A critical parameter to describe beam's quality in many applications is its brightness: particle flux per unit area per unit solid angle. As a consequence of Liouville's theorem, beam brightness remains an invariant along the beamline if its transport is subject to linear optics (e.g. aberration-free lenses) with no dissipative mechanisms present. Such is the case of linear accelerators (linacs). Additionally, as energy (momentum) is imparted to the beam along a preferential (longitudinal) axis, beam divergence (and its solid angle) are further reduced in accordance with the ratio of beam momenta before and after the acceleration. This process is known in accelerator physics as adiabatic damping.
The beam quality out of a linac is set by its source. Brightest sources of relativistic electrons available today employ photoemission process. Such sources are also known as photoinjectors. The key component of a photoinjector is photogun (shown above in GIF animation), which houses a photocathode in a high accelerating gradient (10-100MV/m). The laser light with the necessary RF timing structure impinges on the photocathode generating a train of electron bunches, which can be further accelerated downstream in a linac.
Recently, our lab has successfully constructed a high-brightness photoinjector for energy recovery linac. This photoinjector will drive a much larger accelerator to produce coherent x-rays via synchrotron radiation. The exceptional properties of these x-rays would be largely determined by the photoinjector (shown above, beam moves right to left). Photoinjectors represent a high-impact technology that enables free-electron lasers, can revolutionize synchrotron sources, colliders, as well as small-scale practical devices.
It is also a young technology, with many unanswered questions and possibilities for improvement, actively pursued by the world's leading research institutions. Cornell ERL photoinjector R&D program is a testbed for developing new technical solutions and novel approaches to realize an ultimate brightness electron source. As of 2012, our photoinjector produces the world's highest average brightness and current beam of any photoemission-based accelerator source. Students play a key role in all aspects of design, construction, and operation of this unique facility.
Some parameters of Cornell ERL photoinjector
- 100 mA max average current
- 5-15 MeV beam energy
- 77 pC/bunch at 1.3 GHz
- 0.3 mm-mrad normalized rms emittance (ultimately 0.1 mm-mrad)
- 2-3 ps bunch duration (can be compressed to 0.1 ps)