# Tapering Enhanced Stimulated Superradiant Oscillator

## TL;DR

This paper introduces a novel high-power, high-efficiency free-electron laser oscillator utilizing the TESSA scheme, which employs a high-intensity seed pulse for efficient energy extraction and is optimized for 1 μm wavelength operation.

## Contribution

The paper presents the first implementation of a TESSA-based oscillator with a high repetition rate electron beam and optical cavity, enhancing laser power and efficiency.

## Key findings

- Achieved high power and efficiency in the TESSA oscillator design.
- Demonstrated optimization for 1 μm wavelength with 1 MHz repetition rate.
- Showed effective recirculation and monochromatization of radiation.

## Abstract

In this paper, we present a new kind of high power and high efficiency free-electron laser oscillator based on the application of the tapering enhanced stimulated superradiant amplification (TESSA) scheme. The main characteristic of the TESSA scheme is a high intensity seed pulse which provides high gradient beam deceleration and efficient energy extraction. In the oscillator configuration, the TESSA undulator is driven by a high repetition rate electron beam and embedded in an optical cavity. A beam-splitter is used for outcoupling a fraction of the amplified power and recirculate the remainder as the intense seed for the next electron beam pulse. The mirrors in the oscillator cavity refocus the seed at the undulator entrance and monochromatize the radiation. In this paper we discuss the optimization of the system for a technologically relevant example at 1 $\mu$m using a 1~MHz repetition rate electron linac starting with an externally injected igniter pulse.