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The electrode structure of the s-DBT tube consists of five main electrodes: cathode, gate, focusing 1 (F1), focusing 2 (F2), and anode.

Cathodes

The current iteration of the s-DBT tube is called Argus 3.0. There were two previous designs of the tube that will not be discussed here. In Argus 3.0, there are 31 CNT cathodes made from molybdenum metal substrates that are 7.25 mm tall. The CNT deposition area on the top of the substrate is 0.325 cm2, with dimensions 2.5 mm × 13 mm. An example of these cathodes is shown in Figure 3.1. Because the deposition area is relatively large for higher current capacity in the s-DBT application, the CNT deposition area was marked off with a mask. The CNTs were deposited through the EPD method described in Chapter 21. The CNTs used in the EPD ink were produced through thermal CVD, termed few-wall nanotubes (FWNTs)2.

Gate

The gate is made up of two parts: the gate frame, and the gate mesh. The gate frame is a 2 mm thick stainless steel electrode that spans the length of the X-ray tube with elliptical holes (3 mm × 17 mm) cut out over each cathode. Each cathode has a corresponding gate mesh welded to the gate frame across the opening, as shown in Figure 3.2. The gate mesh were made from tungsten metal sheets 50 µm or 60 µm in thickness, depending on where they were made. If they were made at UNC they are 50 µm, and if ordered from Elcon Precision LLC (San Jose, CA) they are 60 µm. Metal bars were etched into the tungsten that are 50 µm wide and separated by spaces that are 200 µm wide. Therefore, the bar pitch is 250 µm. The bars are curved on the ends to protect against breakage from thermal expansion. Optical microscope images in Figure 3.3 show the detail of the gate mesh.

Figure 3.1 The left photograph in this figure shows three CNT cathodes of the type placed in the s-DBT system. On the right, in the red box, is a TEM image of the type of FWNTs deposited on the cathodes.

The purpose of the gate is to create the electric field that will induce field emission from the CNTs. The distance from the top of the cathode to the gate frame is 280 µm. The exact distance to the gate mesh would be 280 µm minus the thickness of the gate mesh, because the gate mesh is welded on the side of the gate frame closest to the cathode. Applied electric field is calculated by dividing the potential difference between the gate and cathode, by the distance between the cathode and the gate mesh.

Figure 3.2 On the left is a magnified photograph of a gate mesh immediately after

fabrication. The right photograph shows an example of a gate frame with gate mesh welded over the individual openings. This gate frame was not for the s-DBT tube, but the gate mesh are very similar.

The use of a gate mesh makes this a triode setup as opposed to a diode setup having only an anode and a cathode. A triode setup increases the turn-on electric field due to a non- uniform electric field above the cathode1_{. It also lowers the rate of electron transmission to}

the anode from the cathode because some electrons are blocked by the gate. Current loss is dependent on the percentage of open space in the mesh. The focal spot size is also affected by the gate mesh because it alters the electron beam. Misalignment of cathodes to their individual gate openings can cause non-uniform field emission behavior between cathodes1. Optimizing the gate mesh is a very important part of tube design.

Focusing electrodes

The focusing structure used in the s-DBT system, shown in Figure 3.4, was designed and optimized by Dr. Sultana3. The purpose of the focusing electrodes is to narrow the electron beam after it passes through the gate mesh into the desired focal spot size on the anode. The focal spot size can be altered by changing the voltage on the focusing electrodes. However, applying different voltages can also lower the percentage of the electron beam that is transmitted through the focusing to the anode. Typical s-DBT operation uses both focusing Figure 3.3 Optical microscope images of a tungsten gate mesh. The bar thickness and spacing size are labeled. Also, the curved bar edges can be seen.

electrodes grounded to the same potential as the gate frame to maximize the transmission rate.

As can be seen from Figure 3.4, the focusing structure is made of two Einzel type electrostatic focusing lenses. The openings in the focusing structures are elliptical. Focusing 1 (F1) is lower, closer to the gate electrode. Focusing 2 (F2) is the higher of the two, nearer to the anode. The opening in F1 is larger than that of F2, 17 mm versus 12 mm. The F1 electrode is also much thicker than F2. The distance between the two electrodes is 1.18 mm.

Both F1 and F2 are stainless steel electrodes that, like the gate electrode, traverse the entire length of the X-ray tube. There is an opening in each electrode per cathode. Since the electrode is common amongst all cathodes, it is not practical to change the focusing voltages for each cathode as they are turned on and off sequentially. Therefore, when a focusing voltage is applied to one cathode, it will be held constant for all of the cathodes.

Figure 3.4 Illustration of the complete electron configuration in the s-DBT tube including simulated electron trajectories in the left-hand image. The two views are different cross- sections of the structure. Adapted with permission from Sultana, 2010.

Anode

The anode is the final electrode in the five-electrode structure. A more realistic illustration of the complete electrode configuration, as designed for the prototype s-DBT tube, is shown in Figure 3.5. As the electron beam bombards the anode, X-rays are created. The anode material in the s-DBT tube is tungsten. Unlike most imaging X-ray tubes, the anode is stationary. This limits the usable focal spot size to prevent melting the anode material. One of the reasons tungsten was chosen for the anode is its high melting

temperature. The anode is tilted so that the electron beam intersects with the anode over a larger area to reduce heating while maintaining good spatial resolution.

A high positive voltage is applied to the anode to accelerate the electrons toward it. The X-ray spectrum peaks at the energy corresponding to the net difference of the anode and cathode voltages. The voltage used for breast imaging can range from 25 kV to 40 kV. Figure 3.5 Complete electrode structure from the SolidWorks drawing of the s-DBT tube.

Although there are 31 individual anodes in the s-DBT tube, the same voltage is applied to all of them during imaging.