Herman Winick

Research and development toward a 4.5−1.5 Å linac coherent light source (LCLS) at SLAC

Abstract In recent years significant studies have been initiated on the feasibility of utilizing a portion of the 3 km S-band accelerator at SLAC to drive a short wavelength (4.5−1.5 A) Linac Coherent Light Source (LCLS), a Free-Electron Laser (FEL) operating in the Self-Amplified Spontaneous Emission (SASE) regime. Electron beam requirements for single-pass saturation in a minimal time include: 1) a peak current in the 7 kA range, 2) a relative energy spread of e = λ 4π , where λ[m] is the output wavelength. Requirements on the insertion device include field error levels of 0.02% for keeping the electron bunch centered on and in phase with the amplified photons, and a focusing beta of 8 m/rad for inhibiting the dilution of its transverse density. Although much progress has been made in developing individual components and beam-processing techniques necessary for LCLS operation down to ∼20 A, a substantial amount of research and development is still required in a number of theoretical and experimental areas leading to the construction and operation of a 4.5−1.5 A LCLS. In this paper we report on a research and development program underway and in planning at SLAC for addressing critical questions in these areas. These include the construction and operation of a linac test stand for developing laser-driven photocathode rf guns with normalized emittances approaching 1 mm-mrad; development of advanced beam compression, stability, and emittance control techniques at multi-GeV energies; the construction and operation of a FEL Amplifier Test Experiment (FATE) for theoretical and experimental studies of SASE at IR wavelengths; an undulator development program to investigate superconducting, hybrid/permanent magnet (hybrid/PM), and pulsed-Cu technologies; theoretical and computational studies of high-gain FEL physics and LCLS component designs; development of X-ray optics and instrumentation for extracting, modulating, and delivering photons to experimental users; and the study and development of scientific experiments made possible by the source properties of the LCLS.

A Permanent Magnet Undulator for SPEAR

A 30 period permanent magnet (SmCo5) undulator has been designed, built and tested. The period is 6.1 cm, overall length is 1.95 m, and the gap is variable from 2.7 cm to 6.0 cm. Magnetic measurements at the midplane with a 2.7 cm gap show that the field is sinusoidal with a peak value of .28 T. Construction details and magnetic measurements are presented along with the spectral distribution of radiation produced by 3.0 GeV electrons traversing the undulator.

Wiggler and Undulator Magnets

To scientists using vacuum ultraviolet and x rays the most important characteristics of an ideal radiation source would be a high intensity within a small solid angle and a high intensity within a small wavelength interval, both extending over a broad range of wavelengths. High spatial brightness (large flux within a small solid angle) permits the delivery of a large number of photons per second to a small sample. High spectral brightness (large flux within a narrow wavelength interval) is essential for high‐resolution spectroscopy. A high‐power tunable vuv and x‐ray laser would be ideal, but unfortunately such a laser does not yet exist. Conventional vuv sources (such as gas‐discharge lamps) and x‐ray sources (such as electron‐impact x‐ray tubes) can produce a large flux of radiation, most of which is indeed within a narrow bandwidth at particular fluorescent lines. However, the flux is diffused over a large solid angle and the wavelength is fixed. The continuum radiation from these sources is less inten...

Properties of Synchrotron Radiation

Synchrotron radiation has several important properties including: 1. High intensity 2. Broad spectral range 3. High polarization 4. Pulsed time structure 5. Natural collimation In addition, synchrotron radiation produced by storage rings (rather than synchrotrons) offers: 6. High-vacuum environment 7 Small-source-spot size 8. Stability

The LCLS: A fourth generation light source using the SLAC linac

Recent technological developments make it possible to consider use of the Stanford linear accelerator to drive a linac coherent light source (LCLS)—a laser operating at hard x‐ray wavelengths. In the LCLS, stimulated emission of radiation would be achieved in a single pass of a high‐energy, extremely bright electron beam through an undulator, without the optical cavity resonator normally used in storage ring‐based free‐electron lasers. The x‐ray laser beam would be nearly diffraction limited with very high transverse coherence, and would exhibit unprecedented peak intensity and peak brightness, and sub‐picosecond pulse length. Such an x‐ray source offers unique capabilities for a large number of scientific applications.

