ELBE SRF Gun II - An improved SRF gun for routine user operation, 2009 – Present

Following the successful proof-of-principle demonstration of the world’s first SRF gun operated in a superconducting accelerator—the ELBE linac—a second project was launched with financial support from the German Federal Ministry of Education and Research (BMBF). Its main objectives were to achieve a high average current of 1 mA and a low emittance of 1 mm·mrad at 77 pC for the ELBE linac, as well as to test novel semiconductor photocathodes within the BMBF-funded PCHB project.

The main objective was to achieve the design specifications for the intrinsic quality factor and maximum accelerating gradient of the niobium cavity. To this end, an improved cleaning procedure was applied, together with several design modifications based on the experience gained with the previously installed ACC300 cavity [1, 2]. Another major improvement was the integration of a superconducting solenoid into the gun cryomodule [3]. The solenoid is cooled by the same helium supply line as the cavity, while its precise positioning is controlled by thermally isolated stepper motors. The complete module was assembled and commissioned in the summer of 2014 [4].

The JLabFG cavity

The heart of the SRF Gun II photoinjector is the new JLabFG cavity, which is again a 3.5-cell cavity made of so-called fine-grain niobium. It was developed, fabricated, and tested in collaboration with Thomas Jefferson National Accelerator Facility [5] by late 2013. To reduce its sensitivity to helium pressure fluctuations and microphonics, the half-cell of the cavity was equipped with additional stiffening elements, shown in green in the image below.

Milestones

• Jun. 2010:     JLabFG cavity manufacturing finished

• Dec. 2010:     field- and frequency tuning

• Aug.2011:      first vertical tests (E₀ = 39 MV/m)

• Feb. 2012:     helium vessel welding and vertical test (E₀ = 41 MV/m)

• Nov. 2013:     final vertical test (E₀ = 41 MV/m) and string assembly

• Apr. 2014:      module assembly (see picture below) and commissioning

• Aug. 2014:    cool down and RF commissioning (E₀ = 25 MV/m [6])

• Feb. 2015:     4.5 MeV CW electron beam from a copper cathode [7]

• Mar. 2015:     Cs₂Te cathode operation in the gun (E₀ = 20 MV/m , [8])

• Mar. 2016:     acceleration of 200 pC electron beam up to 30 MeV by the ELBE LINAC

• Feb. 2017:    1st user beam time (several days) for neutron and THz generation [9]

• since 2018:   SRF gun delivers rountinely 4 MeV CW beam with 200-250 pC and 50, 100, 250 kHz rep. rate

• since 2019:   very reliable and stable user operation (1800h per year), on average 15 C per cathode in 500 hr beam time [10], [11]

• June 2024:  Generation of an average current of almost 1 mA, acceleration to 28 MeV by the ELBE LINAC and irradiation of one of the ELBE IR-FELs [13]

Cavity history

(note: E_cath=1.53xE_acc and E₀=2.56xE_acc)

References:

[1] P. Murcek et al., Modified 3½-Cell SC Cavity Made of Large Grain Niobium for the FZD SRF Photoinjector, Proc. SRF 2009 Conference, Sept. 20-25, 2009, Berlin & Dresden, Germany.

[2] P. Murcek et al., Modified SRF Photoinjector for the ELBE at HZDR, 15th International Conference on RF Superconductivity, 25.-29.07.2011, Chicago, USA.

[3] H. Vennekate et al., Emittance Compensation for an SRF Photo Injector, SRF 2013 – 16^th Int. Conf. on RF Superconductivity, September 23-27, 2013, Paris, France.

[4] P. Murcek et al., The SRF Photoinjector at ELBE – Design and Status 2013, SRF 2013 – 16^th Int. Conf. on RF Superconductivity, September 23-27, 2013, Paris, France.

[5] A. Arnold et al., Fabrication, Tuning, Treatment and Testing of Two 3.5 Cell Photo-Injektor Cavities for the ELBE Linac, 15^th International Conference on RF Superconductivity, 25.-29.07.2011, Chicago, USA.

[6] A. Arnold, et al., Commissioning Results of the 2nd 3.5 Cell SRF Gun for ELBE, LINAC 2014 - 27th Linear Accelerator Conference, 31st August to 5th September 2014, Geneva, Switzerland

[7] J. Teichert, et al., First Beam Characterization of SRF Gun II with a Copper Photocathode, 56th ICFA Advanced Beam Dynamics Workshop on Energy Recovery Linacs, Stony Brook, USA, 2015

[8] A. Arnold, et al., RF Performance Results of the 2nd ELBE SRF Gun, 17th International Conference on RF Superconductivity, September 13-18, 2015, Whistler, British Columbia, Canada

[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] R. Xiang, et al., SRF Gun and SRF Linac Driven THz at ELBE Successfully in User Operation, 19th International Conference on RF Superconductivity (SRF'19), Dresden, Germany, 2019, p. 915, doi:10.18429/JACoW-SRF2019-THP032

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

[13] A. Arnold, et al., ELBE SRF gun – the most advanced source of its kind, 22nd International Conference on RF Superconductivity (SRF'25), Tokyo, Japan, 2025