2016/12/6 Yasuhiro Sugimoto @LCWS2016 R&D status of a gas-compressor based 2-phase CO2 cooling system for FPCCD vertex detector 2016/12/6 Yasuhiro Sugimoto @LCWS2016

Introduction

Why we need CO2 cooling? Operation temperature CTI of FPCCD due to radiation damage is a function of temperature A simple simulation of CTI based on Shockley-Read-Hall theory shows around -40℃ is optimal 2-phase CO2 cooling gives constant temperature cooling ~1x1011 e/cm2

Why we need CO2 cooling? Material budget / dead region Heat sources of FPCCD VTX locate mainly at both ends of ladders (on-chip amps and front-end ASICs) Cooling by 2-phase CO2 at both ends of ladders seems an attractive solution 2-phase CO2 can go through very thin cooling tube (OD 2mm or less)  Only 0.3%X0 increase of material budget of the end-plate Gas cooling requires much thicker tube  Dead space between FTD and beam pipe

Advantages of CO2 cooling Large latent heat ~300 J/g (x3 of PFC) High pressure ~1 MPa @ −40℃ (~5 Mpa @15 ℃) Less evaporated gas volume Less temperature drop due to pressure drop  We can use thin cooling tube Much less Global Warming Potential CO2 C2F6 C3F8 Latent heat @-40C 321 J/g ~100 J/g ~110 J/g Triple point -56.4℃ -97.2℃ -160℃ Critical point 31.1℃ 19.7℃ 71.9℃ Pressure @-40C 1 MPa ~0.5 MPa ~0.1 MPa GWP 1 12200 8830

Schematic diagram Liquid pump based system (NIKHEF) Heat load Detector side Liquid pump based system (NIKHEF) Heat load Expensive Heat load Heat load Detector side Gas compressor based system (KEK) Inexpensive (Cooling water line) Close to room temp.

Merit of gas-compressor system We don’t need expensive low temperature chiller Cooling water near room temperature, which must be available in the detector hall of ILC, is enough to liquefy CO2 We don’t need strict thermal insulation for transfer tubes between liquefier plant and detector Flexible transfer tube off the shelf can be used Transfer tubes can be placed along the cable chain  Big advantage in push-pull operation Transfer tube for Belle-II Cable chain

R&D at KEK Collaboration R&D goal Progress Members from LC-TPC, FPCCD VTX, Belle-II VTX, and KEK cryogenic group Started in 2010 R&D goal Development of 2-phase CO2 cooling system using a gas compressor for CO2 circulation for practical applications Progress Development of a “blow system”(2011~) 1st prototype of circulating system (2013~) 2nd prototype of circulating system (2015~)

2nd Prototype

2nd Prototype Consists of 3 units: liquefier unit, flow meter unit, cooling unit 3 units are connected with metal core flexible tubes (1/4” for liquid, 3/8” for gas) 20m 5m 15m

2nd Prototype Flowmeter Unit Liquefier Unit Cooling Unit

2nd Prototype Gas compressor Haskel gas booster AGD-7 Driven by compressed air Exhausted air is used for cooling of gas cylinder Pdischarge=7Pair+Psuction

Phase diagram 200 J/g Condenser HEX Transfer tube + flow meter Needle valve Gas compressor Detector HEX Heater 200 J/g

Cooling power Cooling power measurement using dummy load Cooling temperature: ~−40℃ Flow rate: 1.4 g/s Dry out power ~170 W  Less than expectation of 280 (=200x1.4) W >1kW for cooling temperature of +15℃ Possible reasons of deficit of the cooling power at −40℃ Heat load of transfer tube (Liquid CO2:15℃, ambient temperature:27℃) and other low temperature part T4 T5 T2 Dummy load T3 T (℃) P (W)

Cooling power Reality（？） 120 J/g

Transfer tube Dp (MPa) Pressure drop in metal core flexible tube is one of concerns However, measured pressure drop is reasonably low Smaller than resolution of flow meters at low flow rate At high flow rate, pressure drop is dominated by flow meter for liquid CO2, and still smaller than the resolution for CO2 gas 5.4 MPa liquid 1.0 MPa gas Flow rate (g/s) Dp (MPa) 5.9 MPa liquid 5.0 MPa gas Flow rate (g/s) (Resolution: 0.15 MPa for liquid, 0.075 MPa for gas)

R&D for improvement

Pressure control Pressure control of 2-phase CO2 by manual back-pressure valve is somewhat unstable Automatic pressure controller was studied Back pressure is controlled by external voltage setting It works very well Setting Pressure Temperature Flow rate

Degradation of O-rings O-rings made of elastomer are used for gas compressor and safety valves in our system Degradation of O-rings, called as explosive decompression (ED), causes gas leak Explosive decompression At high pressure environment, CO2 gas immigrates into elastomer When pressure is reduced suddenly, CO2 dissolved inside the elastomer comes out as micro bubbles, expands, and damage the elastomer from inside

Degradation of O-rings O-ring used for a safety valve

Degradation of O-rings How to mitigate the risk? Safety valve Replace with metal seal safety valves Gas compressor Find out better material for O-rings Higher Shore durometer hardness Different materials: Kalrez, Chemraz, etc. Frequent overhaul Safety valve with elastomer seal (left) and metal seal (right)

Other subjects to be done Development of low mass heat exchanger Very massive stainless plate HEX is used now HEX made of double-layer tube will be studied Small and silent liquefier Remote control of the system Thermal contact between cooling tube and endplate, endplate and ladders

summary

Summary We have successfully developed a prototype of 2-phase CO2 cooling system using a gas compressor for CO2 circulation Cooling power at −40℃ has been measured, which is satisfactory for FPCCD VTX cooling Degradation problem of O-rings has to be solved R&D for many issues are still to be done