Electromagnetic characterization of nonevaporable getter properties between 220–330 and 500–750 GHz for the Compact Linear Collider damping rings

Due to its effective pumping ability, nonevaporable getter (NEG) coating is considered for the vacuum chambers of the Compact Linear Collider (CLIC) electron damping rings (EDR). The aim is to suppress fast beam ion instabilities. The electromagnetic (EM) characterization of the NEG properties up to ultra-high frequencies is required for the correct impedance modeling of the damping ring (DR) components. The properties are determined using rectangular waveguides which are coated with NEG. The method is based on a combination of complex transmission coefficient $S_{21}$ measurements with a vector network analyzer (VNA) and 3D simulations using CST Microwave Studio® (CST MWS). The frequency ranges discussed in this paper are 220–330 and 500–750 GHz.

Figure 1

WR3.4 (left top and bottom) and WR1.5 (right top and bottom) 25 mm long waveguides from VDI made of Al with thin gold plating.

Figure 2

$S_{21}$ comparison between CST simulation (magenta) for a bare gold waveguide with perfectly smooth surface and $S_{21}$ measurements along the WR3.4 waveguide (blue). In CST simulation the conductivity value used for gold is $4.1\times 10^7\mathrm{S}/\mathrm{m}$.

Figure 3

$S_{11}$ reflection parameter measured in the WR3.4 waveguide. $S_{11}$ is smaller than -30 dB and the impact on the transmitted signal $S_{21}$ is expected to be negligible.

Figure 4

Effective conductivity of gold between 220–330 GHz as extracted from the intersection of measured $S_{21}$ and CST simulation which assumes a roughness-free waveguide.

Figure 5

A WR1.5 rectangular waveguide connected to the VNA with the corresponding frequency extenders.

Figure 6

$S_{21}$ comparison between CST simulation (magenta) for a DC conductivity value of $0.57\times 10^6\mathrm{S}/\mathrm{m}$ and measurements of four WR3.4 waveguides. Two curves are shown for each waveguide. One curve corresponds to the transmission measured in the forward direction, the other to the transmission measured in the reverse direction by flipping the waveguides by 180°.

Figure 7

Measurements of the reflected parameters $S_{11}$ in all four WR3.4 waveguides. $S_{11}$ is just below $-20\mathrm{dB}$ for waveguides 5–12, 5–15, and 5–16. The matching is significantly better for waveguide 1 with a reflection below $-30\mathrm{dB}$.

Figure 8

NEG effective conductivity between 220–330 GHz as extracted from the intersection of measurements and CST simulations assuming a uniform coating of $3\mu \mathrm{m}$.

Figure 9

$S_{21}$ CST simulation (magenta) for a DC conductivity value of $0.57\times 10^6\mathrm{S}/\mathrm{m}$ compared with transmission coefficient measurements along two WR1.5 waveguides. The waveguides are labeled as 4–26 (blue) and 4–27 (red).

Figure 10

Measurements of the reflection parameters $S_{11}$. The return loss remains below $-15\mathrm{dB}$ between 500–700 GHz and only at frequencies above 700 GHz $S_{11}$ is around $-15\mathrm{dB}$.

Figure 11

NEG effective conductivity between 500–750 GHz as extracted from the intersection of measured data and CST simulation assuming a uniform coating of $3\mu \mathrm{m}$.

Figure 12

Front view of a WR3.4 waveguide in CST separated into various blocks with different NEG thickness. Along the width of the groove, $5\mu \mathrm{m}$ and $10\mu \mathrm{m}$ are used. On the side walls $1.5\mu \mathrm{m}$, $1.7\mu \mathrm{m}$, and $2\mu \mathrm{m}$ are used according to the SEM results.

Figure 13

NEG effective conductivity between 220–330 GHz as extracted from the intersection of measurements and CST simulation for a nonuniform profile.

Figure 14

NEG effective conductivity between 500–750 GHz as extracted from the intersection of measurements and CST simulation with a nonuniform profile according to the SEM results.

Figure 15

Extracted NEG conductivity between 10–11 GHz, 220–330 GHz, and 500–750 GHz. For the measurements in blue, the X-band coating setup was used to apply the NEG film while for the measurements in red the coating setup for small dimension objects was used. The DC measurements are also included in the plot and shown at 0 GHz. A difference is observed even for the DC conductivity (blue and red at 0 GHz) depending on the coating setup used. The NEG conductivity seems to reduce as frequency increases by a factor 2–3 times compared to the DC value.