Monte Carlo simulations and measurements of the radiation environment at a laser-plasma accelerator

In current experiments at the high-power laser facility at the Lund Laser Centre, electrons accelerated to hundreds of MeV over short distances by means of laser wakefield acceleration in produced plasmas. To determine the amount of secondary radiation generated when accelerated electrons interact with surrounding materials is of interest to ensure a safe working environment. In this thesis, the radiation levels inside the laboratory are simulated using GEANT4, a C++ class library for particle physics and particle tracking using Monte Carlo methods. For persons directly outside the room of the vacuum chamber, conservative simulation results indicate that even for electrons accelerated to a relatively large average energy of 500 MeV, in bunches containing an average charge of 100 pC, in excess of 10 8 shots would need to be fired in a single year to reach doses of the order of the limits set by the Swedish Radiation Safety Authority. At a pulse rate of 0.1 shots per second, this corresponds to continuous operation for 27 700 hours. As a test of the simulations, experimental dose data were collected using dosimetric instruments – measuring doses from electron, gamma and neutron radiation – showing agreement with simulated doses to within reasonable error.At the Lund High Power Laser facility, electrons are accelerated to energies of several hundred MeV in just a few millimeters—reaching velocities of 99.9999 % of the speed of light. When these electrons hit the surrounding walls and equipment in the laboratory, secondary radiation is generated as other particles are knocked out in electromagnetic and nuclear interactions. In this thesis, this secondary radiation is simulated to make sure that persons working in the laboratory are not subjected to harmful radiation levels. Acceleration of electrons is achieved by sending short laser pulses of high power onto a gaseous target. The power in a single laser pulse reaches the order of tens of terawatt, equivalent to the power output of thousands of nuclear power plants. As the laser pulse hits the gas, the optical fields are strong enough to pull the electrons from the nucleus, separating negative and positive charges from each other, thus ionizing the gas. The ionization occurs for many atoms at the same time which means that part of the gas is turned into a plasma. Now it no longer behaves like a gas of individual molecules. Instead, the plasma has a more complex collective behavior similar to that of a fluid, but strongly driven by electromagnetic interactions. As the laser pulse travels through the plasma, electrons are subjected to a strong force which pushes them away, both forwards and radially outwards. This force is strong enough to create a bubble following the laser pulse which is completely empty of electrons and therefore carries a large positive charge. If a small fraction of electrons are placed inside this bubble, they will be strongly attracted by the positive charge of the bubble and become trapped inside it. Trapped electrons are now quickly accelerated to high energies when the bubble follows the laser pulse. Simulations of the radiation environment start with the accelerated electron beam and does not include the laser-plasma interactions themselves. The accelerated electrons originate from the center of a vacuum chamber made out of aluminum. When an accelerated electron enters the aluminum, a large number of processes can occur, producing secondary particles which in turn may undergo secondary processes, producing even more particles. The accelerated electrons and any secondary particles that manage to pass through the aluminum will then undergo similar processes as they hit the concrete walls in the laboratory. All particles generated in these processes, as well as the primary accelerated electrons, are tracked until they have lost all their energy and stop. To simulate these particle interactions and the production of secondary particles, so called, Monte Carlo methods are used, where a large number of primary electrons and secondary particles (e.g. photons and neutrons) are traced using statistical methods. This means that for each step in the simulation all the possible events are given a certain weight according to their relative probabilities of occurring. These probabilities are either determined from theoretical models or from experimental data. To pick one event from the complete probability distribution of all possible events, a random number generator is used. Results from the simulations indicate that using the current experimental setup, the radiation levels during one year are more than 20,000 times below the safe limit when the correct safety precautions are taken. They also indicate that the experiments are not without danger as a person standing directly in the electron beam would reach the yearly radiation limit within less than an hour of operation