Photoinjectors

Gun Ia | Gun 1b | Gun II | Gun III | Gun IV | Performance Data

The basic principle of the PI is simple: short bunches of electrons are generated by laser pulses incident on a photocathode located inside an rf accelerating structure. The structure is operated at a high accelerating field to make the electron bunch relativistic in a short distance. Thanks to the combination of the high surface field on the cathode and the high yield of electrons possible by photo emission, a very large current density, J ~ 10 to 100000 A/cm2, is possible. This current density is much larger than that possible by thermionic emission (about 10 A/cm2). The normalized thermal rms brightness Bn, (for a cathode effective temperature T) is proportional to the current density. Therefore a PI can deliver a very large brightness. For example, photoinjectors using the 'emittance compensation' technique, can provide peak currents of over 100 amperes with a normalized rms emittance of one pi-mm-mrad. A series of S-band photoinjectors have been designed, fabricated and tested at Brookhaven National Laboratory's Accelerator Test Facility. The metal photocathode RF gun was developed at the Instrumentation Division of BNL. Look also at this link.

Gun Ia:

The first gun, or Gun I, a 1 1/2 cell device, is shown below:

Schematic drawing of the BNL Gun I.

The 'BNL Gun I' design as well as somewhat modified versions are (or were) in operation at numerous laboratories around the world. The rf gun is a resonant pi-mode 1 1/2 cell cavity operating at 2856 MHz. It uses a metal photocathode that forms part of the wall of the 1/2 cell. Cathodes can be changed by using the 'choke joint' access port.

The 78.75 mm long cavity is 83.08 mm inner diameter and its beam aperture diameter is 20 mm. It has an unloaded Q of 11900 and a shunt impedance of 57 MOhm/m, which corresponds to a beam energy of 4.65 MeV at a structure peak power of 6.1 MW. At this power, the peak surface electric field is 119 MV/m and the cathode field is 100 MV/m. These operating conditions can be achieved after a few days of careful rf conditioning.

The laser illumination of the photocathode is a bit more complicated than expected and utterly critical to the successful performance of the gun..  For details on the photocathode laser itself, see the Nd:YAG laser description.  For the description of the "gun hutch" optics (the photocathode illumination control and diagnostics area), see Gun Hutch Optics.

At this high electric field the contribution of space-charge forces to emittance growth can be quite small. Of course, as one increases the field its radial component may become a significant contributor to emittance growth in the gun. These conditions can be simulated with the computer code PARMELA. The parameters under our control are the electric field on the cathode, the laser pulse length and the laser spot size:

Emittance calculated by PARMELA vs. various gun parameters.

Even more detailed simulations of the beam and electromagnetic field dynamics in the photoinjector can be done using 3-D Particle-In a Cell (PIC) codes, such as MAGIC. The results are in general agreement with PARMELA

Wake fields trailing the electron bunch propagating in the gun cavity.

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Gun Ib:

The second photoinjector of the ATF used a solenoid lens for initial focusing of the beam, and had a solid copper back (not dismountable). Under proper conditions, a significant part of the correlated emittance growth due to space-charge can be recovered.

The ATF photoinjector Gun Ib shown with the solenoid lenses

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Gun II:

This gun was developed in collaboration under a CRADA with Grumman (Later Northrop-Grumman, later still - Advanced Energy Systems). One of the guns developed as a result of this collaboration is now powering the BNL Center for Radiation Chemistry Research.

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Gun III:

In collaboration with UCLA and SSRL, we have developed another version of the gun. This new gun, designated at BNL as Gun III and by the collaboration as 'The Next Generation Photoinjector', has been installed and tested at the ATF, at UCLA and at the Gun Test Facility at SLAC. The measurement program was one of the ATF Approved Experiments, New Generation Photocathode RF Gun Test Program. A normalized rms emittance of 2.6 mm mrad rms was measured for a charge of 1 nC with a pulse length of 10 ps. This is in agreement with PARMELA simulations for a Gaussian laser intensity distribution and demonstrates the success of the emittance compensation scheme. 

