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The Neptune Laboratory

(Extensive technical documentation is also available)


The Neptune laboratory at UCLA is directed jointly by Profs. Joshi, Pellegrini and Rosenzweig, in the departments of Physics and Electrical Engineering, and is funded by the U.S. Department of Energy through the Office of High Energy Physics. The facility comprises a versatile, state-of-the-art photoinjector and a terawatt CO2 laser; it is hence equipped especially for research into laser-driven acceleration concepts, though a great deal of basic research in beam physics and laser optics is also carried out. A particular interest of the Neptune collaboration is the development and demonstration of a next-generation plasma beat-wave accelerator (PBWA), which was one of the founding goals of the facility. To date, the first phase of an ongoing series of PBWA experiments has been completed at Neptune. As with many advanced acceleration schemes, the achievement of high-quality PBWA output depends strongly on the quality of the injected electron beam; the mission of the PBPL is hence to achieve the best possible beam brightness through detailed understanding of the beam dynamics in the photoinjector and beamline elements.

The Neptune photoinjector has been designed to produce optimized, emittance-compensated performance over a range of charges, expanding the potential of the laboratory to a wide range of possible experiments. This injector is combined with a linac to produce an output beam containing up to a nanocoulomb of charge at energies up to 15 MeV. While the bunch length of the photoinjector beam is fixed by the drive laser at ~6 ps, bunches as short as 0.3 ps have been measured after compression using a magnetic chicane or ballistic (rf) bunching. Possible advanced accelerator experiments at Neptune range from direct laser acceleration (with operation in the ultra-low charge and emittance mode) to IFEL and FEL microbunching (moderate charge and emittance) to monoenergetic PBWA and plasma wakefield acceleration using ultra-short, low-charge bunches. Beam physics experiments undertaken at Neptune have involved space-charge emittance compensation, the effects of space-charge on emittance measurements, beam compressibility, and emittance growth processes in bends. The Neptune beamline has also served as a laboratory for detailed comparison of beam parameters with a number of simulation codes, particularly the UCLA version of PARMELA.

This page outlines the various components of the Neptune photoinjector beamline: the rf gun and linac, electron beam diagnostics, and ancillary systems (drive laser, vacuum, and rf power). Research and ongoing experiments at Neptune are described in the Experiments section of this website.

Table: Achieved beam parameters for NEPTUNE. Parameters such as charge and laser energy can go beyond listed values, possibly at the expense of other parameters.

Beam Energy
15 MeV
Energy spread
Energy jitter
Beam size (rms)
100 µm
Beam Charge
300 pC
Bunch length (rms)
4 ps
CO2 Laser Power
200 GW
CO2 Rayleigh range
2.2 cm
Pointing jitter
<100 µm
Alignment precision
<1 mrad

Extensive technical documentation is also available

RF Photoinjector Gun

The RF gun at Neptune is a product of the BNL-SLAC-UCLA collaboration on high-brightness beam development and is similar to guns that have been operated at BNL and SLAC. It is a 1.625 cell pi-mode standing wave cavity, cell-to-cell on-axis coupled. The nominal on-axis peak field is 100 MV/m, although it is possible to obtain 115 MV/m with the available power; the central launch phase of the laser beam is 45 deg (where 90 deg corresponds to maximum field). The nominal energy of the beam at gun exit is 4.6 MeV; in practice, energies from 3.5 to 5 MeV have been obtained. The gun is coupled to the wave guide in the full cell only, with a symmetrizing port located opposite the coupling slot to cancel dipole components of the field which can lead to emittance growth. Several different cathode types have been employed. Initially, the cathode was a disk of solid copper, and subsequently a single copper crystal was embedded in the cathode center; currently, a magnesium cathode is in use, which has raised quantum efficiencies by a factor of 2 to 3. Emittance compensation during injection is accomplished by a yoked magnetic solenoid mounted around the gun exit port. Measured normalized beam emittances have ranged from 4 to 10 pi mm mrad, with nominal value near 5 pi mm mrad.

PWT Linac

The linac is a 7+2/2 cell pi-mode standing wave structure and the first fully operational version of a plane-wave transformer (PWT). Under normal operating conditions, roughly 9 MeV of acceleration is produced in the linac (45 MV/m peak field), but more acceleration is possible. The design of this novel device is discussed in [ref] and summarized here. The structure is similar to a disc-loaded linac, except that the outer wall is moved to a large radius, leaving a gap between the disks and the wall. This gap serves as a coaxial, plane-wave transmission line, which provides extremely strong cell-to-cell coupling and excellent mode separation. In the absence of support rods (which are needed to hold the disks in place), this structure also has a very high shunt impedance Rs and high unloaded Q, with the ratio Rs /Q approximately the same as a more standard structure. The introduction of the four support (and water-cooling) rods into the cavity causes enhanced rf power losses, and Rs is actually smaller in this case than that found in a standard structure. Even so, Q is enhanced ,and the PWT has a long fill time. Both the gun and PWT are approximately critically coupled, and the PWT barely fills during the 3.5 microsecond RF pulse.

