Paul R. Bolton

Slice emittance measurements at the SLAC gun test facility

Abstract A goal of the Gun Test Facility (GTF) at SLAC is to investigate the production of high-brightness electron beams for the Linac Coherent Light Source (LCLS) X-ray FEL. High brightness in the RF photocathode gun occurs when the time-sliced emittance is nearly the same as the cathode thermal emittance and when the slices are all lined up, i.e., their Twiss parameters are nearly identical. In collaboration with the BNL Source Development Lab (SDL), we have begun a systematic study of the slice emittance at GTF. The technique involves giving the bunch a near linear energy chirp using the booster linac and dispersing it with a magnetic spectrometer. Combined with knowledge of the longitudinal phase space, this establishes the energy–time correlation on the spectrometer screen. The slice emittances are determined by varying the strengths of the quadrupoles in front of the spectrometer. Spectrometer images for a range of quadrupole settings are then binned into small energy/time windows and analysed for the slice emittance and Twiss parameters. Results for various gun parameters are presented.

ABCD formalism and attosecond few-cycle pulse via chirp manipulation of a seeded free electron laser

An ABCD formalism is identified to characterize a seeded Free Electron Laser (FEL) with three chirps: an initial frequency chirp in the seed Laser, an energy chirp in the electron bunch, and an intrinsic frequency chirp due to the FEL process. A scheme of generating attosecond few-cycle pulses is proposed by invoking an FEL seeded by high-order harmonic generation (HHG) from an infrared laser. The HHG seed has generic attosecond structure. It is possible to manipulate these three chirps to maintain the attosecond structure via post-undulator chirped pulse compression.

Free electron laser seeded by ir laser driven high-order harmonic generation

Coherent x-ray production by a seeded free electron laser (FEL) is important for next generation synchrotron light sources. The authors examine the feasibility and features of FEL emission seeded by a high-order harmonic generation (HHG) of an infrared laser. In addition to the intrinsic FEL chirp, the longitudinal profile and spectral bandwidth of the HHG seed are modified significantly by the FEL interaction well before saturation. This smears out the original attosecond pulselet structure. The authors introduce criteria for this smearing effect on the pulselet and the stretching effect on the entire pulse. They discuss the noise issue in such a seeded FEL.

New Design Study and Related Experimental Program for the LCLS RF Photoinjector

We report the results of a recent beam dynamics study, motivated by the need to redesign the LCLS photoinjector, that lead to the discovery of a new effective working point for a split RF photoinjector. We consider the emittance compensation regime of a space charge beam: by increasing the solenoid strength, the emittance evolution shows a double minimum behavior in the drifting region. If the booster is located where the relative emittance maximum and the envelope waist occur, the second emittance minimum can be shifted to the booster exit and frozen at a very low level (0.3 mm-mrad for a 1 nC flat top bunch), to the extent that the invariant envelope matching conditions are satisfied. Standing Wave Structures or alternatively Traveling Wave Structures embedded in a Long Solenoid are both candidates as booster linac. A careful measurement of the emittance evolution as a function of position in the drifting region is necessary to verify the computation and to determine experimentally the proper position of the booster cavities. The new design study and supporting experimental program under way at the SLAC Gun Test Facility are discussed.

FEL research and development at the SLAC sub-picosecond photon source, SPPS

Abstract An upgrade project to the SLAC linac allows ultra-short electron bunches to be interleaved with the routine high-energy physics program operation, for use with an undulator to produce short-pulse, high-brightness X-rays. The linac upgrade comprises of the installation in the summer of 2002 of a bunch compressor chicane of similar design to the Linac Coherent Light Source (LCLS) project. A final compression stage in the high-energy Final Focus Test Beam (FFTB) line compresses the 28 GeV, 3.4 nC electron bunch to 80 fs FWHM, where a 5 m section of undulator (K = 4.45) will produce 1.5 A X-rays with 3 × 107 photons per pulse and a peak brightness of 4 × 1024 photons mm−2 mrad−2 s−1 (0.1% BW). The facility will allow us to test the dynamics and associated technology of bunch compression and gain valuable experience for the LCLS using the SLAC linac. New ultra-short electron bunch diagnostic techniques will be developed hand in hand with the same ultra-fast laser technology to be used for LCLS. Issues of high peak power (27 GW) X-ray transport and optics can be addressed at this facility as well as pump-probe and ultra-fast laser timing and stability issues.© 2003 Elsevier Science B.V. All rights reserved.PACS: 29.27Eg; 29.27Fh

