学　术

　　人力资源部教师发展中心开展“名师讲堂”系列活动，定期邀请国内外知名专家学者、国家级教学名师等作专题报告，旨在加强广大师生的学术思想交流和碰撞，促进青年教师成长，开拓学生视野。

　　“学者论坛”第48期活动特别邀请来自以色列理工学院的Levi Schächter教授主讲，欢迎广大师生参与。具体安排如下：

　　主　题：Particle Acceleration by Cherenkov Radiation Amplified by an Active Medium

　　主讲人：APS Fellow, 以色列理工学院　Levi Schächter 教授

　　时　间：2016年11月18日（周五）16:00-18:00

　　地　点：物理电子学院204会议室

　　主持人：物理电子学院　段兆云教授

　　内容简介：

　　Linear Regime: We present detailed development of the linear theory of wakefield amplification by active medium and its possible application to a two-beam accelerator (TBA) is discussed. A relativistic train of triggering microbunches traveling along a vacuum channel in an active medium confined by a cylindrical waveguide excites Cherenkov wake in the medium. The wake is a superposition of azimuthally symmetric transverse magnetic modes propagating along a confining waveguide, with a phase velocity equal to the velocity of the triggering bunches. The structure may be designed in such a way that the frequency of one of the modes is close to active-medium resonant frequency, resulting in amplification of the former and domination of a single mode far behind the trigger bunches. Another electron bunch placed in proper phase with the amplified wakefield may be accelerated by the latter. Importantly, the energy for acceleration is provided bythe active medium and not the drive bunch as in a traditional TBA. Based on a simplified model, we analyze extensively the impact of various parameters on the wakefield amplification process.

　　Non Linear Regime: A trigger bunch of electrons traveling inside or in the vicinity of a dielectric medium generates a Cerenkov wake. If the dielectric medium is active, a small fraction of the spectrum of the wake is amplified and far behind the trigger bunch where the active medium is fully depleted, the amplitude and the phase of the wake are virtually constant. In this range, a second bunch of electrons trailing behind the trigger bunch can be accelerated. For optimal operation, the trigger bunch should be density modulated at the resonant frequency of the medium. However, we demonstrate that even if the bunch is uniform along many wavelengths we may still take advantage of the saturation characteristics to obtain conditions adequate for acceleration. Further we demonstrate that for large enough number of electrons it is possible to have a coherent amplified wake after a saturation length which is determined analytically and tested numerically. In addition, we show that almost 100% of the stored energy in the active medium can be transferred to the acceleration of the trailing bunch electrons. The relatively large energy spread due to the beam loading is well suited to a medical accelerator. When the beam loading is weak, the gradient is virtually constant but the acceleration efficiency drops to about 2% for typical parameters.

　　主讲人介绍：

　　Levi教授，1983年、1985年、1988年分别在以色列理工学院获得电子工程学士、硕士和博士学位。1996年-2013年多次前往美国康奈尔大学和斯坦福大学访学。现任以色列理工学院电子工程学院Gerard Swope主席、校学术委员、校友会董事、美国物理学会Fellow、国际无线电科学联合会以色列分部秘书。长期从事加速器物理、辐射源等方面的研究，发表论文300余篇，申请专利多项，出版专著3册，获得Hershel Rich prize，Kidron Foundation Prize等奖项。

　　主办单位：人力资源部教师发展中心

　　承办单位：物理电子学院

　　　　　　　　　　　　　　　　　　　人力资源部教师发展中心

　　　　　　　　　　　　　　　　　　　　　2016年11月14日