rf breakdown tests of mm-wave metallic accelerating structures

We are exploring the physics and frequency-scaling of vacuum rf breakdowns at sub-THz frequencies. We present the experimental results of rf tests performed in metallic mm-wave accelerating structures. These experiments were carried out at the facility for advanced accelerator experimental tests (FACET) at the SLAC National Accelerator Laboratory. The rf fields were excited by the FACET ultrarelativistic electron beam. We compared the performances of metal structures made with copper and stainless steel. The rf frequency of the fundamental accelerating mode, propagating in the structures at the speed of light, varies from 115 to 140 GHz. The traveling wave structures are 0.1 m long and composed of 125 coupled cavities each. We determined the peak electric field and pulse length where the structures were not damaged by rf breakdowns. We calculated the electric and magnetic field correlated with the rf breakdowns using the FACET bunch parameters. The wakefields were calculated by a frequency domain method using periodic eigensolutions. Such a method takes into account wall losses and is applicable to a large variety of geometries. The maximum achieved accelerating gradient is $0.3\mathrm{GV}/\mathrm{m}$ with a peak surface electric field of $1.5\mathrm{GV}/\mathrm{m}$ and a pulse length of about 2.4 ns.

Figure 1

Picture of the mm-wave traveling wave copper structure operating in the standing wave regime (a), and of the traveling wave copper and stainless steel structures, at the bottom-left and top-right, respectively (b).

Figure 2

The mm-wave traveling wave accelerating structure with detail of the output part, including coupler and output waveguides. Solid model (a) and picture (b).

Figure 3

Geometry of one quarter of the vacuum part (simulated by Ansys HFSS) of the regular cell (a) and of the coupler of the SW structure (b).

Figure 4

Geometry of one quarter of the vacuum part (simulated by HFSS) of the regular cell (a) and of the coupler of the TW structure (b).

Figure 5

Plot of the electric field (a) and of the magnetic field (b) of the eigenmode solutions of one quarter of period of the first copper cell ($\text{gap}=0.9\mathrm{mm}$), at the synchronous frequency. Considering an infinite traveling wave structure these fields are excited by a bunch of charge of 2.7 nC and $50\mu \mathrm{m}$ long.

Figure 6

Dispersion curve (red) and speed of light line (blue), with intersection at the synchronous point (for clarity we show a very high group velocity structure).

Figure 7

Plot (a): Unfolded dispersion curve of the first and second mode (red and green) and speed of light line (blue), with intersection at the synchronous points. Plot (b): Folded dispersion curve of the first and second mode (red and green) and folded speed of light line (cyan-dashed), with opposite intersection for the high order mode.

Figure 8

Wakefield of the only trapped mode (blue), electron bunch shape (red), for the first copper structure, with $\mathrm{gap}=0.9\mathrm{mm}$.

Figure 9

Pulse of the power induced by the 2.7 nC bunch.

Figure 10

Coupler reflection (a) and transmission (b), as a function of the gap aperture, for couplers matched for specific apertures ($a=0.15$, 0.2, 0.25, 0.3, 0.45 mm). These plots show that matching the coupler to $a=0.15\mathrm{mm}$ does not produce significant mismatches when the gap is changed, where “$a$” is the half gap.

Figure 11

Integrated loss factors for the TW structure, for all the analyzed gaps.

Figure 12

Output power signals from the waveguide generated by a 3.2 nC bunch of $50\mu \mathrm{m}$, taking into account the reflection at the coupler, for the copper structure (a) and the stainless steel structure (b).

Figure 13

Synchronous frequency (a), decay length (b), group velocity (c), peak and accelerating electric field (d), peak power traveling along the cells, and power exiting from the waveguide considering the reflection (e), pulse energy with and without considering the coupler reflection (f) with respect to the gap, for 3.2 nC and $50\mu \mathrm{m}$ long bunch.

Figure 14

Plot of the electric field (a) and of the magnetic field (b) of the eigenmode solutions of one quarter of period of the TW cell ($\text{gap}=0.2\mathrm{mm}$), at the synchronous frequency. Considering an infinite traveling wave structure these fields are excited by a bunch of charge of 3.2 nC and $50\mu \mathrm{m}$ long.

