Date: - 20-10 -2004

OM-GEN-

Introduction

To

Cryogenic Instrumentation in SM18

Prepared by –

Prashant K. Awale and Bangalore Arunkumar, BARC Department of Atomic Energy, India

Index / Contents

Chapter NoChapter NamePage No.

  1. Introduction to Cryogenic Instrumentation ………………03–06
  2. Sensors used in SM18 Cryogenic system ………………07-09
  3. Temperature Instrumentation ……………………………10-17
  4. Flow Instruments ……………………………………….17-18
  5. Level Instrumentation ……………………………………19
  6. 13 KA Current lead Instrumentation ……………………..20
  7. Cryo OK for 1.9K/4.2K/HV@ Cold ……………………...21
  8. References ………………………………………………...21
  9. Acknowledgement ………………………………………..21

Chapter 1

//Introduction to Cryogenic Instrumentation//

Abstract:Cryogenic Instrumentation is the vast and unique filed of measurement and is one of the many challenges for the cryogenic system of SM18 magnet test facility as well as for LHC. This write-up/document is written in order to understand the cryogenic instrumentation aspects pertaining to SM18 magnet test facility and also addressed is some of the low temperature instrumentation aspects both for LHC as well as for SM18. Study of various low temperature instrumentation sensors conducted by the experts from CERN is also discussed here in brief. Also enclosed here is the list of various instruments and sensors (along with their range, make and location) that are being use in cryogenic system of SM18 magnet test facility. TT821 Cernox temperature sensor that is installed in each magnet is also discussed.

Introduction:

For testing of approximately 1800 superconducting magnets for the LHC machine at CERN prior to their installation inside LHC tunnel, an extensive Magnet testing set up is available in SM18. This set up comprises of total 6 test clusters with 2 test benches per cluster. i.e. overall 12 test benches are available in SM18 and all this test benches are in operation. Magnets are subjected to both warm as well as cold tests. For cold tests these magnets are cooled down to 1.9K. For the controlled cool down and warm up of these magnets, an extensive cryogenic system is installed and is in operation in SM18 (1) (Refer reference 1 “Cryogenic Infrastructure for Testing of LHC Series Superconducting Magnets” further details).

This document/write-up is written in order to understand the Cryogenic Instrumentation aspects pertaining to SM18 Magnet test facility.

Cryogenic instrumentation may be regarded as a unique field of measurement requiring the development of new techniques. It should be considered a separate field of effort because of increasingly higher accuracies required (like in case of cryogenic temperature control at 1.9K for LHC), the inherent remoteness of the measurements and the peculiarities of the cryogenic fluids themselves. The latter consideration is among the strongest in setting cryogenic instrumentation apart.

In addition to the obvious characteristics of low boiling points, cryogenic fluids are characterized by extremely low heat of vaporisation. Cryogenic fluids like liquid helium require much less energy for vaporisation. The combination of low boiling point and low heat of vaporisation increases the possibility that the cryogenic fluid will become boiling, two phase system. The influence this has on pumping, liquid density and level determination is quite obvious. Any sensor adding energy to the system is in fact creating a vapour/liquid interface at the very point of measurement. An added consequence is that when the system is at equilibrium in the two phase region, the measurement of both temperature and pressure is redundant because the system has only one degree of freedom. (2) (e.g. TT 147 and PT147 in CFB / SSL test at 4.2K with helium pressure of 1350millibar).

Instrumentation for the cryogenic system of LHC is one of the many challenges and most of these is overcome by rigorous research and development, proper design, selection of commercially available instruments and then tailoring it to meet the actual system requirements. Instrumentation plays a major role in terms of monitoring the healthiness of the process, measurements of various process parameters (like Temperature, Pressure, Flow, Level, Conductivity etc.), controlling the desired process parameter within the specified set limits and to take care of the safety aspects in case of control failure (e.g. Safety relief valves etc).