The PWT was designed at UCLA and built in the Physics Dept. machine shop. Engineering issues to be solved included effective internal cooling, incorporating all-metal sealing surfaces, and optimization of the iris apertures to reduce the maximum surface gradient. The continued successful operation of the PWT, despite its unusual mode and frequency spectrum, demonstrates its usefulness as an rf linac.

Beamline and Magnets

An electron beam is delivered to the final focus location using three quadrupole triplets and ten steering magnets. All of these, as well as the emittance-compensation solenoid and the chicane, were designed at UCLA and constructed in the UCLA Physics Dept. machine shop. The PBPL has supplied quadrupoles and solenoids to a number of other advanced accelerator laboratories, with quadrupole designs ranging from very large-bore electromagnets through which the MARS CO2 laser beam can pass to permanent-magnet designs which are being developed for use in the final focus on the secondary beamline as well as at Lawrence Livermore National Laboratory.

The magnetic chicane, used at Neptune for beam compression, is a dual-purpose installation that also serves as a dipole switching magnet by operating the last two segments alone and in parallel. Adjustable trim coils are used to compensate for nonuniformity between the four magnet sections.

Beam Diagnostics

The choice of diagnostics is driven by the characteristics of the beams generally used for experiments and by the constraints of the beam transport. Most of the diagnostics described here give a shot-by-shot response, which allows for removal of the effects of drive laser fluctuations.

Spot Size Measurements

Beam spot sizes are measured using a mixture of phosphor and YAG screens imaged by CCD cameras. YAG crystals are more sensitive and able to image low-charge or expanded beams; phosphor screens are slow, but are able to reach higher resolution for bright beams, which can cause blooming in YAGs. Measurement resolution is limited by camera pixel size to about 30 to 40 microns, sufficient for acceleration and transport, which can be improved to better than 20 microns with camera optics. Both YAG and phosphor screens are normal to the beamline (preventing depth-of-focus problems) and imaged from the downstream side using a 45-degree front-surface mirror. Video signals from the CCDs are viewed by the machine operator and can be digitized for shot-to-shot analysis of beam size and position.

Charge Measurements

Beam charge is measured for every shot using two integrating current transformers (ICTs). This nondestructive measurement is supplemented by Faraday cups mounted on all beam dumps. Each ICT consists of a capacitively terminated, ferrite-loaded toroid which is mounted over a ceramic DC break in the beamline. It integrates the current over approximately a nsec and outputs a ten nsec pulse whose charge and peak current is proportional to the beam charge. One ICT is located immediately after the gun, while the other is just before the final focus quadrupoles, allowing optimization of the beam focusing and steering through the beamline by comparing the two signals. The Faraday cups are terminated with high impedance for short-pulse integrated charge measurements and are optimized for high capacitance (low charging voltage) and low radiation production.

Emittance Measurements

The behavior of high-brightness, low-energy electron beams is dominated by space charge, causing inaccuracy in emittance measurements using traditional quadrupole scan techniques. For unambiguous emittance measurements at Neptune, we have developed a slit-based emittance measurement system, essentially a one-dimensional pepperpot which separates the electron beam into eight horizontally spaced beamlets. The intensity distribution of the beamlets at a downstream phosphor screen is measured to give the phase space distribution of the beam; the width of each beamlet gives a measure of the width of the transverse momentum distribution at each slit, and the centroid of the beamlets gives the correlated offset of the momentum distribution at each slit. Since charge in each beamlet is small, space charge effects are nearly eliminated. The Neptune system uses slits of width 50 microns separated by 0.75 mm, much smaller than the beam size at the slits, in a sheet of stainless steel of thickness 5 mm. This ensures that the intercepted portions of the beam are stopped or completely scattered; the drift length following the slits is chosen to keep the beamlets from overlapping at the screen. Slit parallelism and flatness are critical; electron discharge machining was used for construction of the slits, and they are mounted on a rotatable actuator driven by a stepper motor, for precision alignment. A phase-space reconstruction algorithm takes the video data from the downstream phosphor image and calculates the rms Twiss parameters from the centroids and widths of the beamlets.

Longitudinal Bunch Length Measurements

The longitudinal profile of the Neptune beam can be measured using coherent transition radiation (CTR), which is able to diagnose sub-picosecond longitudinal structure and which has been used for the various beam compression experiments undertaken at Neptune. The technique used for collecting and analyzing the CTR was developed in collaboration with Prof. Uwe Happek of the University of Georgia. The electron beam is intercepted by a 45-degree foil, producing CTR light with the same temporal profile as the beam. A polarizing transmission Michelson interferometer is used to measure the autocorrelation function of the light pulse by scanning over many shots; the pulse length is then inferred from the autocorrelation trace. The useful measurement regime of the apparatus is limited on the long-wavelength side by diffraction losses and on the short-wavelength side by the spacing of the wire gratings which form the polarizing mirrors; this filtering must be taken into account when reconstructing the pulse length from the autocorrelation trace. A version of this technique could be used to measure microbunching in an FEL experiment.

