.
• Phase space distributions are reconstructed from time-of-flight and electron emission data, with projections used for emittance calculation.

Experimental results

Research questions

• RQ1 How does beam current affect longitudinal bunch structure and emittance in high-current ion beams at 1.4 MeV/u?
• RQ2 To what extent do space charge forces cause beam filamentation and energy spread broadening in the absence of RF damping?
• RQ3 Can a coincidence technique using secondary electrons and diamond detectors achieve sufficient time resolution (≤25 ps) to measure longitudinal phase space with high precision?
• RQ4 How do inhomogeneities in the gas stripper affect the measured energy spread and phase space distribution?
• RQ5 What is the impact of detector geometry and field non-uniformity on the accuracy of longitudinal emittance measurements?

Key findings

• The system achieved a timing resolution of 25 ps, corresponding to a phase width of 0.3°, enabling high-precision longitudinal phase space measurements.
• For a 0.1 mA Ar⁺ beam, the measured longitudinal emittance was consistent with simulations, while for a 5 mA beam, strong filamentation and increased emittance were observed due to space charge effects.
• The energy spread increased from 1.7% (FWHM) at low current to 2.8% at high current (5 mA), with a clear correlation in the phase space plot, indicating beam instability.
• The measured energy spread was larger by a factor of 2 than predicted by particle tracking simulations, likely due to inhomogeneities in the accelerating field for secondary electrons and detector timing jitter.
• The pepper-pot system successfully measured transverse emittance within a single macro-pulse, with beamlets imaged on a sapphire screen and processed via CCD camera with background subtraction.
• A 5% background level in the pepper-pot image was attributed to scattered light, requiring correction to avoid systematic errors in emittance calculation.