# Review of Liquid Argon Detector Technologies in the Neutrino Sector
## 3 Dual-Phase Experiments
### WA105 and ProtoDUNE-DP
The pattern of the strips, details of which are shown in Figure 41 (bottom-right inset), has been optimised to give good position resolution as well as equal charge sharing-and therefore sensitivity-between the \(x\) and \(y\) views [78; 81]. Although the field cage is fixed to the underside of the top-cap using eight FR4 supports, the CRP is not in turn directly attached to the field cage. Instead, it is suspended from three cables which pass into the cryostat interior via dedicated feedthroughs [78]. These cables can be externally adjusted, and so this setup allows the height and relative flatness of the CRP to be optimised independently and without needing to access the rest of the TPC. Ease of access is also a guiding factor in the design of the WA105 signal feedthroughs. Each one is a 2 m long nitrogen-filled tube sealed at both ends by vacuum-tight PCBs [74; 78], and rather than mounting the cold front-end electronics boards (which perform initial amplification and multiplexing) directly on the TPC or anode plane, they are located within the feedthrough, in the space between-and connected to-the PCBs. The bottom PCBs are in turn connected to the anode plane readouts, and the top PCBs to the warm intermediate electronics positioned externally. This feedthrough design therefore allows the front-end electronics to be inserted into or removed from the detector without requiring access to the entire interior, while still situating them in thermal contact with the gaseous Ar, where they can benefit from the cold environment [78]. The WA105 detector was operated during 2017, collecting cosmic muon data and performing a detailed characterisation of the LArTPC design, components and operation. The successful completion of this first stage of the dual-phase DUNE R&D program provided valuable experience and insight for the construction and operation of ProtoDUNE-DP. This detector, shown in Figure 42, is considerably larger than WA105-with a drift volume of 6 \(\times\) 6 \(\times\) 6 m [82] and an active LAr mass of just over 300 tonnes. (In actual fact, this is the size of detector originally proposed as the dual-phase LArTPC demonstrator [83], before it was appreciated that the construction and operation of a smaller iteration would be beneficial.) Figure 41: (**Left**) The fully assembled 3 \(\times\) 1 m WA105 charge readout plane, viewed from underneath in order to see the bottom surfaces of the individual 50 \(\times\) 50 cm LEM units. The extraction grid is not shown. (**Right**) The “sandwich” structure of the combined LEM and anode plane, with a spacing of 2 mm between them. Insets show the fine structure of the LEM (top) and anode (bottom) surfaces. Both taken from [78]. Similarly to that of WA105, the ProtoDUNE-DP cathode is a stainless steel grid: the design is shown in Figure 43. The entire structure consists of 4 \"units\", each of which measures 3 \(\times\) 3 m and are electrically connected (and isolated from each other) via resistors once assembly of the entire cathode is completed. This division of the cathode into smaller units is done in order to reduce the impact of discharges on the surrounding electronics and cryostat structure [74]. The DUNE-DP cathode is envisioned to use an identical design, but with a 12 \(\times\) 60 m assembly of 80 units. The layout of the grid results in the cathode having 60% optical transparency [74], allowing the placement of PDS components below it. The nominal drift field in ProtoDUNE-DP will be 500 V/cm, provided via a field cage identical in design to those used on SBND and (Proto)DUNE-SP: rolled aluminium profiles mounted onto a modular framework of pultruded fiber-reinforced polymer beams [74]. The interior of the ProtoDUNE-DP drift volume, showing the assembled field cage and cathode, is shown in Figure 44.  
The ProtoDUNE-DP cathode is biased at \(-\)300 kV [82]-twice the voltage used by (Proto)DUNE-SP.