Time: Friday, Nov. 20, 10:00 am
Venue: Rm. 413, Bld. 11#, Yuquan Campus & Ding Talk Video Conference (Group "Plasma Seminar")
Speaker #1: 宁小川
Title: The focusing condition of an intense ion beam in resistive plasmas
Abstract: Ion beam transport in a plasma has been widely studied in fundamental physics and applications, such as inertial confinement fusion, accelerator physics and cosmic rays. Of considerable important researches, a particularly important topic is the transverse focusing. Many approaches have been put forward to achieve this goal. A modern one is using a background plasma containing abundant electrons to neutralize the beam charge first, which can facilitate the reduction of strong space-charge forces. Then the neutralized beam is injected into an external magnetic lens. Recently, we find that the beam may also be focused by the self-generated fields while propagating in a large-scale resistive plasma. In the transverse direction, the beam is governed by the focusing force J_b×B and the diffusion force -∇p_th. Whether the ion beam can be focused or not is the result of the competition between these two forces. It has been found in theory and confirmed in simulation that both the beam density and velocity have a significant impact on focusing. Increasing either its density or velocity can promote the beam to focus. In addition, the beam radius is also an important parameter to the focusing. It can determine the focusing distance to some extent. Qualitatively, a smaller radius can make the focusing distance shorter.
Speaker #2: 程中明
Title: Relativistic doughnut-shaped light solitons and electron acceleration by vector laser beams.
Abstract: We simulated the interaction between vector laser beams and plasma by the JPIC3D code. We found that angularly-polarized lasers can be easily captured by underdense plasma and forms a doughnut-shaped soliton. The soliton-formation distance is related to the plasma density and laser duration. We also studied two new ways to accelerate elections to relativistic energy in a short distance with radially-polarized lasers. The first method is that lasers are self-focused to a higher intensity in underdense plasmas, then accelerate electrons around the plasma rear surface. The second method is that the laser first pulls out an isolated electron bunch from the plasma surface, then the laser is reflected and focused, further catches up with the electron bunch and boosts the election bunch into a higher energy.