Superconducting Radio Frequency Photo Electron Injector (SRF gun)

The success of many proposed energy-recovery linacs (ERLs)—as potential replacements for storage rings—and high-power free-electron lasers (FELs) strongly depends on the development of suitable electron sources. Consequently, high-brightness electron injectors capable of continuous-wave (CW) operation at megahertz repetition rates and bunch charges of up to 1 nC are a major focus of contemporary accelerator research and development.

Currently, the injector field is dominated by two types of electron sources. The first is the DC injector, or DC gun, which uses a static electric field to initially accelerate the electrons and can therefore readily provide CW beams. However, the comparatively low electric-field strength at the cathode surface and the short accelerating gap limit the achievable beam quality and maximum extractable bunch charge [1].

The second, more advanced type is the normal-conducting radio-frequency injector, or NCRF gun, which can produce high-quality electron beams. Nevertheless, its limited duty cycle may restrict the performance of superconducting accelerators. Efforts to increase the duty cycle are ongoing, but they come at the cost of more complex cooling systems, higher RF power requirements, and reduced power-conversion efficiency [2].

SRF guns represent a further step in the development of photoinjector technology. By combining the well-established principles of NCRF guns with superconducting RF technology, the dissipated RF power can be reduced by several orders of magnitude, enabling CW operation at high average beam currents.

The concept was first proposed in 1988 by Chaloupka and co-workers [3]. Four years later, the first experiments were conducted at the University of Wuppertal [4]. Another decade later, in 2002, the world’s first electron beam from an SRF gun was generated by the Drossel SRF gun at Forschungszentrum Rossendorf (FZR, now HZDR) in Dresden. This proof-of-principle experiment also demonstrated that a normal-conducting semiconductor photocathode could be successfully operated inside a superconducting half-cell cavity [5].

Table 1: Design parameters for ELBE SRF guns I and II

Inspired by this success, several R&D projects were launched within the Helmholtz Association in Germany, including projects at HZB and DESY, as well as at other research facilities worldwide. These projects aimed to develop SRF guns for future accelerator-based light sources [6].

At HZDR, this work was continued in 2004 with the development of a 3.5-cell SRF gun, referred to as ELBE SRF gun I, in collaboration with DESY, HZB, and MBI [7]. The project pursued two main objectives: the installation of a high-brightness photoinjector for the ELBE accelerator at HZDR, with the design parameters listed in Table 1, and a broader contribution to SRF gun R&D as a promising technology for future accelerator facilities.

An important part of this research is the further development of suitable photocathodes that meet the source requirements without adversely affecting the superconducting cavity. In addition, the investigation of novel emittance-compensation methods adapted to this specific configuration remains a major focus of the current research at HZDR.

Although the performance of ELBE SRF gun I fell short of expectations, it successfully demonstrated the first lasing of ELBE’s free-electron laser [8]. To achieve the full performance potential of the SRF gun, a new and improved niobium cavity was developed and fabricated in collaboration with Thomas Jefferson National Accelerator Facility (Jefferson Lab) in the United States. The cavity was installed in a new gun cryostat, referred to as ELBE SRF gun II, and subsequently commissioned.

Starting in 2016, user demand increased for higher bunch charges of up to 200 pC at a repetition rate of 100 kHz, as well as for improved beam stability, particularly for the generation of THz radiation. ELBE SRF gun II has therefore been used routinely for this application [9–11].

Content

Drossel - The world's first working SRF gun

ELBE SRF Gun I - The world's first SRF gun injecting in to a LINAC

 ELBE SRF Gun II - Our SRF gun for routine user operation

Cathode preparation

Publications

Projects

References:

[1] N. Nishimori, et al., DC Gun Technological Challenges, Proc. ERL09, Cornell University, Ithaca NY (June 8-12, 2009).

[2] D. H. Dowell et al., First operation of a photocathode radio frequency gun injector at high duty factor, Appl. Phys. Lett. 63, 2035 (1993).

[3] H. Chaloupka, et al., A proposed superconducting photoemission source of high brightness, Nuclear Instruments and Methods A285 (1989) 327.

[4] A. Michalke, Ph.D. thesis, University of Wuppertal, 1992, WUB-DIS 92-5.

[5] D. Janssen, et al., First operation of a superconducting RF gun, Nuclear Instruments and Methods A507 (2003) 314.

[6] A. Arnold, J. Teichert, Overview on superconducting photoinjectors, Physical Review Special Topics – Accelerators and Beams 14 (2011) 024801.

[7] A. Arnold, et al., Development of a superconducting radio frequency photoelectron injector, Nuclear Instruments and Methods A577 (2007) 440.

[8] J. Teichert, et al., Free-electron laser operation with a superconducting radio-frequency photoinjector at ELBE, Nuclear Instruments and Methods in Physics Research A 743 (2014) 114.

[9] B. Green, et al., High-Field High-Repetition-Rate Sources for the Coherent THz Control of Matter, Scientific Reports 6, 22256 (2016)

[10] Hassan A. Hafez, et al., Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions, Nature Vol. 561, 507–511 (2018)

[11] J. Teichert, et al., Successful user operation of a superconducting radio-frequency photoelectron gun with Mg cathodes, Phys. Rev. Accel. Beams 24, 033401 (2021)