用于自由电子激光的S波段J型波导馈电加速管

S-band J-type waveguide feeding accelerating structure for free electron laser

随着波导式耦合行波加速管设计梯度的日益提高,为了防止加速管输入耦合器电场的横向动量在束流通过加速器耦合器时引起束流品质的下降,侧壁开有两个对称耦合孔的对称双馈圆柱腔获得了广泛的应用。研制的S波段J型波导馈电加速管即为双馈圆柱腔中的一种,研制样管在老练平台上老练时的最高加速梯度达到30 MV/m。然而因为四极场的存在,开有两个耦合孔的圆柱耦合腔内,仍然会引起轴向电场的幅度和相位在横向的梯度,从而使束流发射度变差。在理论上对J型波导馈电的跑道式耦合腔进行了研究,通过与圆柱腔进行比对模拟计算,证明跑道式耦合器可以很好地改善轴向电场在横向平面内非近轴区域沿圆周的场强一致性,从而减小四极场的影响。重要的是,J型波导馈电跑道式耦合腔的机械加工、测试比圆柱腔更加容易实现,是未来双馈加速器发展的一个理想方向。

With the development of the required gradient of the waveguide coupled TW accelerating structure,for preventing the undesirable transverse momentum contributions being imparted to the beam during traversal of the linac coupler cavities,the use of symmetric dual feed cylindrical cavities which have diametrically opposed side wall coupling apertures has been widely accepted.As one type of the dual feed cylindrical cavities,the prototype of the S-band J-type waveguide feeding accelerating structure developed has got a top accelerating gradient of 30 MV per meter during RF conditioning.However,the transverse gradient of the amplitude and the phase of the longitudinal electric field caused by the existence of the quadrupolar field in the dual feed cylindrical cavity,will also degrade the beam emittance.So the J-type waveguide feeding racetrack cavity is studied theoretically.With comparison of the simulation results to the cylindrical cavity,the racetrack cavity can improve the rotational symmetry of the longitudinal electric field in the non-near axis area in the transverse plane very well,which will reduce the impact of the quadrupolar field.More importantly,the J-type waveguide feeding racetrack cavity is easier in the aspect of mechanical fabrication and measurement,which seems to be a very good direction for the future dual feed accelerating structures.