For these type of critical cryogenic applications, a huge number of cryogenic probes are required and some of the important features these probes must have for the trouble free operation of the system are: Good accuracy, repeatability, long term stability (i.e. to keep the specifications over the lifetime of the equipment and under its environmental conditions), maintenance free with very very high MTBF so that the probe can last till the life time of the equipment, rugged as far as possible and most important is to withstand all the extreme process and ambient conditions like cryogenic temperature of around 1.8K, Magnetic field of the order of 9 tesla (wherever applicable), ionising radiation field and temperature and pressure cycling.

Over and above this “Redundancy and Diversity” are the principles employed in instrumentation design, especially wherever accidents and failures are envisaged due to pressure build up etc. e.g. Multiple safety devices (for relieving the excess pressure build up) are installed in a single header where pressure build may take place due to quenching of magnets etc. From diversity point of view, Bursting Discs (Rupture discs) are also provided in parallel.

Before going to instrumentation aspects, it is good to know the SM18 cryogenic process. (3) (refer to Basic Cryogenic Document- Cryogenics for LHC Dipole prepared by Mr Uttam Bhunia- Reference #3). Refer below the block diagram of the cryogenic system layout for SM18 (Fig#1)

Liquid Super fluid helium is used to cool magnets to 1.9K as helium is the only gas that makes a good super-fluid due to its very weak intermolecular forces. Helium condenses to liquid at 4.2 K and turns into super-fluid at 2.17 K. Super-fluid helium has very high thermal conductivity and hence is a very good coolant, has very low coefficient of viscosity and can penetrate tiny cracks, deep inside the magnet coils to absorb any generated heat.

As seen from the Cryogenic system layout below, total 12 CFB’s (Cryogenic Feed Box) are installed between Cooldown Warmup line (CWL) and Cryogenic Compound Line (CCL). This CFB is a system that enables a superconducting magnet to be completely connected up, cooled to 1.9K and maintained at that temperature whilst the magnet performance is being tested.

As cryogenic Instrumentation is the vast field and therefore this write up/document talks mostly of low temperature measurement techniques that were studied and implemented in CERN for LHC / SM18 operations as well as the cryogenic system level and flow measurements. Also discussed in brief about the various instruments those are used in SM18 cryogenic system.

1

Fig#1: Block diagram of the cryogenic system layout for SM18:

1


Fig.2 Actual magnet cooling circuit for power test
(Courtesy: Cryo-operation group, SM18)