Energy Measurement

Beam energy before injection into an experiment is measured at the magnetic dipole after the linac, which is part of the magnetic chicane and is also used to send the beam into a secondary beamline through a dogleg. (This secondary beamline is known as the S-Bahn and is discussed separately.) When used as a spectrometer, a calibrated phosphor screen is inserted at the beginning of the dogleg and is used to locate the beam and infer the energy spread. This measurement is used as a diagnostic on the RF phase in the linac, with normal operation at the phase giving minimum energy spread on the beam. A second energy measurement is typically made after any acceleration experiment, though the type of spectrometer used will vary according to the output beam characteristics. For the IFEL experiment currently underway, we are using a water-cooled magnetic Brown-Buechner spectrometer that images electrons with output energies from 10 to 50 MeV along a phosphor-coated window. The video image obtained gives a calibrated energy spectrum using the intensity of the phosphorescence as a measure of electron number.

Drive Laser

The photocathode drive laser illuminates the gun cathode with a pulse of ultraviolet (266 nm) light having a minimum temporal length of about 1 psec (FWHM), at an energy up to 300 microjoule/pulse. This is accomplished using chirped pulse amplification and compression of a 1.064 mm pulsed mode-locked Nd:YAG laser (using the 38.08 MHz master rf signal) amplified by a Nd:glass regenerative amplifier at 5 Hz. To amplify the pulse in the regen the laser is matched into a 500 m long fiber to produce pulse lengthening and a frequency chirp. The chirped pulse is then amplified by a factor of 1 million in the regen and sent to a grating pair where it is compressed by removing the chirp correlation. The pulse length can be arbitrarily lengthened by detuning the grating compressor, allowing a choice of pulse lengths for emittance optimization. The resulting picosecond pulse is frequency up-converted using two stages of BBO doubling crystals. The conversion efficiency is typically 10%. Because of the many nonlinear stages in this laser configuration the pulse-to-pulse fluctuation amplitude is at best 10% rms. There are also pulse length fluctuations associated with these effects, but they are not of great consequence operationally. The time of arrival of the laser with respect to the rf wave is controlled by a phase-shifters (manual and electronic) in the low level 2.856 GHz rf system. The fluctuations in the arrival time of the laser pulse on the cathode with respect to this rf signal are suppressed by use of a Lightwave electronics feedback system on the oscillator. The fluctuations have been measured to be below one picosecond.

RF System

The RF system at Neptune operates at the SLAC frequency of 2.856 GHz and relies on standard S-band technology. Synchronization with the drive laser is ensured by using the laser modelocker output (at 38.08 MHz) as the RF clock, after frequency multiplication by 75. About one watt of continuous low-level RF is delivered to a pulsed (<12 microsec) solid state amplifier, which in turn drives the input of the XK-5-type klystron. The klystron is pulsed using a custom modulator with pulse length variable between 3.5 and 7 usec, achievable by varying the number of capacitors in the modulator circuit. For normal (short-pulse) operation, a PFN composed of 10 capacitors is charged to 41 kV and discharged using a thyratron switch into the klystron pulse transformer, where the pulse is voltage-multiplied by 12.

Up to 20 MW of output power is supplied by the klystron and delivered to the beamline through a waveguide system pressurized to 28 psi with sulfur hexafluoride. Power is divided in a 2:1 ratio between the linac and gun, with typical values of 4-5 MW in the gun and 10-12 MW in the linac. Both gun and linac are equipped with high-power variable attenuators, and there is also a phase shifter on the linac input waveguide, which allows the operation of the linac at any desired phase relative to the gun. The injection phase of the drive laser at the gun can also be varied with a phase shifter on the low-level RF line.

Forward and reflected power levels in the system are measured using directional couplers in different portions of the waveguide and calibrated crystal detectors. RF power values can be calculated from oscilloscope traces of the crystal detector voltages and displayed automatically in the control room.

Control System

The photoinjector is operated through a mixture of computer and manual controls, and is in the process of moving to greater automation. Beamline magnet and actuator control, data acquisition, and video manipulation are accomplished using LabView 6.0 running on a Macintosh G4 computer. Magnets and actuators are switched and controlled via CAMAC modules, with readback of signals and diagnostics primarily through digitizing oscilloscopes which communicate via GPIB with the control computer. 16 channels of video are switched in the control room and may be digitized via a frame grabber for online, shot-by-shot analysis of beam spots or emittance slits. RF power levels and phases are controlled manually at present, but a computer-based active feedback system on the linac RF phase has been built and will be used in the future.

The trigger and timing system uses a video signal as a master clock, and relies on four Stanford Research Systems timing boxes for precise trigger signals. The drive laser oscillator is mode-locked to the master clock and its signal is frequency multiplied to serve as the RF clock.

We hope to upgrade the control system in the near future. Planned improvements include readback of magnet currents, temperature stabilization of the pulse-stretching fiber in the drive laser, more RF pickoff points, better automation of RF and quantum efficiency measurements, and an increase in the overall number of digital and video measurement channels.