The objective is to produce a laser-photocathode electron gun at s-band with the following improvements:

• - Reduced multipole components of the rf field for a smaller emittance. The emittance contribution of the multipole fields has been reduced by about an order of magnitude through careful symmetrization of the field. (Cold Test Results of the BNL/SLAC/UCLA 1.6 Cell Photocathode RF Gun, D.T. Palmer, R.H. Miller, H. Winick, X.J. Wang, K. Batchelor, M. Woodle and I. Ben-Zvi, Proc. 1995 Particle Accelerator Conference, Dallas May 1-5.)
• - Improved surface electric field distribution (lower peak field by 30 percent as compared to original BNL gun).
• - Improved connection of the photocathode to the gun. We have gone from the previous choke-joint at a high electric field point to a low electric field, high current point.
• - Simplified rf input coupling.
• - Simpler machining for reduced cost.

In the figure above, the beam exit port (2 3/4" conflat) is oriented down and left. The large flange on the other side is the photocathode holder, which is also the back wall of the first cell (the so called 0.6 cell, the second cell is full length). The waveguide input coupler can be seen extending right and down. The power is coupled from the waveguide to the full cell. Coupling to the 0.6 cell is done through the intercell aperture. Opposite the waveguide input coupler is a symmetrical opening used as a pump-out port and field sample probe for the full cell. On top one can see a tuner port (with a mini-conflat flange) and near the cathode flange is another mini-conflat flange for the laser input port. (It is oriented at 72 degrees to the cathode).
Completed Gun	The gun mounted on the solenoid
The gun mounted on the solenoid (Computer Generated Graphic)	Measured emittance vs. charge data for Gun III

For the results using this gun, please see the following publications:

D.T. Palmer, R.H. Miller, H. Winick, X.J. Wang, K. Batchelor, M. Woodle, and I. Ben-Zvi, "Microwave Measurements of the BNL/SLAC/UCLA 1.6 Cell Photocathode RF Gun", 1995 Particle Accelerator Conference, (1995), BNL 61851.

D.T. Palmer, R.H. Miller, H. Winick, X.J. Wang, K. Batchelor, M. Woodle, and I. Ben-Zvi, "Simulations of the BNL/SLAC/UCLA 1.6 Cell Emittance Compensated Photocathoce RF Gun Low Energy Beam Line", 1995 Particle Accelerator Conference, p.2432, (1995), BNL 61852.

D. T. Palmer, X. J. Wang, R. H. Miller, M. Babzien, I. Ben-Zvi, C. Pellegrini, J. Sheehna, J. Skaritka, H. Winick, M. Woodle, and V. Yakimenko. Commissioning results of the next generation photoinjector. Advanced Accelerator Concepts Workshop, Lake Tahoe, CA, October 13-18, 1996. December 1996. BNL 63808

D. T. Palmer, X. J. Wang, R. H. Miller, M. Babzien, I. Ben-Zvi, C. Pellegrini, J. Sheehan, J. Skaritka, H. Winick, M. Woodle, V. Yakimenko, Emittance Studies of the BNL/UCLA 1.6 cell Photocathode rf Gun, 1997 Particle Accelerator Conference, May 12-16, 1997, Vancouver, BC, Canada BNL 64466.

D. T. Palmer, X. J. Wang, I. Ben-Zvi, R. H. Miller, Beam Dynamics Enhancement due to Accelerating Field Symmetrization in the BNL/SLAC/UCLA 1.6 cell S-Bank Photocathode RF Gun, 1997 Particle Accelerator Conference, May 12-16, 1997, Vancouver, BC, Canada BNL 64467.

D. T. Palmer, X. J. Wang, I. Ben-Zvi, R. H. Miller, J. Skaritka, Experimental Results of a Single Emittance Compensation Solenoidal Magnet, 1997 Particle Accelerator Conference, May 12-16, 1997, Vancouver, BC, Canada BNL 64469.

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Gun IV:

This gun was developed in collaboration with KEK and SHI. Copies of this gun can be found at the BNL Deep UV FEL Project, at Argonne National Laboratory LEUTL  and the University of Tokyo. 

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Last Modified: December 3, 2007
Please forward all questions about this site to: Vitaly Yakimenko

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