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| MOAAU02 |
Lasing of a Cherenkov FEL Driven by a Low Electron Beam Current
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- K.-J. Boller, I. de la Fuente, P. J.M. van der Slot
Mesa+, Enschede
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We have designed and constructed a compact (0.5 x 1.5 m), 100 kV Cerenkov FEL operating at around 20 to 25 GHz*. The electron beam is produced by a gridded thermionic electron gun, which yields a maximum current of about 500 mA. This current is injected into a cylindrical waveguide lined with Al2O3 and about 60 % of the beam is transported through the lined waveguide section. A transported current of only 250 mA was sufficient to observe lasing. The total power generated for an interaction length of 25 cm was about 3 W. Although not designed as an oscillator, reflections at various positions along the waveguide provide a low-Q resonator. The power observed was in good agreement with results obtained with a model of the CFEL when reasonable values are used for beam quality, resonator feedback and liner fluctuations**. Measurement of the tuning with voltage showed excellent agreement with theory, and indicates operation at the fundamental TM01 mode as well as on higher order modes.
* Proceedings of the 2004 FEL Conference, Trieste, Italy, 2004, p314-317** J. Appl. Phys. 100, 053108 (2006)
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Slides
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| MOPPH068 |
The NCAS-FEL: an FEL Oscillator with High Slippage
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163 |
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- K.-J. Boller, E. van Geijin
Mesa+, Enschede
- J. Jalink, W. J. van der Zande
Radboud University Nijmegen, Institute of Molecules and Materials, Nijmegen
- P. J.M. van der Slot
Twente University, Laser Physics and Non-Linear Optics Group, Enschede
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In normal operation of FEL oscillators with little or no slippage, the cavity length needs to be slightly smaller than the synchronous value due to the lethargy in the gain build-up. If the FEL experiences high slippage, i.e., when the slippage becomes comparable or even larger than the length of the electron pulse, a different cavity detuning may be required. We use the one dimensional Medusa1D code to study the NCAS-FEL* as an example of an FEL oscillator having high slippage. Medusa1d is basically equal to the fully three dimensional Medusa code** with the 3D effects stripped out. It includes multiple wiggler segments, electron beam transport elements, harmonics and a simple oscillator model. Medusa1D can also include 3D effects using the fitting formula of Xie***. The NCAS-FEL is a RF-linac based FEL dedicated for high resolution spectroscopic studies in the wavelength range from 100 micron to 1.5 mm. The design of the NCAS-FEL is currently under way, and we report here initial results of this design and focus on the effects of high slippage on cavity detuning and the development of coherence within the optical pulse.
* Proceedings of FEL 2006, Berlin, Germany (2006) p485 - 487 ** Phys. Rev. ST-AB 8, 110701 (2005)*** Nucl. Instrum. Methods Phys. Res., Sect. A 445, 59 (2000)
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| TUAAU05 |
Modelling Mirror Aberrations in FEL Oscillators Using OPC
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- K.-J. Boller, J. G. Karssenberg, P. J.M. van der Slot
Mesa+, Enschede
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Thermal distortion in mirrors used in high average power FEL oscillators, like the JLAB FEL and the 4GLS VUV-FEL, will influence the mode quality and affect the FEL performance. In order to quantify these effects, these distortions needs to be characterised. Mirror aberrations are generally described using Zernike polynomials and also in case of thermal distortions, it has been shown that these polynomials can be used to describe the mirror distortion*. The Optical Propagation Code (OPC)** is a general optical propagation package in the paraxial approximation, that works together with gain codes like Medusa and Genesis 1.3 to model FEL oscillators. We have extended OPC to include phase masks, that can either be generated by an external program or internally using Zernike polynomials. This allows OPC to model mirror aberrations. We will present a few examples, illustrating the capabilities of OPC.
* Nucl. Instrum. Meth. A407 (1998)401** J. Appl. Phys. 100, 093106 (2006)
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