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Metal Photocathodes for Free Electron Laser Applications


Synchrotron x-ray radiation sources have revolutionized many areas of science from elucidating the atomic structure of proteins to understanding the electronic structure of complex materials such as the cuprate superconductors. These advances have been possible because of the high brightness of synchrotron radiation. This high brightness comes from the very small size and divergence of the electron beam, and the interference of light produced by transverse acceleration of the beam in the periodic magnetic structures in the straight sections of a storage ring. The radiation produced is however a sum over the emission from individual electrons as there is no phase relationship between the positions of each electron; the emission therefore scales as the number of electrons. In a Free Electron Laser (FEL), the main difference to the synchrotron radiation mechanism is that the light field acts on the electron beam, over a long distance in an undulator, and causes electron bunching at the optical wavelength. Electrons in different parts of the electron bunch are therefore correlated, and so emit coherently, with a brightness that scales as the square of the number of electrons. This coherent emission process results in the FEL having a brightness typically 10 orders of magnitude higher than a synchrotron radiation source. Although the FEL concept has been around since the mid 1970's, only within the last few months has the world's first x-ray FEL lased. This FEL, the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAC), produces x-rays up to an energy of around 8.5 keV. A much lower energy FEL (FLASH) capable of operation to a few 100 eV started to operate at the DESY laboratory several years ago.

In order to lase, the electron beam in a FEL must have a transverse geometric emittance less than the wavelength of the light to be produced. For the generation of x-ray wavelengths, this is one of the most difficult challenges in the design and construction of a FEL. The geometric emittance can be "compressed" by acceleration to very high energy, but with the penalty of very large physical size and very large cost. The motivation for this work was provided by the desire to investigate the fundamental origin of the emittance of an electron beam as it is born at a photocathode. If this initial, or "thermal" emittance can be reduced, the energy, scale and cost of accelerators potentially would be reduced. As the LCLS used copper as its photocathode, this material was the one studied in this work. Copper was used in the LCLS as it represented a "robust" material that could stand the very high accelerating gradients used in the photoinjector of the FEL. Metals are also prompt photoemitters, and so can be used to produce very short electron bunches. This can be a useful property for creation of extremely short FEL pulses, and also for creation of beams that are allowed to expand under space charge forces, but in a way that results in linear fields, allowing subsequent recompression.

An ideal photocathode for FEL photoinjector should have high quantum efficiency (QE), small emittance, fast temporal response, long lifetime, and minimal complexity. High QE of cathodes require less power for driving laser and also reduce the risk of damaging the cathode materials. Small emittance reduce the scale of the accelerator, therefore, the cost. Metal photocathodes such as copper exhibit long lifetime and fast response, but have quite low quantum efficiency \((<10^{-4})\). The aim in this work was to understand the quantum yield of the metal, and the transverse momentum spectrum, as the product of the latter and the cathode beam spot size gives the transverse emittance. Initial x-ray diffraction work provided evidence that the LCLS photocathode consisted of large low index single crystal grains, and so work focused on the study of single crystals that could be produced with atomically ordered surfaces, rather than a polycrystalline material. Present theories of quantum yield and transverse emittance assume the basic premise that the metal is entirely disordered, and work here shows that this is fundamentally incorrect, and that the order of the surface plays a critical role in determining the characteristics of emission. In order to investigate these surfaces, I constructed a laser - based ultra-low energy angle resolved photoemission system, capable of measuring the momentum spectrum of the emission and wavelength and angle dependent electron yield. This system has been commissioned, and data taken on low index surfaces of copper.

Results from this work on single crystal copper demonstrates that emitted electrons from the band structure of a material can exhibit small emittance and high quantum efficiency. We show that the emission from the Cu(111) surface state is highly correlated between angle of incidence and excitation energy. This manifests itself in the form of a truncated emission cone, rather than the isotropic emission predicted from the normal model. This clearly then reduces the emittance from the normal values. It also results in extremely strong polarization dependence, with p-s asymmetry of up to 16 at low photon energy. It also directly suggests ways through changing materials, or by material design to significantly reduce emittance, at the same time increasing electron yield. These results show the benefits that could be gained from electronic engineering of cathodes and should have direct impact in the design of future FEL photoinjectors.

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