John M. Byrd

First infrared beamlines at the ALS: final commissioning and new end stations

The design and initial commissioning of the first IR beamline at the ALS has been described previously. We report the final commissioning and first results of the mid-IR spectromicroscopy beamline 1.4.3. In addition, several improvements and two new branchlines are presented. Beamline 1.4.2 is connected to the front end under vacuum and consists of a Bruker Rapid- and Step-Scan vacuum FTIR bench. The modulated light is then coupled into a UHV surface science chamber for grazing incidence reflection studies. Several more external ports are available from the Bruker bench. Beamline 1.4.1 receives light from a separate port on the beamline 1.4 front end and connects to an optical table for photoluminescence and other experiments using photons with energies up to 6 eV.

Design Studies for a VUV–Soft X-ray Free-Electron Laser Array

Several recent reports have identified the scientific requirements for a future soft X-ray light source [1, 2, 3, 4, 5], and a high-repetition-rate free-electron laser (FEL) facility responsive to them is being studied at Lawrence Berkeley National Laboratory (LBNL) [6]. The facility is based on a continuous-wave (CW) superconducting linear accelerator with beam supplied by a high-brightness, high-repetition-rate photocathode electron gun operating in CW mode, and on an array of FELs to which the accelerated beam is distributed, each operating at high repetition rate and with even pulse spacing. Dependent on the experimental requirements, the individual FELs may be configured for either self-amplified spontaneous emission (SASE), seeded high-gain harmonic generation (HGHG), echo-enabled harmonic generation (EEHG), or oscillator mode of operation, and will produce high peak and average brightness X-rays with a flexible pulse format ranging from sub-femtoseconds to hundreds of femtoseconds. This new light source would serve a broad community of scientists in many areas of research, similar to existing utilization of storage ring based light sources.

Generation of Picosecond X-Ray Pulses in the ALS Using RF Orbit Deflection

A scheme is proposed for producing ps length pulses of x-ray radiation from the Advanced Light Source (ALS) using two RF deflecting cavities. The cavities create vertical displacements of electrons correlated with their longitudinal position in the bunch. The two cavities separated by 180 degrees of vertical phase advance. This allows the vertical kick from one cavity to be compensated by the vertical kick of the other. The location of the cavities corresponds to the end of one straight section and the beginning of the following straight section. Halfway between the cavities a bending magnet source is located. The radiation from the bend can be compressed to ∼ 1 ps in duration.

Design Studies for a High-Repetition-Rate FEL Facility at LBNL

Lawrence Berkeley National Laboratory (LBNL) is working to address the needs of the primary scientific Grand Challenges now being considered by the U.S. Department of Energy, Office of Basic Energy Sciences: we are exploring scientific discovery opportunities, and new areas of science, to be unlocked with the use of advanced photon sources. A partnership of several divisions at LBNL is working to define the science and instruments needed in the future. To meet these needs, we propose a seeded, high-repetition-rate, free-electron laser (FEL) facility. Temporally and spatially coherent photon pulses, of controlled duration ranging from picosecond to sub-femtosecond, are within reach in the vacuum ultraviolet (VUV) to soft X-ray regime, and LBNL is developing critical accelerator physics and technologies toward this goal. We envision a facility with an array of FELs, each independently configurable and tunable, providing a range of photon-beam properties with high average and peak flux and brightness.

Observing beam motion using infrared interferometry

Phase noise in the RF master oscillator driving synchrotron oscillations of the beam has been identified as one of the dominant sources of noise in the infrared beamline at the Advanced Light Source. We present measurements of the effect of the electron beam motion in a Fourier transform interferometer (FTIR) detector. This form of detector may be sensitive to very small beam motions.

Operation and performance of a longitudinal feedback system using digital signal processing

A programmable longitudinal feedback system using a parallel array of AT&T 1610 digital signal processors has been developed as a component of the PEP‐II R&D program. This system has been installed at the Advanced Light Source (LBL) and implements full speed bunch by bunch signal processing for storage rings with bunch spacing of 4 ns. Open and closed loop results showing the action of the feedback system are presented, and the system is shown to damp coupled‐bunch instabilities in the ALS. A unified PC‐based software environment for the feedback system operation is also described.

Design and implementation of IIR algorithms for control of longitudinal coupled-bunch instabilities

The recent installation of third-harmonic RF cavities at the Advanced Light Source has raised instability growth rates, and also caused tune shifts (coherent and incoherent) of more than an octave over the required range of beam currents and energies. The larger growth rates and tune shifts have rendered control by the original bandpass FIR feedback algorithms unreliable. In this paper we describe an implementation of an IIR feedback algorithm offering more flexible response tailoring. A cascade of up to 6 second-order IIR sections (12 poles and 12 zeros) was implemented in the DSPs of the longitudinal feedback system. Filter design has been formulated as an optimization problem and solved using constrained optimization methods. These IIR filters provided 2.4 times the control bandwidth as compared to the original FIR designs. Here we demonstrate the performance of the designed filters using transient diagnostic measurements from ALS and DAΦNE.

A model for producing stable, broadband terahertz coherent synchrotron radiation in storage rings

We present a model for producing stable broadband coherent synchrotron radiation (CSR) in the terahertz frequency region in an electron storage ring. The model includes distortion of bunch shape from the synchrotron radiation (SR), enhancing higher frequency coherent emission and limits to stable emission due to a microbunching instability excited by the SR. We use this model to optimize the performance of a source for CSR emission.

Nonlinear longitudinal dynamics studies at the ALS

We present an account of our efforts to understand unexpected observations in the course of performing measurements of the longitudinal beam transfer function. As the amplitude of excitation was increased, we observed a notch in the middle of the peak amplitude response of the transfer function. Our observations are explained by a bifurcation in the amplitude due to the nonlinearity of the oscillations, demonstrated by measurements of the longitudinal bunch profile using a streak camera.

A 20fs synchronization system for lasers and cavities in accelerators and FELs

A fiber-optic RF distribution system has been developed for synchronizing lasers and RF plants in short pulse FELs. Typical requirements are 50-100fs rms over time periods from 1ms to several hours. Our system amplitude modulates a CW laser signal, senses fiber length using an interferometer, and feed-forward corrects the RF phase digitally at the receiver. We demonstrate less than 15fs rms error over 12 hours, between two independent channels with a fiber path length difference of 200m and transmitting S-band RF. The system is constructed using standard telecommunications components, and uses regular telecom fiber.