High-gradient rf tests of welded $X$-band accelerating cavities

Linacs for high-energy physics, as well as for industry and medicine, require accelerating structures which are compact, robust, and cost-effective. Small foot-print linacs require high-accelerating gradients. Currently, stable-operating gradients, exceeding $100\mathrm{MV}/\mathrm{m}$, have been demonstrated at SLAC National Accelerator Laboratory, CERN, and KEK at X-band frequencies. Recent experiments show that accelerating cavities made out of hard copper alloys achieve better high-gradient performance as compared with soft copper cavities. In the scope of a decade-long collaboration between SLAC, INFN-Frascati, and KEK on the development of innovative high-gradient structures, this particular study focuses on the technological developments directed to show the viability of novel welding techniques. Two novel X-band accelerating structures, made out of hard copper, were fabricated at INFN-Frascati by means of clamping and welding. One cavity was welded with the electron beam and the other one with the tungsten inert gas welding process. In the technological development of the construction methods of high-gradient accelerating structures, high-power testing is a critical step for the verification of their viability. Here, we present the outcome of this step—the results of the high-power rf tests of these two structures. These tests include the measurements of the breakdown rate probability used to characterize the behavior of vacuum rf breakdowns, one of the major factors limiting the operating accelerating gradients. The electron beam welded structure demonstrated accelerating gradients of $90\mathrm{MV}/\mathrm{m}$ at a breakdown rate of ${10}^{-3}/(\mathrm{pulse}\mathrm{meter})$ using a shaped pulse with a 150 ns flat part. Nevertheless, it did not achieve its ultimate performance because of arcing in the mode launcher power coupler. On the other hand, the tungsten inert gas welded structure reached its ultimate performance and operated at about a $150\mathrm{MV}/\mathrm{m}$ gradient at a breakdown rate of ${10}^{-3}/(\mathrm{pulse}\mathrm{meter})$ using a shaped pulse with a 150 ns flat part. The results of both experiments show that welding, a robust, and low-cost alternative to brazing or diffusion bonding, is viable for high-gradient operation. This approach enables the construction of multicell standing and traveling-wave accelerating structures.

Figure 1

Solid model, one-half of the welded single-cell cavity: (1) middle high-gradient cell, rf vacuum chamber, (2) special screws for clamping, (3) input rf flange, (4) secondary vacuum chamber, (5) Conflat vacuum flange for the secondary vacuum chamber, (6) downstream Conflat vacuum flange, and (7) water cooling pipe.

Figure 2

Electric and magnetic fields on the cavity mid-plane which are scaled to a $100\mathrm{MV}/\mathrm{m}$ accelerating gradient in the middle cell; (a) shows the mid-plane electric field, with a maximum of $202.9\mathrm{MV}/\mathrm{m}$ and (b) shows the mid-plane magnetic field, with a maximum of $325\mathrm{kA}/\mathrm{m}$.

Figure 3

Left, single-cell cavity after TIG welding. Right, single-cell cavity after EBW. (1) Welding joints, (2) input rf flange, (3) downstream Conflat vacuum flange, and (4) Conflat flange for pumping the secondary vacuum chamber.

Figure 4

Bead-pull setup at SLAC with TIG welded cavity installed for measurements. (1) 1C-SW-A2.75-T2.0-TIG-Cu-Frascati-#1 cavity, (2) ${\mathrm{TM}}_{01}$ mode launcher, (3) nitrogen purge, and (4) fish-line pulley system for the bead pulling.

Figure 5

LabView screen of the bead-pull control program showing the on-axis electric field profile of the TIG cavity measured with the non-resonant bead-pull technique.