Electro-Optic Sampling of Single Electron Beam Bunches of Ultrashort Duration

The effect of ultrafast electron beam bunch dynamics on single shot electro-optic sampling detection schemes is examined. It is shown that ultrashort electron bunch fields of adequate magnitude can dynamically impose additional bandwidth on laser probe pulses. The significance of this effect is evaluated by comparing the dynamics of the laser probe to that of the nonradiative field of a single electron bunch for a given crystal material. Dynamic effects can be distinguished with ultrafast temporal resolution of the transmitted probe spectrum. Furthermore, velocity matching of probe and bunch fields in a co-propagation scheme is less restrictive. Such time resolved spectra then can noninvasively determine single bunch dynamics and represent a new type of electro-optic sampling.

Analysis of slice emittance measurements for the SLAC Gun Test Facility

The Linac Coherent Light Source (LCLS) at SLAC requires the rf photo-injector to produce a beam with a normalized, projected emittance of 1 micron in a 10 ps long bunch with a charge of 1 nC. In addition, a small longitudinal emittance is needed to attain the desired 3 kiloamperes peak current after compression in two chicane bunchers. To achieve this excellent beam quality, we are performing systematic studies of both the transverse and longitudinal beam properties from the rf photocathode gun at the SLAC Gun Test Facility (GTF). Time resolved emittances (slice) are determined by using a bunch with a linear energy chirp which is dispersed by a magnetic spectrometer. By varying the strength of a quadrupole lens upstream of the spectrometer allows measurement of the individual slice emittances. Spectrometer images at the various quadrupole settings are binned in small energy/time windows and analyzed for the slice parameters. Our measurements indicate a temporal resolution of approximately 100 femtoseconds. In addition, the longitudinal phase space distribution is determined by measuring the energy spectrum over a range of linac phases. The correlated and uncorrelated components of the phase space distribution are determined by fits to the energy spectra analogous to a quad scan in the transverse dimension. The combined analysis of the transverse and longitudinal data gives not only the slice and longitudinal emittances, but also any correlations due to wakefields or other effects.

Comparison of parmela simulations with longitudinal emittance measurements at the SLAC gun test facility

At the Gun Test Facility (GTF), we have been testing an S-band RF gun similar to the one to be used in the Linac Coherent Light Source (LCLS) Photo-Injector. The beam transverse properties have been extensively characterized on that gun and it was shown that this gun is capable of providing slice emittances of less than 1 mm.mrad for 100A slices [1]. The longitudinal beam properties are now also being investigated for 2 principal reasons: (1) the transverse beam properties are correlated to the longitudinal one; an excessively large correlated energy spread at the gun exit would damage the emittance compensation; and (2) the uncorrelated rms energy spread as small as 10keV at the gun exit is required for a good lasing in the LCLS. To measure the longitudinal emittance, the booster phase scan technique has been used at the GTF as described in [2]. We have now performed simulations of this experiment to better understand the non-linear effects and to reconstitute the longitudinal emittance.

Ion Production via Optical Field Ionization of Atoms and Ions: Results from Early Work

Optical field ionization of atoms, ions and molecules rapidly progresses sequentially through higher charge states. The time‐dependent field of a single high intensity laser pulse can multiply ionize a multielectron atom during its risetime such that the peak intensity predictably ‘sees’ ions with the highest possible charge state (as determined by the peak intensity level). Common OFI models are reasonably predictive and closed form expressions for threshold and saturation laser intensities (in terms of relevant ionization potentials) bracket a useful range for ion production. They can also facilitate the design of ion sources and laser diagnostics. Results are given from the early OFI studies in gases and aluminum vapor conducted at the Lawrence Livermore National Laboratory.

Techniques for Electro‐Optic Bunch Length Measurement at the Femtosecond Level

Electro optic methods to modulate ultra‐short laser pulses using the electric field of a relativistic electron bunch have been demonstrated by several groups to obtain information about the electron bunch length charge distribution. We discuss the merits of different approaches of transforming the temporal coordinate of the electron bunch into either the spatial or frequency domains. The requirements for achieving femtosecond resolution with this technique are discussed. These techniques are being applied to the Linac Coherent Light Source (LCLS) and the Sub‐Picosecond Photon Source (SPPS) currently under construction at SLAC.