Computation of Bremsstrahlung Radiation and Ionization Phenomenon in
Computation of Bremsstrahlung Radiation and Ionization Phenomenon in a CEBAF LINAC 1 Stoecker , Mason Matt 1 Virginia Tech, 2 Thomas Jefferson National Accelerator 2 Poelker Abstract Bremsstrahlung is a process observed at accelerators associated with deceleration of charged particles. In the transport arcs of the Continuous Electron Beam Accelerator Facility (CEBAF) where electron beams are steered 180 o, the process is called synchrotron radiation and it is well understood. In contrast, this work focuses on bremsstralung in the straight sections of CEBAF – the linear accelerator sections, aka LINACs, which are composed of cryomodules and the warm girder regions between the cryomodules - where the electron beam can collide with the residual gas inside the beampipe. Electron beam collisions with gas atoms and molecules results in photon production, electron beam energy loss, gas ionization, and ion trapping which can further enhance the bremsstrahlung process. These phenomena can trigger Fast Shut Down trips of the accelerator, interrupting the beam delivery to the experiment halls. Fast Shut Down trips serve to protect the accelerator from damage but reduce the time CEBAF is used to perform nuclear physics experiments. The focus of this project was to evaluate the magnitude of gasbremsstrahlung at CEBAF, to determine if steps should be taken to minimize it. This was accomplished using realistic assumptions of the vacuum conditions in the LINAC and computer calculations based on widely accepted model of electron beam interaction with thin targets. It was shown that the water vapor in the warm girder regions is most problematic, compared to the dominant gas species H 2 inside the cryomodules. Furthermore, gas ionization can lead to significant enhancement of gas-bremsstrahlung because ions can be attracted and trapped within the electron beam. Vacuum Model of the LINAC Ionization Conclusion The LINAC is composed of two types of vacuum regions, cryomodules which provide beam acceleration using superconducting radio frequency cavities, and warm girder regions in between each cryomodule. The full CEBAF LINAC consists of 25 cryomodules and warm regions. Actual measured vacuum pressure along the LINAC could be used for calculations, but for this project, typical pressures were simply assigned to each region: the warm girder regions were set to 10 -7 Torr and the cryomodules were set to 10 -11 Torr. Another important phenomenon when dealing with the residual gas inside the LINAC is the effect of ionization of the gasses by the electron beam. Ions can be trapped by the beam’s electrostatic potential. Eventually, the number of ions equals the number of electrons, and this is called “neutralization”. The phenomenon is well known [Pon 99, Ful+16] for beams inside storage rings. For CEBAF, the time it takes for the ions to neutralize the electrostatic field of the electron beam is the relevant parameter: the neutralization time should be short compared to the frequency of Fast Shut Down trips. In conclusion, there is more gas-bremsstrahlung at 12 Ge. V CEBAF compared to 6 Ge. V CEBAF. The status of the vacuum at CEBAF, while generally good, deserves more attention in this context. Relatively speaking, the amount of bremsstrahlung and ionization generated in the warm regions is orders of magnitude more than the cryomodule region. The vacuum in the warm girder regions could be improved via bakeout to eliminate water vapor. Another solution is to institute clearing gaps which would allow the ionized residual gas to leave the beam. This represents the first analytical study to estimate size of the problem of gas-bremsstrahlung at CEBAF, aimed at putting the topic into correct context. Indeed, it seems gas-bremsstrahlung could impact CEBAF operations. Next Steps Figure 1: A MOLFLOW simulation with a texture and volume overlay for considering conditions in the warm girder regions due to presence of vacuum ion pumps Figure 6: Determining the amount of ions produced in one pass through the LINAC Figure 7: Ionization cross section of the two most common residual gas species in the LINAC Energy Dissipation Bremsstrahlung and Schiff Spectrum The Schiff spectrum describing bremsstrahlung produced from thin targets was used to model gas-bremsstrahlung in the CEBAF LINAC. It is an integration of the bremsstrahlung cross section over all angles. A major reason for performing these calculations was to determine the beam energy loss associated with gas-bremsstrahlung. The first transport arc of CEBAF can only deliver beam with an energy spread of 0. 1%. Electrons that lose more energy than 0. 1% of the primary beam energy will strike the vacuum chamber walls. This means that in Figure 3, all of the electrons represented by the graphs will be lost on the way to the experiment halls. • Steps should be taken to determine the percentage of Fast Shut Down trips that result from gas-bremsstrahlung • It would be worthwhile to measure the gasbremsstrahlung radiation directly, to validate the calculations presented here • A clearing gap could be a simple way to eliminate Fast Shut Down trips associated with gas-bremsstrahlung Table 1: Power dissipation stemming from gas-bremsstrahlung, electrons and photons Works Cited Introduction When the electron beam at CEBAF passes through the Linear Accelerator (LINAC) sections, it interacts with residual gasses in the beampipe. These interactions result in phenomena that are detrimental to beam delivery. Bremsstrahlung, German for "braking radiation“ is electromagnetic radiation that is produced from the deceleration of charged particles (at CEBAF, the electron beam) after passing through matter, in this case the field of the residual gas molecules. The electron beam can also ionize residual gas atoms and molecules, which are then attracted to the field of the electron beam resulting in a phenomenon known as Ion Trapping, which can increase the density of matter in the beam, and cause more bremsstrahlung. Figure 2: The Schiff bremsstrahlung cross section which depends primarily on the energy of the electron beam and the mass of residual gas (Z), which has a quadratic dependence Results Acknowledgements I would like to thank my mentor Matt Poelker for the project and clear objectives, Riad Suleiman for the assistance in understanding the computational and nuclear phenomenon, Marcy Stutzman for assistance with MOLFLOW , Lisa Surles-Law who is an awesome program director, and my mother for always believing in me. Objectives • Determine the amount of bremsstrahlung radiation emitted under typical CEBAF operating conditions • Determine analytically the amount of ions produced in an generalized LINAC • Determine neutralization times in relation to the ionization • Obtain values for the amount of energy dissipated via these processes Figure 3: Energy loss experienced by the electron beam via the process of bremsstrahlung. Electrons that suffer this energy loss will not be delivered to the experiment halls. (note, the majority of the electron is not represented) Figure 4: Bremsstrahlung photon energy spectrum, for 6 and 12 Ge. V CEBAF, with and without enhancement via ion trapping. All plots assumed 100 u. A electron beam current. Figure 5: Neutralization time for the two major residual gas species with respect to the Lorentz Factor. Poster Session at JLAB CEBAF center on 08/02/2019 at 11: 00 am
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