Microwave measurements of the BNL/SLAC/UCLA 1.6 cell photocathode RF gun

The longitudinal accelerating field E/sub z/ has been measured as a function of azimuthal angle in the full cell of the cold test model for the 1.6 cell BNL/SLAC/UCLA 3 S-band RF gun using a needle rotation/frequency perturbation technique. These measurements were conducted before and after symmetrizing the full cell with a vacuum pump out port and an adjustable short. Two different waveguide to full cell coupling schemes were studied. The dipole mode of the full cell is an order of magnitude less severe before symmetrization for the /spl theta/-coupling scheme. The multi-pole contribution to the longitudinal field asymmetry are calculated using standard Fourier series techniques. The Panofsky-Wenzel theorem is used in estimating the transverse emittance due to the multipole components of E/sub z/.

A 2 to 4 nm high power FEL on the SLAC linac

Abstract We report the results of preliminary studies of a 2 to 4 nm SASE FEL, using a photoinjector to produce the electron beam, and the SLAC linac to accelerate it to an energy up to 10 GeV. Longitudinal bunch compression is used to increase ten fold the peak current to 2.5 kA, while reducing the bunch length to the subpicosecond range. The saturated output power is in the multi-gigawatt range, producing about 1014 coherent photons within a bandwidth of about 0.2% rms, in a pulse of several millijoules. At 120 Hz repetition rate the average power is about 1 W. The system is optimized for X-ray microscopy in the water window around 2 to 4 nm, and will permit imaging a biological sample in a single subpicosecond pulse.

Synchrotron radiation as a new tool within photon‐beam technology

Synchrotron radiation is now playing an increasingly important role in recent developments of new light sources. Synchrotron light emitted from a relativistic electron beam has a radiation pattern which makes it a unique source. The advantages with this type of radiation can be summarized as (a) continuous spectrum extending from the ir to the x‐ray region, (b) strongly polarized, (c) highly collimated, (d) pulsed structure allowing time‐resolution spectroscopy, and (e) high intensity making feasible the use of monochromators with narrow band pass. The Stanford Synchrotron Radiation Project has been in operation since May 1974 as a U.S. National Facility for uv and x‐ray research in many disciplines using the radiation from the storage ring SPEAR at the Stanford Linear Accelerator Center. The radiation spectrum is characterized by the critical energy which varies as E3 (E=electron‐beam energy) and is 11 keV for E=4 GeV. Useful flux is available out for approximately five times the critical energy. Five mo...

Sulfur K-edge X-ray absorption studies using the 54-pole wiggler at SSRL in undulator mode

Abstract A series of experiments have been conducted to further characterize and utilize the 54-pole wiggler at SSRL in undulator mode. The magnetic field was varied between 1.45 and 5.0 kG. Below 2.0 kG the two graphite heat absorbing filters in the beam line could be safely removed, providing high flux and brightness in the 2.3–3 keV region of the electromagnetic spectrum. Spectral distribution and intensity as a function of magnetic field and methods for harmonic rejection were evaluated. With a magnetic field of 1.45 kG, i.e. adjusted to position the third harmonic of the undulator spectrum at the sulfur K edge, X-ray absorption spectra having excellent signal-to-noise ratio were recorded for a 5 mM aqueous sulfate solution, for intact living vanadocyte blood cells, and for the protein ferredoxin. These results clearly demonstrate the significant new experimental opportunities provided by the undulator in the aquisition of data otherwise difficult to obtain, such as e.g. sulfur K edge X-ray absorption spectra of dilute solutions and biological substances.

Emittance studies of the BNL/SLAC/UCLA 1.6 cell photocathode RF gun

The symmetrized 1.6 cell S-band photocathode gun developed by the BNL/SLAC/UCLA collaboration is in operation at the Brookhaven Accelerator Test Facility (ATF). A novel emittance compensation solenoid magnet has also been designed, built and is in operation at the ATF. These two subsystems form an emittance compensated photoinjector used for beam dynamics, advanced acceleration and free electron laser experiments at the ATF. The highest acceleration field achieved on the copper cathode is 150 MV/m, and the guns normal operating field is 130 MV/m. The maximum rf pulse length is 3 /spl mu/s. The transverse emittance of the photoelectron beam were measured for various injection parameters. The 1 nC emittance results are presented along with electron bunch length measurements that indicated that at above the 400 pC, space charge bunch lengthening is occurring. The thermal emittance, /spl epsiv//sub 0/, of the copper cathode has been measured.