The Field Cage for the Large TPC Prototype

The FLC TPC Group at DESY in collaboration with the Department of Physics of the University of Hamburg will construct the field cage for the large TPC prototype (LP). This field cage is designed to fit into the superconducting magnet (PCMAG) which will be provided by the activity JRA1 in the electron test beam at DESY.

To fit into PCMAG, the dimensions of its field cage are restricted by the magnet's geometry. The outer diameter of the field cage is foreseen to be 77cm leaving a gap of 4cm to the inner wall of the magnet, which has a diameter of 85cm. This space will be sufficient for the installation of silicon strip detectors. These will give two independent external reference points when shooting electrons through the TPC. Figure 1 shows a sketch of the fieldcage in a technical drawing of the PCMAG.

Large TPC Prototype in PCMAG

Figure 1: Sketch of the large TPC prototype in the PCMAG including silicon strip detectors (view from top)

The length of the field cage will be 61cm. The reason why the field cage is comparably short to its diameter is the magnetic field which is homogeneous within 3% only in a range of ±30cm around the magnet's center. Figure 2 shows a drawing of the PCMAG and the TPC prototype with the magnetic field.

Magnetic field of PCMAG

Figure 2: Drawing of the PCMAG and the TPC prototype with the magnetic field.

The field cage should be very lightweight but nevertheless stable and flexible to use. Therefore its structure will be made of composite materials which have already been used for the construction of smaller prototypes, for example at DESY.
The radiation length of the wall profile was estimated to be ca. 1.31% of X0. For this estimation, the copper layers of the field strip foil have been approximated by an even copper layer. The left side of Figure 3 shows an estimation of the fraction of the radiation length for the different materials of the wall structure in units of X0. On the right side, the thicknesses of the different layers are shown in a sketch of the profile, including the epoxy layers that glue the other materials together.

A detailed technical drawing of the current planning status can be found in the PDF file in the right column.

Radiation Lenght

Figure 3: Estimation of the radiation length of the wall profile. Left side: fraction of the wall radiation length of the different materials; right side: wall profile.

Further, the field cage has to ensure a very homogeneous electric field from the cathode to the anode of the TPC prototype. This is achieved by the use of field shaping bands at the wall, the so called field strips. These strips are connected by a resistor chain so that their potentials fall with a constant slope from the cathode to the anode. Due to the gaps between two strips, the potential in this this area gets distorted by so-called punch throughs of the ground potential of the outer shielding (see Figure 4, left side).

Potential at the field strip foil

Figure 4: Potential at the field strip foil.
Left side: without mirror strips. Right side: with mirror strips.

To avoid the field distortions due to the gaps between the field strips, a layer of mirror strips will be placed behind the field strips. The mirror strips lie on an intermediate potential between the two adjoining field strips. In this way, the punch through of the ground potential is avoided (see Figure 3, right side). The design of the field strip foil is shown in Figure 5. The mirror strips are connected through vias on "islands" between the field strips. These islands are connected by two resistors to the adjoining field strips. To confirm this design, field calculations have been performed with a program based on finite element methods.

Layout of the field strip foil

Figure 5: The layout of the field strip foil.

The construction of the Large Prototype TPC field cage is fnished and the prototype is installed in the testbeam area at DESY

More information about the field cage for the Large Prototype can be found in the following EUDET memos:


 

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