………………………………………………………………

Chapter 2

List of some of the important Sensors used in

SM18 Cryogenic system: (Refer Figure# 2) Table#1

Temperature sensors
Sl. No / Sensor No. / Parameter / Range / Location / Make and Type
1 / TT821 / Temperature inside magnet / 1.6 to 300K / Inside magnet on SS collar / CERNOX™ (CX)
2 / TE147 / Temp on line M2 (Temp. refered for Cryo OK of 1.9K) / 1.4 to 2.4K / In CFB on line M2 / Helium 3 bulb Vapor Pressure, Ingovi make.
3 / TE148 / Temp. on line M2 / 0 to 20K / In CFB on line M2 / Carbon Resistance (CRT), AB make
4 / TE149 / I/L temp of magnet in CFB / 1.6 to 100K / In CFB on line N / Resistance carbon glass (Lake shore ref: CGR-1-1000-1).
5 / TE150A / O/L temp. of magnet on line M2 in CFB / 73K to 300K / In CFB on line M2 / Pt100 Class#A RTD, Make ABB Automation
6 / TE150B / O/L temp. of magnet on line M2 in CFB / 1.6 to 100K / In CFB on line M2 / Resistance carbon glass (Lake shore ref: CGR-1-1000-1).
7 / TE103 / Temp in CFB on line E / 73K to 300K / In CFB on line E / Pt100 Class#A RTD, Make ABB Automation
8 / TE161 / He. Gas temp / 73K to 300K / On line coming from CWS to CFB / -do-
9 / TE130A,B / Temp of 13KA current leads / 20K to 300K / On CL / -do-
10 / TE129 / Temp at the bottom of CL / 20K to 300K / On CL / -do-
Pressure Instrumentation
Sl. No / Sensor No. / Parameter / Range / Location / Make and Type
1 / PT147 / Pr. Of helium bulb on line M2 / 0 to 300mbar / CFB
(Refer TE147). / Druck LPX2380 SPL.
2 / PT102 / Helium pr. At CFB Top / 0 to 2 bar / CFB / Rosemount Model:3051TA2A2B21CQ4
3 / PT121 / Helium Guard Pressure / 0 to 2 bar / Inside Helium gaurd / -do-
4 / PT142 / Pr Upstream of FV142 / 0 to 60mbar / -do- / -do-
5 / PT143 / Pr Upstream of FV143 / 0 to 2 bar / -do- / -do-
6 / PT151 / On BP return line to CWS / 0 to 5 bar / CFB / -do-
7 / PT162 / Helium pr. At CFB Top / 0 to 20 bar / CFB / -do-
8 / PT181A / Pr. In Vacuum vessel / Vacuum
(10-10 mbar to 1 bar) / Top platform of CFB / Inficon, pirani/penning gauge
9 / PT181B / -do- / 0 to 1 bar / -do- / Rosemount Model:3051TA1A2B21CQ4
10 / PE126 / Pr(Vacuum) downstream of valve FV126 / 0 to 1 bar / -do- / Inficon (vacuum) Type:PSG 400 (ref: 350-000)
11 / PT127 / Pr. (vacuum) upstream of FV127 / 0 to 1 bar / -do- / -do-
12 / PE182 / Turbo Pump P002 suction pr.(vacuum) / - / -do- / Ingovi, vacuum.
14 / PDT239 / ΔP across FE239 (warm He gas flow from CWS) / 0 to 80mbar / -do- / Rosemount Model; 3051CD2A02A1CS5Q4
15 / PDT265 / ΔP across FE265 (cold He gas flow from CWS) / 0 to 300mbar / -do- / -do-
16 / PT185
Flow Instrumentation
1 / FT239 / Warm helium gas flow in CFB from CWS / 0 to 100gm/sec / In CWS / Differential head type V- Cone meter
2 / FT265 / Cold helium gas flow in CFB from CWS / -do- / -do- / -do-
3 / FT136A/B / He. Gas mass flow for cooling the resistive upper section of current leads / - / On CFB top plarform / Brooks mass flowmeter ref: 5863
4 / FT160 / -do / -do-
5 / FT145 / Liquid He. Flow in the magnet for cooling to 1.9K / Not in use
Level Instrumentation
1 / LE100A / Liquid Helium level in CFB Helium vessel / Lact = 300mm / Inside CFB / AMI make, Superconducting Nb-Ti level sensor.
Φ = 6.35mm
2 / LE100B / -do- / -do- / -do- / -do-
3 / LE140 / Liquid helium level in X+Y line. / Lact = 85mm / -do- / -do-
4 / LE148 / Liquid helium level in line M2. / Lact = 40mm / -do / -do-
Most of the Important Cryo Valves (e.g. CV145,CV150,CV103,CV104..) are of Velan make Model: SCGR-DN 6-32.
Most of the security (safety valves- Pressure relief valves) are of Circle Seal controls Corona, California make.
Helium gas leak detector used for detecting the helium leak if any from the system is of Pfeiffer make.

......

Chapter 3

Temperature Instrumentation:

Temperature measurement is a key issue in the Large Hadron Collider (LHC), as it will be used to regulate the cooling of the superconducting magnets. The compromise between available cooling power and the coil superconducting characteristics leads to a restricted temperature control band, around 1.9 K.

