Effort towards symmetric removal and surface smoothening of 1.3-GHz niobium single-cell cavity in vertical electropolishing using a unique cathode

A detailed study on vertical electropolishing (VEP) of a 1.3-GHz single-cell niobium coupon cavity, which contains six coupons and four viewports at different positions, is reported. The cavity was vertically electropolished using a conventional rod and three types of unique cathodes named as Ninja cathodes, which were designed to have four retractable blades made of either an insulator or a metal or a combination of both. This study reveals the effect of the cathodes and their rotation speed on uniformity in removal thickness and surface morphology at different positions inside the cavity. Removal thickness was measured at several positions of the cavity using an ultrasonic thickness gauge and the surface features of the coupons were examined by an optical microscope and a surface profiler. The Ninja cathode with partial metallic blades was found to be effective not only in reducing asymmetric removal, which is one of the major problems in VEP and might be caused by the accumulation of hydrogen (${\mathrm{H}}_2$) gas bubbles on the top iris of the cavity, but also in yielding a smooth surface of the entire cavity. A higher rotation speed of the Ninja cathode prevents bubble accumulation on the upper iris, and might result in a viscous layer of similar thickness in the cavity cell. Moreover, a higher electric field at the equator owing to the proximity of partial metallic blades to the equator surface resulted in a smooth surface. The effects of ${\mathrm{H}}_2$ gas bubbles and stirring were also observed in lab EP experiments.

Figure 1

1.3-GHz Nb coupon cavity with six disk-type Nb coupons at beam pipe, iris, and equator positions shown by red arrows. Inset: Photograph of a coupon.

Figure 2

Schematics of (a) NC-1 with Al blades, (b) NC-2 with insulating blades, and (c) NC-3 with partial metallic blades. (d) Ninja cathode inside a cavity cell. Red color in the figures represents Al cathode whereas white and grey colors indicate the insulating parts.

Figure 3

Schematic of VEP setup for a one-cell cavity.

Figure 4

(a) Schematic showing the arrangement of Nb samples near the Al cathode and electrical connections to measure EP currents from individual samples, (b) EP bath, and (c) stirrer for the acid agitation.

Figure 5

Coupon and cavity current densities obtained with (a) rod cathode and (b) NC-1. The red dotted vertical lines in (b) show the times when the blades faced the top iris coupon. The grey solid vertical lines in (b) represent the logged signal of cathode rotation.

Figure 6

Top iris viewport during VEP. (a) Bubbles in the case of the rod cathode, (b) bubbles before passing a blade of NC-1, and (c) a huge amount of bubbles, which removed sticking bubbles, while passing a blade of NC-1.

Figure 7

(a) Black dots show positions of thickness measurements. Trends of the removal thicknesses in VEP performed with (b) rod cathode and (c) NC-1.

Figure 8

Optical microscope image of a typical non-EPed coupon surface. Inset: Zoomed-out image of the surface.

Figure 9

Optical microscope images of the coupons after VEP with the rod cathode (left) and NC-1 (right). Coupons located at (a) top beam pipe, (b) bottom beam pipe, (c) top iris, (d) bottom iris, (e) and (f) equator. Insets show zoomed-out images of the surfaces.

Figure 10

Roughness Ra (up) and Rz (down) of coupons after VEP was performed with the rod cathode and NC-1.

Figure 11

Current density profiles for the Nb samples located at different positions relative to the cathode.

Figure 12

Optical microscope images of the Nb samples after lab EP-1. Insets show zoomed-out images.

Figure 13

Current densities for the Nb samples at a stirring speed of 210 rpm.

Figure 14

Optical microscope images of the Nb samples EPed at a stirring speed of 210 rpm. Insets show zoomed-out views of the surfaces.

Figure 15

Coupon current densities with NC-2 at rotation speeds of 1 and 50 rpm.

Figure 16

Polarization curves obtained for the beam pipe, iris, and equator coupons, and cavity with NC-2 at 0 rpm (a)–(d) and 50 rpm (e)–(h). Solid vertical lines represent a transition from the etching region to the oscillation region followed by the polishing region.

Figure 17

Polarization curves obtained for the beam pipe, iris, and equator coupons, and cavity with NC-3 at 0 rpm (a)–(d) and 50 rpm (e)–(h). The solid vertical lines represent a transition from the etching region to the oscillation region followed by the polishing region.

Figure 18

(a) Black dots show positions of thickness measurements. Comparison of removal thicknesses along the cavity length for the VEP experiments performed with (b) NC-2 and (c) NC-3.

Figure 19

Optical microscope images of the coupon surfaces after VEP with NC-2 (left) and NC-3 (right). Coupons located at (a) top beam pipe, (b) bottom beam pipe, (c) top iris, (d) bottom iris, (e) and (f) equator. Insets show zoomed-out images of the surfaces.

Figure 20

Comparison of average roughness Ra (up) and Rz (down) of the coupons VEPed with NC-2 and NC-3.