Superconducting Radio Frequency Photo Electron Injector (SRF gun)

The success of most of the proposed energy recovery LINACs (ERL) -- as future storage ring replacements (SRR) -- and high power infrared free-electron lasers (FELs)  depends strongly on the development of appropriate sources. Thus, high brightness electron injectors for continuous wave (CW) operation with megahertz pulse repetition rates and bunch charges of up to 1 nC are a hot topic of contemporary accelerator research and development.

At present, the state-of-the-art injector field is dominated by to types of electron sources. One type is the DC injector (DC gun) which uses a DC field to initially accelerate electrons and therefore easily provides CW beams. However, the low electric field strength at the cathode’s surfaceand the short accelerating gap limit the beam quality and the maximum extractable bunch charge [1]. The other more advanced kind is the normal conducting radio frequency (RF) injector (NCRF gun) which produces high quality eletron beams. Nevertheless, its low duty cycle may limit the performance of superconducting accelerators. Efforts to increase the duty cycle are underway but at the expense of complex cooling requirements, higher klystron power and lower power conversion efficiency [2].

SRF guns represent a new step in the development of photoinjector technologies. By merging the well-established NCRF technology and superconductivity, the dissipated RF power is reduced by several orders of magnitude and the CW operation for high average currents can be realized. The concept was first proposed in 1988 by Chaloupka and co-workers [3]. Four years later, first experiments were done at the University of Wuppertal [4]. Another 10 years later, in 2002, world’s first electrons were delivered by the Drossel SRF gun at FZR (now HZDR) in Dresden. Within this proof-of-principle experiment, it was also possible to show that a normal conducting semiconductor photo cathode can be operated in a superconducting half-cell cavity [5].

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


ELBE mode

High charge mode

RF frequency

1.3 GHz (CW)

beam energy

9.5 MeV

drive laser

262 nm

photocathode (quantum efficiency)

Cs2Te (≥1%)

repetition rate

13 MHz

≤ 500 kHz

pulse length (FWHM)

4 ps

15 ps

laser spot size

2 mm

5 mm

bunch charge

77 pC

1 nC

average current

1 mA

0.5 mA

normalized transverse emittance (rms)

1 mm mrad

2.5 mm mrad

Inspired by this success, several R&D projects were launched within the Helmholtz Association in Germany (e.g. HZB, DESY) and other research facilities worldwide, which plan the application of SRF guns in future accelerator-based light sources [6]. At the same time, the work at HZDR was continued in 2004 by developing a 3½-cell SRF gun (referred to as ELBE SRF Gun I) in collaboration with DESY, HZB and MBI [7]. The project has two main goals: The installation of a high-brightness photoinjector (see table 1) for the ELBE accelerator at HZDR as well as the overall contribution to SRF gun R&D as a promising future technology. Part of this research is to enhance the used photo cathodes in order to meet the requirements for the designed sources while leaving the superconducting cavity unimpaired. Furthermore, the investigation of new methods of emittance compensation, applicable for this particular setting, is of great focus of the current work at HZDR.

Although the performance of the ELBE SRF Gun I was behind the expectations, it was used to demonstrate first lasing with ELBE’s free-electron laser [8]. In order to realize the full performance of the SRF gun, a new and improved niobium cavity has been developed and built in cooperation with Thomas Jefferson National Accelerator Facility (Jefferson Lab) in the United States. This cavity has been installed in a new gun cryostate (ELBE SRF Gun II) and commissioned over the last years. Starting in 2016, users' demand for higher bunch charge (200 pC @ 100 kHz) and higher stability increased to generate THz radiation. So, especially for this application SRF gun II is now used routinely [9, 10, 11].



DrosselDrossel - The world's first working SRF gun

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

Drossel ELBE SRF Gun II - An improved SRF gun for routine user operation

DrosselCathode preparation




[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)