TPC Field Cage Bo Yu DUNE Engineering Meeting

TPC Field Cage Bo Yu DUNE Engineering Meeting Sept. 8, 2015

Requirements / Assumptions • Provide the nominal drift field of 500 V/cm • Withstand -180 k. V near the cathode • Define the drift distance between the APAs and CPAs to <1 cm* • Use materials with comparable CTEs to that of stainless steel along the length of the TPC, minimal CTE in the drift direction • Limit the electric field exposed to LAr to under 30 k. V/cm • Prevent damage to the TPC In case of a HV discharge anywhere on the field cage, or cathode. • Provide redundancy in the resistor divider • The divider current must be >> the ionization current in the TPC drift cell, yet less than the power supply current limit when all dividers are connected in parallel • Constructed in modular form that can be easily installed in the cryostat • proto. DUNE: low mass section in front of the beam window • If a laser calibration system is adopted, allow laser beams to enter into the active volume *base on the requirement of “the fiducial volume of the detector shall be known with a precision of at least 1%”. Other requirements are being formulated. 2 09/08/2015 Bo Yu | Field Cage Status

Field Cage Overview The reference design is the light weight, PCB based field cage. The copper edges are covered by a thick layer of polymer with high dielectric strength (solder mask) to reduce the electric field exposed to the LAr. Surge suppressor will be installed in parallel with resistors along the divider. A total of 2000 m 2 coverage is required for a 10 kton detector. A corner of the 35 ton TPC field cage during a trial assembly. 3 09/08/2015 Bo Yu | Field Cage Status

Field cage of the 35 ton TPC In order to avoid exposing the high electric field at the edges of the copper strips, the solution for the 35 ton TPC is to cover the edges of the copper strips with solder mask, a standard PCB fabrication technique. The catch is that we need a thicker layer than on typical PCBs. The standard solder mask material for rigid FR 4 boards are not very resilient in thick sections and may be damaged during assembly. External E field: 4. 4 k. V/cm Red: left edges Green: right edges 09/08/2015 Electric field [V/m] Slot in the PCB 4 Electric field along a line at the edges of the copper strips Bo Yu | Field Cage Status Maximum E field in liquid ~ 15 k. V/cm lower strip upper strip

Field Cage PCB Features The PCBs have copper strips on both sides. The strips outside the TPC are shifted by one strip pitch for most of their length, forming additional capacitance between strips. Size 4 screws ¼” pins Inside copper strip pattern Holes for the blind rivets Outside copper strip pattern 5 09/08/2015 Bo Yu | Field Cage Status Clearance holes for the resistor leads Total inter-strip capacitance from all four sides of the field cage ~ 3. 5 n. F. Nominal strip to ground capacitance ~ 10 p. F

Field Cage Corner • Made of double sided 0. 2 mm thick FR 4 substrate. • The vertical edge is mechanically and electrically connected to the larger side panel. Spring contacts on the top edge ensure electrical connection to the top panel. Notch for the CPA frame Spring contact thermal relief Eyelet holes to attach to the side panel 6 09/08/2015 Bo Yu | Field Cage Status

Field Cage Transient Response in a Discharge • The resistors along the divider provide a linear DC voltage gradient. • However, at shorter time scale (<<1 s), the electrical behavior of the divider is determined by the varies capacitances on and between each electrodes. This divider is no longer linear at this time scale. • A perfect capacitive divider requires the capacitance of each node to ground to be 0. In reality, there is always a finite capacitance from each node to ground. These capacitances “resist” change in the voltages on the nodes. • In the event of a HV breakdown between the cathode to ground (cryostat), the cathode voltage quickly collapses to ground, but the first field cage strip to ground capacitance keeps its voltage from changing instantaneously to follow the cathode voltage, results in a momentary larger voltage differential between the cathode and the first field cage strip. • This voltage differential can be a significant fraction of the cathode operating bias, large enough to cause HV breakdown between the two electrodes, or worse yet, destroy the resistors between the two electrodes. • The natural solution to this problem is to additional capacitance between the nodes of this divider. • See Micro. Boo. NE docdb 3307 for summary of the analyses 7 09/08/2015 Bo Yu | Field Cage Status

Maximum Voltage Difference Tube-to-Tube for a discharge from the cathode or any of tubes Micro. Boo. NE: Added Capacitors: gaps 1 -14: 10 n. F, gaps 15 -21: 5 n. F, gap 22: 2. 5 n. F Micro. Boo. NE: Red: no added capacitors Blue: add 22 capacitors 35 ton field cage w/ 3. 5 n. F : <8 k. V Discharge probability Analysis by S. Rescia 8 09/08/2015 Bo Yu | Field Cage Status

Surge Suppressor Studies • • • An alternative to adding capacitance between divider nodes is to use surge protection Extensive tests have been done by Micro. Boo. NE (docdb 3242, ar. Xiv: 1406. 5216 v 2) on the use of varistors and GDTs (gas discharge tubes) as a mean of limiting the over voltage condition in the event of a HV discharge in the TPC. Both types will work for the purpose of restricting the voltage differential between field cage rings in LAr temperature. • • A GDT quickly shorts the terminals when the voltage differential exceeds a threshold A varistor changes its resistance to keep the voltage differential near the threshold voltage. The smooth transition and well defined clamping voltage of the varistors are preferred to the abrupt switching of the GDTs. The varistors would also function as redundant “resistors” in a divider chain. Left: varistor Right: GDT 9 09/08/2015 Bo Yu | Field Cage Status

Alternate Field Cage Design A more robust field cage can be constructed with roll-formed metallic profiles. The example below shows the electric field on the highest biased field cage electrodes can be controlled to under 12 k. V/cm using this profile even with only a 20 cm ground clearance. If we can find a safe way of dealing with the ends of the profiles, this construction could allow a reduction in the top and bottom TPC clearance and make more efficient use of the LAr. 10 09/08/2015 Bo Yu | Field Cage Status

