The terahertz gap is a region of the electromagnetic spectrum where high average and peak power radiation sources are scarce while at the same time scientific and industrial applications are growing in demand. Free-electron laser (FEL) coupling in a magnetic undulator is one of the best options for radiation generation in this frequency range, but slippage effects require the use of relatively long and low-current electron bunches to drive the terahertz FEL, limiting amplification gain and output peak power. Here we use a circular waveguide in a 0.96-m strongly tapered helical undulator to match the radiation and electron-beam velocities, allowing resonant energy extraction from an ultrashort 200-pC 5.5-MeV electron beam over an extended distance. Electron-beam spectrum measurements, supported by energy and spectral measurement of the terahertz FEL radiation, indicate an average energy efficiency of ~10%, with some particles losing >20% of their initial kinetic energy.

太赫兹间隙是电磁频谱的一个区域，其中高平均和峰值功率辐射源稀缺，同时科学和工业应用的需求也在增长。磁波荡器中的自由电子激光 (FEL) 耦合是该频率范围内产生辐射的最佳选择之一，但滑移效应需要使用相对较长且电流较低的电子束来驱动太赫兹 FEL，从而限制了放大增益并输出峰值功率。在这里，我们在 0.96 米的强锥形螺旋波荡器中使用圆形波导来匹配辐射和电子束速度，从而允许从超短 200-pC 5.5-MeV 电子束中提取更长距离的共振能量。电子束光谱测量，