J.F. Schmerge

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.

Compact far-IR FEL design

Abstract A compact size far-infrared free-electron laser (FIR FEL) is currently being built at Stanford. A microwave gun products 1–3.3 MeV electrons, which are sent into a 50 cm long wiggler of 1 cm period through a hole on the upstream mirror to generate radiation at a wavelength of 100 to 1000 μm. A superconducting solenoid along with an array of permeable material is used to generate a 9.6 kG rms wiggler field with a 2.0 mm gap. The electron beam consists of 10 ps micropulses with 10 A peak current, 1% energy spread and unnormalized emittance for 90% of the particles of 2π mm mrad. A 10 dB small signal gain has been calculated with the parameters mentioned above. An overview of the design details as well as a discussion on the uniqueness of our wiggler are presented.

Photocathode rf gun emittance measurements using variable-length laser pulses

The Gun Test Facility (GTF) at the Stanford Linear Accelerator Center (SLAC) was created to develop an appropriate injector for the proposed Linac Coherent Light Source (LCLS) at SLAC. The LCLS design requires the injector to produce a beam with at least 1 nC of charge in a 10 ps or shorter pulse with no greater than 1 (pi) mm-mrad normalized rms emittance. The first photoinjector under study at the GTF is a 1.6 cell S-band symmetrized gun with an emittance compensation solenoid. Emittance measurements, reported here, were made as function of the transverse laser pulse shape and the Gaussian longitudinal laser pulse length. The lowest achieved emittance to data with 1 nC of charge is 5.6 (pi) mm-mrad and was obtained with both a Gaussian longitudinal and transverse pulse shape with 5 ps FWHM and 2.4 mm FWHM respectively. The measurement is in agreement with a PARMELA simulation using measured beam parameters. There are indications that the accelerator settings used in the results presented here were not optimal. Simulations indicate that a normalized emittance meeting the LCLS requirement can be obtained using appropriately shaped transverse and temporal laser/electron beam pulses. Work has begun on producing temporal flat top laser pulses which combined with transverse clipping of the laser is expected to lower the emittance to approximately 1 (pi) mm-mrad for 1 nC beams with optimal accelerator settings.

The free-electron laser as a laboratory instrument

A free-electron laser (FEL) with a component cost, including the accelerator, of approximately

Reduction of Thermal Emittance of rf Guns

300k, has lased at a wavelength of 85 /spl mu/m with /spl ap/12 ps micropulse duration, achieving a power growth four orders of magnitude greater than the coherent spontaneous emission, and with a small-signal, single-pass gain of 21%. The price is about an order of magnitude less than other FELs for the far infrared, and transforms the device from the role of a national facility to that of a laboratory instrument. Cost reduction was achieved by employing several novel features: a microwave cavity gun for the accelerator, a staggered-array wiggler, and an on-axis hole in the upstream cavity mirror for electron ingress and radiation egress. >

The design for the LCLS RF photoinjector

Abstract The transverse emittance from optimized RF photoinjectors is limited by the thermal emittance. The thermal emittance can be lowered by a factor >2 by using a semiconductor photocathode.

Preliminary emission characteristic measurements for a

Abstract We report on the design of the RF photoinjector of the Linac Coherent Light Source. The RF photoinjector is required to produce a single 150 MeV bunch of ∼1 nC and ∼100 A peak current at a repetition rate of 120 Hz with a normalized rms transverse emittance of ∼1 π mm-mrad. The design employs a 1.6-cell S-band RF gun with an optical spot size at the cathode of a radius of ∼1 mm and a pulse duration with an rms sigma of ∼3 ps. The peak RF field at the cathode is 150 MV/m with extraction 57° ahead of the RF peak. A solenoidal field near the cathode allows the compensation of the initial emittance growth by the end of the injection linac. Spatial and temporal shaping of the laser pulse striking the cathode will reduce the compensated emittance even further. Also, to minimize the contribution of the thermal emittance from the cathode surface, while at the same time optimizing the quantum efficiency, the laser wavelength for a Cu cathode should be tunable around 260 nm. Following the injection linac the geometric emittance simply damps linearly with energy growth. PARMELA simulations show that this design will produce the desired normalized emittance, which is about a factor of two lower than has been achieved to date in other systems. In addition to low emittance, we also aim for laser amplitude stability of 1% in the UV and a timing jitter in the electron beam of 0.5 ps rms, which will lead to less than 10% beam intensity fluctuation after the electron bunch is compressed in the main linac.

300k FIR FEL

Abstract If the free-electron laser is to move from the category of “national facility” to the designation of a “laboratory instrument” it must meet several conditions, including a reduction in cost. For the far infrared, an FEL can be constructed for a component cost of approximately

Electron beam characterization for a compact far-infrared free-electron laser

300 000 including the accelerator. Such a device has been assembled, using a 1 1/2 cell RF cavity gun for the accelerator and a staggered-array wiggler consisting of permeable pole pieces in the field of a superconducting solenoid. Spontaneous emission measurements have been performed, and laser gain has been observed. Measurements have been in good agreement with theory.

SLAC rf photocathode gun test facility

A compact, far-infrared free-electron laser (FIR-FEL) is operating at 85 /spl mu/m at Stanford University, where the electron beam is obtained from a 1 1/2 cell, thermionic RF (2.856 GHz) cavity gun. This gun was not designed for FEL operation, and under the condition at which it was intended to operate, with a peak acceleration gradient below 85 MV/m, it would not be suitable. We have explored new parameter ranges, and have found that at high peak gradients, from 100-140 MV/m, the gun will function satisfactorily as an FEL accelerator. For example, thirty-nine percent of the total gun current was transmitted through a 1% energy window at a beam energy of /spl gamma/=9.72. At /spl gamma/=9.56, a 304 mA macropulse current with an estimated rms micropulse length of 3.4 ps was obtained, and the normalized rms emittance was measured to be 11.6 /spl pi/-mm-mrad for the 1% energy-spread electrons, corresponding to a beam brightness of 2.3/spl times/10/sup 11/ A/m/sup 2/. For these parameters, the calculated small small-signal gain for our 0.5 m-long wiggler is 110% at a wavelength of 85 /spl mu/m. In this paper we characterize the electron beam from a 1 1/2 cell, thermionic cathode, RF cavity gun in a parameter range where it can be used as the accelerator for a far infrared FEL. >