Arne Freyberger

CEBAF energy recovery experiment

A successful GeV scale energy recovery demonstration with high ratio of accelerated-to-recovered energies (50:1) was recently carried out on the CEBAF recirculating linear accelerator. Future high energy (multi-GeV), high current (hundreds of milli-Amperes) beams would require gigaWatt-class RF systems in conventional linacs - a prohibitively expensive proposition. However, invoking energy recovery alleviates extreme RF power demands; required RF power becomes nearly independent of beam current, which improves linac efficiency and increases cost effectiveness. Furthermore, energy recovering linacs promise efficiencies of storage rings, while maintaining beam quality of linacs: superior emittance and energy spread and short bunches (sub-pico sec.). Finally, energy recovery alleviates shielding, if the beam is dumped below the neutron production threshold. Jefferson Lab has demonstrated its expertise in the field of Energy Recovery Linacs (ERLs) with the successful operation of the Infrared FEL, where 5 mA of average beam current have been accelerated up to 50 MeV and the energy stored in the beam was recovered via deceleration and given back to the RF power source. To date this has been the largest scale demonstration of energy recovery.

Beam characterization in the CEBAF-ER experiment

Energy recovering a 1 GeV beam through CEBAF (Continuous Electron Beam Accelerator Facility) presents many operational challenges. As a result, it is important to have a quantitative understanding of the beam behavior throughout the machine. The emittance provides a figure of merit in this context inasmuch as it characterizes the extent to which beam quality is preserved during energy recovery. A solution to the problem of obtaining a highresolution emittance measurement in the extraction region of the CEBAF-ER experiment (CEBAF with Energy Recovery) is presented. The method makes use of a single scanning quadrupole and a downstream wire scanner. In addition, by using multiple wire scans, a scheme for measuring the emittance and momentum spread of the first pass beam in the injector and Arcs 1 and 2 was implemented. And by using a novel technique employing wire scans in conjunction with PMTs (Photomultiplier Tubes) to accurately measure the beam profile at the dump, we can quantify the extent to which we have successfully transported beam to the energy recovery dump.

Feasibility of near-field ODR imaging of multi-GeV electron beams at CEBAF

We have evaluated the feasibility of using the optical diffraction radiation (ODR) generated as a 1- to 6-GeV CW electron beam passes nearby the edge of a single metal conducting plane as a nonintercepting (NI) relative beam size monitor for CEBAF. Previous experiments were successfully done using near-field imaging on the lower-current, 7-GeV beam at APS, and an analytical model was developed for near-field imaging. Calculations from this model indicate sufficient beam size sensitivity in the ODR profiles for beam sizes in the 30- to 50- micron regime as found in the transport lines of CEBAF before the experimental targets. With anticipated beam currents of 100 muA, the ODR signal from the charge integrated over the video field time should be -500 times larger than in the APS case. These signal strengths will allow a series of experiments to be done on beam energy dependencies, impact parameters, polarization effects, and wavelength effects that should further elucidate the working regime of this technique and test the model. Plans for the diagnostics station that will provide reference optical transition radiation (OTR) images will also be described.

Beam Physics for the 12 GeV Cebaf Upgrade Project

Beam physics aspects of the 12 GeV Upgrade of CEBAF are presented. The CEBAF Upgrade to 12 GeV is achieved via 5.5 recirculations through the linacs, and the installation of 10 new high-gradient cryomodules. A new experimental hall, Hall D, is envisioned at the end of the North Linac. Simulation results for a straight-ahead and a recirculated injector are summarized and compared. Beam transport designs are discussed and evaluated with respect to matching and beam breakup (BBU) optimization. Effects of synchrotron radiation excitation on the beam properties are calculated. BBU simulations and derived specifications for the damping of higher order modes of the new 7-cell cavities are presented. The energies that provide longitudinal polarization in multiple experimental halls simultaneously are calculated. Finally, detailed optics of the Hall D transport line has been obtained.

Energy Spread Monitoring for the JLAB Experimental Program: Synchrotron Light Interferometers, Optical Transition Radiation Monitors, and Wire Scanners

The hypernuclear physics program at JLAB requires an electron beam with small transverse size (σ ∼ 100 μm) and an upper limit on the RMS energy spread of δEE < 3 × 10−5. To measure and monitor these parameters, a beam size and energy spread measurement system has been created. The system consists of a set of wire scanners, Optical Transition Radiation (OTR) detectors, and Synchrotron Light Interferometers (SLI). The energy spread is measured via a set of wire scans performed at specific locations in the transport line, which is an invasive process. During physics operation the energy spread is monitored continuously with the OTR and/or the SLI. These devices are non‐invasive [or nearly non‐invasive in the case of OTR] and operate over a very wide range of beam energies (1–6 GeV) and currents (∼100 μA down to few μA). All components of this system are automated in an EPICS accelerator control environment. The paper presents our operational experience with the beam size and energy spread measurement system ...

The CEBAF e+ Footprint

The Continuous Electron Beam Accelerator Facility (CEBAF) at the Jefferson Laboratory (JLAB) is capable of accelerating e− to 6 GeV in energy. Presently CEBAF is being upgraded to a maximum energy of 12 GeV. In addition to e− scattering, the user community has expressed interest in performing e+ scattering experiments with the upgraded CEBAF accelerator. This paper describes the existing and planned CEBAF accelerator complex, possible e+ production locations and the expected e+ beam qualities. Possibilities for production of e+ at the JLAB free electron laser (FEL) is also briefly described.

Admittance Test and Conceptual Study of a CW Positron Source for CEBAF

A conceptual study of a Continuous Wave (CW) positron production is presented in this paper. The Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab (JLAB) operates with a CW electron beam with a well-defined emittance, time structure and energy spread. Positrons created via bremsstrahlung photons in a high-Z target emerge with a large emittance compared to incoming electron beam. An admittance study has been performed at CEBAF to estimate the maximum beam phase space area that can be transported in the LINAC and in the Arcs. A positron source is described utilizing the CEBAF injector electron beam, and directly injecting the positrons into the CEBAF LINAC.

Simulation of a cw positron source for cebaf

A positron source for the 6 GeV (and 12 GeV upgrade) recirculating linacs at Jefferson Lab is presented. The proposed 100 nA CW positron source has several unique characteristics; high incident electron beam power (100 kW), 10 MeV/c incident electron beam momentum,CW incident beam and CW production. Positron production with 10 MeV/c electrons has several advantages; the energy is below neutron threshold activation so the production target and the optical system will not become activated during use; CEBAF requires a very low energy spread, so the absolute energy spread is bounded by the low incident energy. These advantages are offset by the large angular distribution of the outgoing positrons. Results of simulations of the positron production and capture are presented. Energy flow, power deposition and thermal management of the elements present a challenge and are included in the simulations.

Mechanical and Thermal Design of the CEBAF Hall A Beam Calorimeter

A calorimeter is being fabricated to provide 0.5% - 1.0% absolute measurement of the beam current in the Hall A end station of the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab (JLAB). Modern powder metallurgy processes have produced high density, high thermal conductivity tungsten-copper composite materials that minimize electromagnetic and hadronic energy loss while maintaining a rapid thermal response time. Heat leaks are minimized by mounting the mass in vacuum on glass ceramic mounts. A conduction cooling scheme utilizes an advanced carbon fiber compliant thermal interface material. Transient finite difference and finite element models were developed to estimate heat leaks and thermal response times.