1. First tests on the roll-formed field cage.

2. Proposed field cage for DUNE single phase
The present design of the DUNE single phase field cage is based on “palissades” of roll-formed metallic profiles held together through fiber glass i-beams.
Each “palissade” is as long as a full drift and as wide as the APA/CPA, will have his own resistor-chain for voltage degrading and will be electrically insulated from the adjacent ones.
The shape of the profiles has been studied to reduce the e-field towards the walls of the cryostat below 30 kV/cm, allowing to minimize the LAr dead volume
The profile material choice (stainless steel vs aluminum) is under investigation.
A ground plane made of punched stainless steel plated is also included below the LAr surface to avoid E-field leakage in the gas phase. It could be placed also above the corrugated membrane walls to further smooth the E-field in the LAr dead volume.

3. Roll-formed profiles
Open roll-formed metallic profiles are chosen for LAr purity considerations.
With this design the electric field on the highest biased field cage electrodes can be controlled to under 12kV/cm using this profile even with only a 20 cm ground clearance.
A safe way of dealing with the ends of the profiles based on PE caps should allow a reduction in the top and bottom TPC clearance and make more efficient use of the LAr

4. FEA of the field cage corner with PE Caps
20cm ground clearance, -180kV, UHMW PE cap thickness: 5mm
The exposed field in the LAr is ~ 25kV/cm
Plot of the E field on the symmetry plane bisecting the metal profiles
The field in the LAr is ~ 50% higher than the PE cap surface due to dielectric constant difference

5. Field Cage Elements

6. Conceptual design of the ProtoDUNE TPC cage
Ground Planes
Cathode Planes
Anode Planes
Field Cage

7. Roll-Formed Field Cage Test Setup
To validate the field cage concept in pure LAr
Designed to fit in the ICARUS 50 liter cryostat (60 cm diameter, 1.1 m height)
Roll-formed metal profiles with UHMW PE caps
Choice of metal (Al, SS) and surface finish
Pultruded fiberglass I-beams form 4 mini panels
All profiles are at same potential to simplify HV connection
Perforated ground planes 66mm away
Requires 1/3 of FD bias voltage to reach same E field (~ 60 kV)
Ground planes can be connected to external amplifiers to monitor micro-discharges
PMT or Video camera to detects any light from discharges

10. Preliminary test in open air dewar
Waiting for the punched ground plates, the field cage is equipped solind stain less steel wall.
It has been immersed in the open air dewar filled with commercialLAr to visually check that the system is correctly mounted and operated.
The cage is immersed in LAr with the top ring at a depth below the LAr surface ranging from 12 to 20 cm (between successive refill of the LAr bath).
A grounded grid is mounted 10 cm above the top ring to avoid electric field leakage into the gas phase and is always kept immersed in Lar.

11. Preliminary test in open air dewar
After filling or refilling with LAr, when Lar is thermalised, continuous streams of bubbles are produced coming from the edges of the top grid and from the outer ground plates (Heat input is from HV cable conductor and ground conductors). 
The system was successfully slowly ramped-up to 85 and stably operated for several hours without sparks.
Rare random sparks occurred at higher HV (up to 100 kV) but were localized along the HV cable following the bubbles produced at the inner conductor, and only when the Lar level was lowered toward the top ground grid and the bube frequency was increasing.

12. Test of the cage in purified LAr
More recently the cage was mounted in the 50 liter cold vessel and evacuated for few days.
Filling and purification was done with usual procedure:
50 liter vessel is cooled down, immersed in a LAr bath
initial filling performed through Oxisorb/Hydrosorb in liquid phase
continuous recirculation done in gas phase (up to 2/3 LAr liters/h)
LAr was found very quite if outer bath is full with heat input only from top flange and (~10 W) and HV cable:
Bubble formations appear when LAr bath lever lowers (with heat input up to ~80 W also from side walls of LAr vessel)

13. Test of the cage in purified LAr
HV applied soon after filling when the Lar is thermalize (visual inspection though camera):
“slow” ramping up ( ~5 kV/min at start with step decreasing at higher voltage)
Current limitation set to ~ “zero” on power supply
The HV was continuously on for 7 days.
The setup fully stable up to 80 kV:
Stable also up to 100 kV with full LAr bath, no sparks ever recorded.
With bubbles appearing to form around the detector elements, few random sparks (one every few hours) appeared but only above 80 kV (visually they seem to develop again around the HV cable and not between the field cage and the ground plates).

14. Next tests with punched ground planes
Punched ground plates samples are now available at CERN
They seem to meet requirements on planarity and corner curvature: green light for the production of the 8 needed for testing.
We will repeat the tests of HV when available.

15. Further tests before final design
End-caps should be designed to stand HV difference in case one field cage “palissade” undergoes a sudden discharge.
A set up to test insulation capability of the plastic end caps is under study, aiming at optimizing their shape and thickness, material.
Prototype profiles and PE caps should also be tested in LAr under full electric field, and if possible, under full bias voltage