Figure 15

Wakefield of the first mode (black dashed), total wakefield (blue), electron bunch shape (red), of the TW copper structure, for 0.2 mm gap (a), for 0.3 mm gap (b), for 0.5 mm gap (c), for 0.9 mm gap (d).

Figure 16

Accelerating gradient (a), output power (b), copper pulse energy (c), stainless steel pulse energy (d), in function of the position of a 3.2 nC electron beam relative to the center of the corrugations. Plots (b)–(d) take into account the coupler reflection.

Figure 17

First experiment: alignment camera view (a), first experiment: Structure and phosphor screen view from the alignment-camera window(b).

Figure 18

SEM microscope picture of breakdowns generated during the experiment involving the first copper structure, driven by a bunch charge of 2.7 nC and $50\mu \mathrm{m}$ length. Output end of the structure (a), 1st iris breakdown damage (b), 9th iris no breakdown damage (c).

Figure 19

Second experiment setup (a): signal to scope (1), remotely controlled rf beam shutter (2), crystal detector (3), kraken vacuum chamber (4), electron beam (5), laser alignment mirror (6), right reflected rf horn (7), left forward rf horn (8), phosphor (9), rf window (10), output rf beam (11), pyrodetector (12), video camera for beam-structure alignment and rf breakdown diagnostic (13), picture of the device (b).

Figure 20

TW copper input coupler, cells 1–7, no damage.

Figure 21

SEM microscope of the breakdowns generated during the experiment involving the TW copper structure, driven by a bunch of charge of 3.2 nC and $50\mu \mathrm{m}$ long. Cells number 16–23, fist signs of damage (a), iris 19–20, fist signs of damage (b), output coupler, massive breakdown damage (c), cells 108–109, breakdown damage (d).

Figure 22

Third experiment setup (a): signal to scope (1), remotely controlled rf beam shutter (2), crystal detector (3), vacuum chamber (4), electron beam (5), laser alignment mirror (6), right reflected rf horn (7), left forward rf horn (8), phosphor (9), rf window (10), output rf beam (11), pyrodetector (12), video camera for beam-structure alignment and rf breakdown diagnostic (13), vacuum feed-through (14), picture of the device (b).

Figure 23

Predicted (blue) and measured (red) pulse energy in function of the horizontal position of a 3 nC electron beam, for stainless steel structures of $\text{gap}=0.7\mathrm{mm}$ (a), $\text{gap}=0.5\mathrm{mm}$ (b).

Figure 24

Predicted (blue) and measured (red) peak pulse energy in function of the gap, for a 3 nC electron beam (Stainless steel structure).

Figure 25

First observation of rf breakdowns on current monitor, electron beam scan with FACET. Sections (a,b) data: $\text{gap}=0.7\mathrm{mm}$, $\mathrm{q}=3\mathrm{nC}$, $E_{z0}=0.17\mathrm{GV}/\mathrm{m}$, $E_{\max }=0.52\mathrm{GV}/\mathrm{m}$, $l_{att}/v_g=0.35\mathrm{ns}$. Sections (c,d) data: $\text{gap}=0.6\mathrm{mm}$, $\mathrm{q}=3\mathrm{nC}$, $E_{z0}=0.16\mathrm{GV}/\mathrm{m}$, $E_{\max }=0.5\mathrm{GV}/\mathrm{m}$, $l_{\text{att}}/v_g=0.35\mathrm{ns}$.

Figure 26

SEM microscope of the breakdowns generated during the experiment involving the TW stainless steel structure, driven by a bunch of charge of 3 nC and $50\mu \mathrm{m}$ long. Output coupler, no damage (a), iris 110–111, no damage (b), input coupler, beam damage, looks similar to pulse heating damage we see in high gradient rf structures (c)–(e), cells 9–10 rf breakdown damage which is aligned with beam damage, rf breakdowns were likely caused by accidental beam damage to few irises (f)–(h).

Figure 27

Irreversible damage, electron beam scan for the same aperture at the beginning of the shift (a,b) and the end of the shift (c,d), with 0.9 mm gap, charge 3 nC.