Additional Views of the Design Corner treatment (UHMW PE caps, and optional PE angles) to minimize the exposed electric field Resistive divider and surge suppressor chain 11 09/08/2015 Bo Yu | Field Cage Status

Field at the Corner, Profile 1071 -180 k. V, 20 cm to ground Radius of curvature: 10 cm Symmetry planes + the top and bottom faces Max E field < 18 k. V/cm Exploring manufacturing options: • CNC machining from solid block • Bend solid profile • Cast solid • Bend roll-formed profile under • tension Metal plating on composite ground 12 09/08/2015 Bo Yu | Field Cage Status

FEA of the field cage corner with PE Caps • 20 cm ground clearance, -180 k. V, UHMW PE cap thickness: 5 mm • The exposed field in the LAr is ~ 25 k. V/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 13 09/08/2015 Bo Yu | Field Cage Status

Pros and Cons 14 PCB Design Metal Profile Unit cost [$/m 2] ~ 2000 ~ 130 Weight ~4 ~ 10 (SS) ~ 3 (Al) Robustness Fair Good Compliance to TPC distortion Poor Good Ease of assembly Good Fair Built-in Inter-strip Capacitance Yes No Max Field in LAr @ 180 k. V & 50 cm ground clearance ~15 k. V/cm ~5 k. V/cm 09/08/2015 [kg/m 2] Bo Yu | Field Cage Status

Roll-Formed Field Cage Test Setup • To validate the field cage concept in pure LAr • Designed to fit in the ICARUS 50 liter cryostat • 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 66 mm away • Requires 1/3 of FD bias voltage to reach same E field • Ground planes can be connected to external amplifiers to monitor micro-discharges • PMT detects any light from discharges 15 09/08/2015 Bo Yu | Field Cage Status

To Do List • Further FEA of the electric field, in particular in the corner • Analysis of the DUNE specific transient responses of the field cage, with the resistive cathode • Test prototype profiles and PE caps in LAr under full electric field, and if possible, under full bias voltage • Design the mechanical and electrical interfaces between the field cage and the CPA • Assuming no show stoppers are found, submit formal VE proposal to the project 16 09/08/2015 Bo Yu | Field Cage Status

Additional Slides 17 09/08/2015 Bo Yu | Field Cage Status

Field Cage Divider Resistivity Range • Argon 39: 1 bq/kg, average energy: 220 ke. V • TPC drift cell: 2. 3 mx 6 mx 3. 6 mx 1. 4=70 t - 70 k. Hz @ 220 ke. V/23. 6 = 6. 5 E 8 e = 0. 1 n. A - Each CPA wall is connected to ~ 124 divider chains - Upper limit: 1 m. A power supply to 124 divider chains: 8µA each divider - If we want one supply to feed 2 CPA walls in an emergency, each CPA must draw <4µA - 180 k. V/60 Gohm = 3µA 18 09/08/2015 Bo Yu | Field Cage Status

Option 1, Toughen the Surface Another type of solder mask designed for flexible PCBs seems more resilient. We can experiment with that. Or cover the inside copper layer with another FR 4 sheet, pull back the outside copper edges into the “shadows” of the inside copper strips. Outside strip edges may still need to be covered by the flexible solder mask. If 35 ton field cage works, and option 2 fails 19 09/08/2015 Bo Yu | Field Cage Status

Option 2, resistive coating Applying a resistive coating over the copper strips will completely remove the edge effect of the strips. The resistance between strips from the coating needs to be much higher than the resistive divider value to ensure a uniform drift field. The coating must maintain good bonding strength to the PCB substrate during repeated thermal cycles, and be resistant to impact and scratches during manufacturing and assembly steps. If a suitable coating can be found, this seems to be the most effective way of reducing edge field. Copper strip Resistive coating Common development with the resistive cathode; 20 09/08/2015 Bo Yu | Field Cage Status

Field Cage + Ground Plane 21 09/08/2015 Bo Yu | Field Cage Status

E Field at a GTT Knuckle When faced with a 180 k. V conductive plane 20 cm above the knuckle tip: • The maximum E field at the tip of the knuckle is 40 k. V/cm. • The field at the ridges of the corrugation is about 20 k. V/cm. • The facing HV plane has a maximum E field of 7. 5 k. V/cm, average about 7. 4 k. V/cm. At 50 cm clearance: • The tip field <15 k. V/cm • At the HV plane: E~3. 3 k. V/cm Based on these field values, the equivalent “flat” membrane distance is about 3 cm above the real membrane flat: 180 k. V/7. 4 k. V/cm ~24 cm, 180 k. V/3. 3 k. V/cm~54 cm. 40 k. V/cm in the liquid near the top of cryostat is high. But on the bottom of the cryostat where the pressure is 2 atm higher and boiling point is 11 K higher than the liquid temperature, the field may be acceptable. 22 09/08/2015 Bo Yu | Field Cage Status

Comparison of Screw Heads 3 k. V/cm nominal field, screw length 0. 25”. 6 -32 screws, G 10 plate 23 09/08/2015 Bo Yu | Field Cage Status

Field Enhancement Factor at the Corner Field enhancement factor at the 90° bend vs radius 4 3. 5 3 2. 5 2 1. 5 3 E 5 V/m on long flat surface, 1. 1 E 6 at bend. R=1 cm 1 0. 5 0 1 24 09/08/2015 Bo Yu | Field Cage Status Bending Radius [cm] 10

Field Cage Circuit Diagram 25 09/08/2015 Bo Yu | Field Cage Status

Detail Views 26 09/08/2015 Bo Yu | Field Cage Status

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