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There are three main structural components of the CDC (Fig. 6.2): a thin carbon-fiber reinforced plastic (CFRP) inner cylinder, two aluminum endplates, and a CFRP outer cylinder similar to the one in the Belle CDC.
The outer cylinder, with a thickness of 5 mm, supports most of the wire tension of about 4 tonnes. The inner cylinder should be thin (0.5 mm thickness) to minimize material, but it should also support the wire tension for the small-cell chamber (i.e., the innermost superlayer), which has its wires strung independently before installation into the main chamber. Tapered aluminum endplates are used for the outer region to reduce the deformation caused by wire tension, while conical aluminum endplates are used for the inner region to match the detector’s polar angular acceptance of 17°–150°. (The endplates of the small-cell chamber are conical as well.) The tapered shape in the outer region reduces the deformation by a factor of two compared to the Belle CDC.
As shown in Fig. 6.3, a step structure is machined in all three endplate sections to allow easier and more precise drilling of the many holes for the wire feedthroughs. This avoids the difficulty encountered in the Belle CDC, where it was quite hard and time-consuming to drill the holes through the long depth in its conical endplate. The step structure here gives a drilling depth of 10 mm for every endplate section; this is shown in Figs. 6.3 and 6.4.
The endplates for the main and conical parts are machined and drilled separately. Just before the wire stringing, the two parts bolted together, as shown in Figs. 6.3 and 6.5. The forwardand backward-connected endplates are attached on the outer CFRP cylinder (Fig. 6.3), at which time these endplates are precisely aligned.
The tension for the sense wires is the same (50 g) as in the Belle CDC. However, the tension of the field wires is reduced to 80 g from 120 g to reduce the deformation of the endplates. The gravitational sags for the sense and field wires differ by 85 µm. We estimated the maximum distortion in the x–t relation function using a Garfield simulation, and found a difference of at most 20 µm at the cell edge. The total wire tension is 4.1 tonnes (vs 3.4 tonnes for the Belle CDC). Using this wire tension and the described structure, a Finite Element Method calculation shows that the maximum stress of 31 MPa at outer edge of the endplate is smaller than the allowed limit of 107 MPa. In addition, the maximum deformation is 3 mm (vs 4 mm for the Belle CDC) at the inner edge of the endplate. We take account of this deformation in the design of the small cell chamber. Pre-stressing of the endplates is necessary when stringing the wires for the main and conical parts. The thin CFRP inner cylinder supports 370 kg of wire tension, which is a factor of six lower than the buckling load of 2300 kg.
The wire stringing is performed separately for the main chamber (including its conical part) and the small-cell chamber. The small-cell chamber has no outer cylinder and is small, so it is easy to string its wires horizontally from the inside on a table without any special jigs. The wire stringing for the main chamber is more challenging. The outer cylinder, already in place during the stringing operation, is constructed in one piece to be strong enough to support the large wire tension and to maintain the operating gas pressure absolutely constant. However, we need to observe directly each wire as it is being strung. We will string the wires vertically from the outside, while a person stands inside the chamber to observe each wire as it placed and to make any needed adjustments.
The feedthrough is used to fix the wires and to ensure insulation between the wire’s high voltage and the endplate’s ground. The shape of the feedthrough is the same as in the Belle CDC. However, the feedthrough material is changed from Delrin to Noryl due to the latter’s more reliable insulation performance at high voltage. Noryl is used successfully in the Belle small-cell chamber, installed in 2003. For our small-cell chamber, we do not have enough space to use feedthroughs for the field wires: only aluminum pins are used to hold the wire tension. The pin is attached directly on the endplate, as was done in the Belle small-cell chamber. There is space between each endplate and its thin aluminum cover to contain the feedthroughs, high voltage cables, signal cables, and readout electronics. All front-end electronics are located on the backward side to reduce the material in the forward side. The forward side is used only for the connection of high voltage cables. If we adopt the charge-division method for the z trigger, a small amount of additional electronics will be located on the forward side, but just outside the detector acceptance.
The CDC is supported by the outer detector. We are considering a huge cylinder, like the Belle inner detector support (IDS), for the forward side. It connects the CDC forward endplate to the forward inner-detector support flange, which is located inside the barrel ECL’s inner cylinder. For the backward side, the support of the CDC is slightly more complicated. A similar backward support cylinder is connected to the CDC backward endplate prior to installation. The outer radius of this support cylinder is smaller than inner radius of the barrel PID. This support cylinder is attached to the backward inner-detector flange though several separate jigs just after the CDC has been installed. The details will be fixed after the barrel PID option is selected.
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