DOE OSTI2011
The ELI-NP project is now working on the design of their conventional facility. Dr. Gheorghe Cata-Danil recently requested that I provide them with information on the location and amount of electron-beam loss in the MEGa-ray source they have proposed for ELI-NP. This memo is intended to document that information, for transmission to ELI-NP. The ELI-NP MEGa-ray source, as presently proposed, consists of two x-band accelerator sections separated by a large chicane, as show in figure 1. The basic parameters of the machine that are pertinent for specifying the radiation source terms are shown in table 1. These are the parameters of the intentionally-produced photobeam. In addition to the photobeam, the electron gun and accelerator will produce 'dark current' that originates throughout the RF structures (that is, distributed along the accelerator axis) and therefore has a distribution of energy below the energy of the photobeam. Because it is emitted from surfaces inside the RF structures, much of it is not transported through the accelerator and is lost in the accelerator RF structures. A large fraction of the total dark current is produced in the photogun and lost at the entrance of the 1st accelerator RF structure. Important sources of radiation during operation are beam alignment screens that are used for observing the image of the electron beam, during adjustment of beam steering and for general diagnostic purposes. Each screen consists of a 1 mm thick Ce:YAG plate that is moved into the path of the beam when desired. This destroys the electron beam, spraying all beam current into the structures downstream of the screen. Only one screen is inserted at a time. These screens may be located after each accelerator RF structure, and after each set of bend magnets, as shown in figure 3. The photobeam energy and currents at each location are listed in table 2; for simplicity, the dark current energy is (conservatively) assumed to be the same as the photobeam energy. In normal operation, the locations of other beam losses and the estimated current and energy of the lost beam are listed in table 3. In addition to these 'normal' beam losses, it is assumed that the beam may be accidentally mis-steered by up to 1 degree at every steering magnet, so that it hits the surrounding beam tube and other structures. This loss can be modeled as occurring at the locations listed in table 2, numbers 1-7 and 16-21. Similarly, it is assumed that any of the dipole magnets, in chicanes or at the beam dump, may fail, resulting in total beam loss straight ahead. These locations can be assumed to be the same as listed in table 2, numbers 8-10, 12-14, 22-24, 26, and 27. The data provided above are estimates of the beam loss, for the purpose of calculating radiation source terms. Dark current estimates are conservative, until we acquire actual operational data. Neither the estimated currents nor the estimated energies listed above contain any intentional margin. when they have performed similar calculations for their own purposes, they have typically added approximately 10% to the beam energy to provide some overall margin in the analysis.