Continuous wave superconducting radio frequency electron linac for nuclear physics research

CEBAF, the Continuous Electron Beam Accelerator Facility, has been actively serving the nuclear physics research community as a unique forefront international resource since 1995. This cw electron linear accelerator (linac) at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility (Jefferson Lab) has continued to evolve as a precision tool for discerning the structure and dynamics within nuclei. Superconducting rf (SRF) technology has been the essential foundation for CEBAF, first as a 4 GeV machine, then 6 GeV, and currently capable of 12 GeV. We review the development, implementation, and performance of SRF systems for CEBAF from its early beginnings to the commissioning of the 12 GeV era.

Figure 1

CEBAF cryounit configuration (from [9]).

Figure 2

General configuration of CEBAF.

Figure 3

Representative performance of CEBAF cavities in 2 K acceptance tests (from [51]).

Figure 4

Cavity performance just below quench during pair testing (from [48]).

Figure 5

Cavity performance at onset of detected x rays during pair testing (from [48]).

Figure 6

CEBAF cavity usable performance per vertical acceptance tests (from [51]).

Figure 7

CEBAF cavity usable performance per tunnel commissioning tests (from [51]).

Figure 8

Distribution of CEBAF cavity gradient limitations, 1995 (from [51]).

Figure 9

Operational projection of CEBAF cryomodule capacity in 1995 (from [51]).

Figure 10

Modeled gradients for arc fault intervals of four days (from [70]).

Figure 11

CEBAF available SRF cryomodule voltage in 1996 (from [63]).

Figure 12

1996 operational envelope of CEBAF at reduced klystron power (from [81]).

Figure 13

Cavity performance improvement with helium processing 1996–1998 (from [85]).

Figure 14

Matched rf load at full beam current (from [85]).

Figure 15

Maximum SRF voltage per cryomodule in CEBAF in 1999 (from [87]).

Figure 16

Distribution of cavity gradients producing 3 arcs per day (from [88]).

Figure 17

CEBAF operating highpoints prior to 2002 (from [88]).

Figure 18

Comparison of fixed arc-rate gradient conditions before and after the hurricane-induced thermal cycle and recovery (from [90]).

Figure 19

Forward power drive signal for an SRF cavity with $250\mu \sec$-duration tune beam with single beam (red) and a two-beam, energy recovering setup (blue) (from [93]).

Figure 20

rf window relocation and dogleg waveguide in C50 cryomodules.

Figure 21

CEBAF maximum beam energy evolution prior to and during the C50 project.

Figure 22

Performance of cavity IA-085 as installed in CEBAF in 1993, in reworked qualification VTA and cryomodule acceptance tests, and as recommissioned in CEBAF in 2008 (from [106]).

Figure 23

Cross section of Mk II CEBAF cryomodule.

Figure 24

Acceptance test performance of cavities in the first intermediate prototype upgrade cryomodule (SL21) in the CMTF (from [118]).

Figure 25

Cross section of Renascence cavity, couplers, helium vessel, and tuner.

Figure 26

Qualification tests of the four Low Loss (LL) cavities for Renascence (from [123]).

Figure 27

Performance of CEBAF seven-cell cavities after $\sim 30\mu \mathrm{m}$ EP (from [146]).

Figure 28

Performance test of cavity C100-6 without helium vessel.

Figure 29

C100 cavity string assembly.

Figure 30

Annotated oscilloscope image capturing the transition from self-excited-loop (SEL) to generator-driven-resonator (GDR) control with piezotuner enabled (from [157]).

Figure 31

Piezotuner control off and on (from [157]).