Scientific Assessment of High-Power Free-Electron Laser Technology (2009) / Chapter Skim
Currently Skimming:
3 Technical Assessment: Scalability to One-Megawatt Power Levels
3 Technical Assessment: Scalability to One-Megawatt Power Levels
Pages 14-36
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 14... ...
This means that the necessary average current recirculating in the energy recovery system must be ~1 A for a megawatt-class FEL and ~0.1 A for a 100-kW-class FEL. Jefferson Laboratory now achieves its 10 kW operation by recirculating electron pulses of about 0.1 nC at a repetition rate of about 75 MHz.
|
From page 15... ...
Beyond a certain limit of the energy spread, these nonlinear distortions of the phase space can cause some of the low-energy particles to get lost in the last RF cavities and not arrive at the exit of the linac. Experience at the Jefferson Laboratory FELs has shown that, for proper energy recovery, the nonlinear distortions must be corrected.
|
From page 16... ...
If we assume a nominal RF for the accelerator of 700 MHz, a 1 A average current would require electron bunches that each contain 1.4 nC of charge repeated at a 700 MHz repetition rate. Lower average currents can be achieved either by reducing the repetition rate of the electron bunches, or by reducing the charge per bunch, or both.
|
From page 17... ...
Currently, the Jefferson Laboratory FEL system is capable of producing an average current of 10 mA for extended periods of time using cesiated GaAs cathodes. While cathodes that meet the quantum efficiency requirement of a megawatt-class system have been developed, their robustness and lifetime are not yet at levels that would make them suitable for long-term use.
|
From page 18... ...
Drive lasers suitable for driving cathodes to produce 100 mA of average current have been produced; however, no attempt has been made to develop a drive laser suitable for 1 A average current. A photocathode drive laser capable of producing a 1 A average current from a cathode with a quantum efficiency of 2 percent will require approximately 100 W of green laser power to the cathode, which is approximately five times the average power of the current state-of-the-art drive laser at Jefferson Laboratory.
|
From page 19... ...
The ERL parameters are up to ~1 A of beam current and ~100 MeV of acceleration. The energy of 100 MeV has been demonstrated (in the Jefferson Laboratory FEL)
|
From page 20... ...
There are several other considerations, including the effects of coherent synchrotron radiation, halo and beam loss, and ion trapping. At the injection end, longitudinal space charge may lead to beam quality deterioration, particularly for very short bunches.
|
From page 21... ...
Beam losses in the Jefferson Laboratory FEL have been quantified at ~10 mA operation. The total beam loss was <1 μA, <100 nA in the worst locations, and about 10 nA in some other locations.
|
From page 22... ...
reported that the cw damage resistance of these reflectors at 1 µm far exceeded the FEL system requirement. Two decades later, IBSD multilayer reflectors of HfO2/SiO2 have been installed in the Jefferson Laboratory FEL and have survived cw intracavity average power densities well over 100 kW/cm 2 with the FEL operated at 1.6 µm wavelength with <0.4 ps (FWHM)
|
From page 23... ...
Mirror degradation caused by absorption of the UV harmonic radiation was observed in the Orsay storage-ring FEL in 1984,26 and more recently in the Jefferson Laboratory FEL. In both FELs, the resonator mirrors exhibited increased absorption at the lasing wavelength, which seriously limited the operating power.
|
From page 24... ...
Before the Navy's FEL average power scale-up program began in 1995, the highest demonstrated average power was ~10 W The first laser in the Navy's scale-up program was a 1 kW oscillator built at Jefferson Laboratory.
|
From page 25... ...
The harmonics of the fundamental lasing wavelength are a unique feature of FELs and make the transport optics and resonator optics more complex. For FELs operating in a restricted space, such as a Navy ship, the use of a ring resonator with intracavity g razing-incidence, beam-expanding mirrors offers a way to reduce the power density on the cavity mirrors and allows for coatings with larger absorption than could be tolerated at normal incidence.
|
From page 26... ...
With sufficient electron beam energy and average current, the RAFEL should be scalable to megawatt output average powers at 1 µm. As with the single-pass amplifier option, an issue for the RAFEL is determining a means to rapidly expand the intense output beam so that its intensity can be handled safely by the first downstream relay reflector.
|
From page 27... ...
If the amplifier is assumed to have a gain of ~105, the drive laser must supply approximately 10 W of average power in mode-locked pulses with a pulse repetition frequency (prf) of 500-700 MHz synchronized to the RF drive, a pulse width of a few picoseconds, and a peak power on the order of 10 kW.
|
From page 28... ...
28 SCIENTIFIC ASSESSMENT OF HIGH-POWER FREE-ELECTRON LASER TECHNOLOGY FIGURE 3.2 Typical atmospheric molecular absorption and scattering versus wavelength. SOURCE: Courtesy of Joung Cook and John Albertine.
|
From page 29... ...
However, it is clear that the unique operational environment of an FEL on a ship will call for specialized diagnostics that are well integrated with the control system -- for example, component motion and vibration sensors and beam position monitors. For high average beam current operation, minimally intercepting or nonintercepting electron beam diagnostics should be developed to monitor and control, for example, coherent synchrotron radiation and related electron microbunching; electron beam halo, transverse emittance, and energy spread; and cathode performance.
|
From page 30... ...
Although there is no consensus on a rigorous definition of "beam halo," the term usually refers to particles extending beyond the normal beam rms radius, from a few rms beam radii to about 10 rms beam radii or more. Because beam halo increases the risk of beam losses with their multiple deleterious effects, control of halo and beam loss is important in high-average-power ERLs.
|
From page 31... ...
The code MATBBU, which was also developed at Jefferson Laboratory, is based on the eigenvalue method.59 Cross-benchmarks among the codes show consistent beam behavior and excellent agreement with theoretical models. In a series of comprehensive measurements at the Jefferson Laboratory FEL, the beam breakup threshold current was experimentally determined and found to agree with simulations to within ±10 percent.
|
From page 32... ...
Litvinenko, and H Owen, "Optics and Beam Transport in Energy Recovery Linacs," Nuclear Instruments and Methods in Physics Research Section A 557: 345-353 (2005)
|
From page 33... ...
Pfluger, "Undulator for the LCLS Project -- From the Prototype to the Full-Scale Manufacturing," Nuclear Instruments and Methods in Physics Research Section A 543: 42 (2005)
|
From page 34... ...
Kabel, and M Dohlus, "Using TraFiC4 to Calculate and Minimize Emittance Growth Due to Coherent Synchrotron Radiation," Nuclear Instruments and Methods in Physics Research Section A 455: 185-189 (2000)
|
From page 35... ...
Smith, "Multipass Beam Breakup in Energy Recovery Linacs," Nuclear Instruments and Methods in Physics Research Section A 557: 176-188 (2006)
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.