Figure 6

Schematic diagram of the experimental setup: (1) 1C-SW-A2.75-T2.0-EBW-Cu-Frascati-#1 (first experiment) and 1C-SW-A2.75-T2.0-TIG-Cu-Frascati-#1 (second experiment), (2) ${\mathrm{TM}}_{01}$ mode launcher, (3) current monitors to measure field emission currents, (4) high-power directional couplers, (5) waveguide 3 dB hybrid, (6) high-power loads, (7) SLAC XL-4 X-band klystron, (8) variable attenuator, (9) traveling-wave tube amplifier, (10) mixer for klystron signal shaping, (11) 11 GHz signal generator, (12) arbitrary function generator for rf pulse shaping, (13) fast digitizer, (14) peak power meter, (15) probes of peak power meter, (16) phase-locked mixers with built-in local oscillators for rf signal measurements, and (17) low pass filters. Not numbered: cross guide directional couplers that couple signal between the low pass filter (17) and probes (15) and (16), and low power rf loads which are connected to high-power directional couplers (4) and cross guide directional couplers.

Figure 7

The TIG structure installed inside the lead box. Experimental setup: (1) 1C-SW-A2.75-T2.0-TIG-Cu-Frascati-#1 cavity, (2) ${\mathrm{TM}}_{01}$ mode launcher, (3) coaxial cables to the current monitors, (4) port for secondary vacuum, and (5) high-vacuum ion pumps.

Figure 8

Breakdown performance of EBW and TIG welded hard copper structures with soft and hard copper clamped structures, shaped pulse with 150 ns flat part. (a) breakdown rates vs accelerating gradient, (b) breakdown rates vs surface peak electric fields, and (c) breakdown rates vs peak pulse surface heating.

Figure 9

Breakdown performance of EBW and TIG welded hard copper structures with soft and hard copper clamped structures, shaped pulse with 400 ns flat part. (a) breakdown rates vs accelerating gradient, (b) breakdown rates vs surface peak electric fields, and (c) breakdown rates vs peak pulse surface heating.

Figure 10

The breakdown probability dependence on pulse length for the EBW hard copper structure, using shaped pulses with 100, 200, and 400 ns flat parts. (a) total breakdown rates vs accelerating gradient, (b) total breakdown rates vs peak surface electric fields, and (c) total breakdown rates vs peak pulse surface heating.

Figure 11

The breakdown probability dependence on pulse length of TIG welded hard copper (1C-SW-A2.75-T2.0-TIG-Cu-Frascati-#1) structure, using shaped pulses with 100, 150, 400, and 600 ns flat parts. (a) breakdown rates vs accelerating gradient, (b) breakdown rates vs surface peak electric fields, and (c) breakdown rates vs peak pulse surface heating.

Figure 12

Drawing of the TIG welded cavity, showing reference numbers and labels for the SEM analysis. (1), (2), and (3) are the irises of end, middle, and input cell, respectively; (4), (5), and (6) are the end, middle, and input cell, respectively; (7) and (8) are the adjoining drift sections; and (9) is the welding joints.

Figure 13

Image of the middle cell side of iris 1 of the EBW structure. High-electric field areas have many small scattered arc pits. In addition, there are a few isolated locations that have concentrations of a very large number of arc pits, which are typically observed in these type of structures.

Figure 14

EBW structure. Image of middle cell side of iris 1. Machining edges are obvious but there are no signs rf field-induced damage.

Figure 15

EBW structure. Images of middle cell side of iris 1. High-magnetic field areas show no signs of pulse heating induced damage.

Figure 16

EBW structure. Images of the waveguide to rf flange joint. No signs of rf-related damage.

Figure 17

EBW structure. Image of the inner seal area. No evidence of arcing or erosion related to high-rf currents.

Figure 18

TIG structure. Images of output side of middle cell. The high-electric field areas have many scattered small arc pits. In addition, there are a significant number of isolated locations that have high concentrations of arc pits, which is typical for these type of structures.

Figure 19

TIG structure. Images of output side iris of input cell at a high-electric field area. This type of damage is typically associated with pulse surface heating in high-magnetic field areas.

Figure 20

TIG structure. Images of the output side outer surface of input cell. Top, high-magnetic fields areas; bottom, magnification in a region of these areas. The small pits are probably not rf-related.

Figure 21

TIG structure. Images of inner seal areas. No evidence of arcing was found.

Figure 22

TIG structure. Images of inner seal areas. The areas that appear raised and have a rougher texture are areas of incipient diffusion bonding.