Research Summaries

Back Investigation of Beam Source and Collective Effects and Instabilities Relevant to High-Power Free-Electron Laser Performance

Fiscal Year 2014
Division Graduate School of Engineering & Applied Science
Department Physics
Investigator(s) Colson, William B.
Sponsor High Energy Laser Joint Technology Office (Joint)
Summary High-power free-electron laser (FEL) weapons require the delivery of high-quality, high-charge bunches of electrons to the FEL at a high repetition rate (or high currents). The performance of the accelerator to supply the electron beam to the FEL is subject to constraints arising from both the requirement for high current, and high bunch charges. The requirement for bunches that are high-quality and high-charge, coupled with high currents, lead to a number of design challenges in the electron beam source.
The need for high currents drives low accelerating gradients and voltages in the injector. This makes it harder to maintain beam quality with standard emittance-compensation techniques. These techniques, while well-developed and successful in practice, typically over-focus the head and tail of the electron bunch. This can lead to halo formation. Historically this has not been a serious issue, as most FEL applications requiring high-quality beams have not been at high average power. Thus, the requirement of high beam quality - required for the FEL to operate - seems to directly drive an effect that must be avoided in high-power accelerators.
The requirement for energy recovery from the electron beam after it leaves the FEL leads to the placement of a "merger" system between the injector and main accelerator. Preserving the beam quality of a high-charge beam through relatively sharp bends can be problematic at low energy due to space-charge effects, apart from any impact from coherent synchrotron radiation (CSR). The impact of CSR upon electron bunch quality has been studied previously, but usually in the context of very high voltage accelerators compared to weapons-class FELs, and for very different geometries. CSR and its interplay with low-energy space-charge has not been thoroughly explored in theory for bend systems relevant to weapons-class FEL mergers, and not at all experimentally.
Photocathode performance also affect halo formation, emittance compensation and CSR. Nonuniform quantum efficiency across the cathode surface results in variations in local charge density within the newly emitted beam. These variations generate local, nonlinear space-charge fields which can drive halo formation, lead to filamentation of the beam and interaction with CSR, and interfere with the emittance compensation process - in essence, making the phenomena described above harder to predict, stabilize and ameliorate.
The program we propose will address these effects through a combination of theoretical modeling, simulation and experimental investigation and validation. Work on cathodes, led by Dr. John Harris, will include investigation of dispenser photocathodes for uniformity of quantum efficiency and longevity. Novel beam formation techniques, such as the use of elliptical laser pulses to obtain beams with linear space charge forces, will be tested and refined in theory and experiment. Beam transport effects for low-voltage, high-charge, high-quality beams will be predicted and compared with experiment over the 1 - 10 MeV energy range, of high relevance to weapons-class infrared FEL designs. Dr. John Lewellen will serve as PI for the project, and will lead the work on beam formation, transport, and CSR interactions.
Publications Publications, theses (not shown) and data repositories will be added to the portal record when information is available in FAIRS and brought back to the portal
Data Publications, theses (not shown) and data repositories will be added to the portal record when information is available in FAIRS and brought back to the portal