The various components of the LHC cryogenic system work at temperatures from ambient down to 1.6 K. Depending on the actual temperature value, different accuracies are required on its measurement. Between 300 K and 25 K, an uncertainty of 5 K can be tolerated to monitor the warmer components and the general cool-down. However, at the nominal operation of superconducting magnets (below 2.2 K, i.e. at 1.9K) only 10 mK inaccuracy is allowed, to give enough room for the regulation band of the cryogenic controller, while avoiding magnet quench and minimizing the cooling effort of the cryogenic system. Table 2 below shows the allowed uncertainty on temperature measurement. The aimed resolution has to be ten times better than the overall accuracy (dT < 1mK, below 2.2 K temperature measurement on the LHC machine).(4)

Table#2: Required overall Temp. accuracy and resolution:
Temperature Span (K) / Accuracy (mK) / Resolution (mK)
1.6 to 2.2 / 10 / 1
2.2 to 4.0 / 20 / 2
4.0 to 6.0 / 30 / 3
6.0 to 25 / 1000 / 100
25 to 300 / 5000 / 500

Also, the accuracy budget is to be evenly shared between the sensor and the signal conditioning.

CERN has done extensive and elaborate study of various temperature sensors available for cryogenic temperature measurements like CERNOX TM (CX), TVO, RhFe (Rhodium Iron Resistance Temperature Detector), AllenBradley(AB) carbon resistor, Pt100 (Platinum RTD with 100 Ω resistance at 0◦C) etc. Other temperature sensors that are available for low temperature measurements are Silicon diodes, germanium sensors, helium three isotope gas bulb vapor pressure measurement etc. The various guidelines for selection and usage were:

  1. Single sensor covering full temperature range from 1.6K to 300K.
  2. Sensor should be able to withstand the thermal and pressure cycling.
  3. High magnetic field environment ( around 9 teslas).
  4. Radiation field.
  5. Accuracy (inclusive of signal conditioning electronics and hardware) shall be better then the required specification as mentioned in Table#1 above.
  6. very good long term stability and very very low long term drifts.
  7. Maintenance free as most of these sensors will be inaccessible while the system is in operation.
  8. Very good sensitivity specifically at low temperatures (around 1.9K).
  9. self heating effect…

CERNOX™ (CX), TVO® and RhFe listed above cover the full temperature range with a single sensor. AllenBradley® (AB) and Pt100 can be combined to cover respectively low and high temperature scales, or used alone in applications not requiring full range measurements.

In terms of resistive values, CERNOX (CX’s) span is the largest among all sensors (3 decades), requiring wide dynamic range signal conditioning. Also covering the whole temperature range, RhFe spans over only 2 decades of resistance, with the advantage of less demanding dynamic range, but with the consequence of limited sensitivity.

Sensors with negative dR/dT, like CX, TVO® and AB, show high resistance and high sensitivity (dR/R / dT/T) at low temperatures, where measurement accuracy has to be at its best. This semiconductor behavior relaxes the constraints on signal conditioner accuracy for low temperature measurement. On the other hand, at low temperature metallic sensors like RhFe exhibit a sensitivity one order of magnitude worse, demanding much more accurate signal conditioning.

Typical characteristics of cryogenic temperature sensors
Sensor type / T Span (K) / R Span (Ω) / dR/dT (Ω/K) / (dR/dT)/(dT/T)
CERNOX(CX) / 1.6 to 300 / 30000 to 30 / -40000 to -0.1 / -2.7 to -1.0
TVO / 1.6 to 300 / 9000 to 900 / -7000 to -0.7 / -1.3 to -0.2
RhFe / 1.6 to 300 / 6 to 110 / +0.7 to +0.4 / +0.2 to +1.0
AB / 1.6 to 100 / 10000 to 100 / -12000 to -0.3 / -3.0 to -0.2
Pt100 / 73 to 300 / 18 to 110 / + 0.4 to +0.4 / +2.0 to +1.0

Of almost all the sensors mentioned above, CERNOX™ (CX) is the sensor best suited for LHC application. CERNOX™ (CX) temperature sensor possess many attributes desirable in a temperature sensor for LHC type of project application including high sensitivity, excellent short- term and long term stability, small physical size, fast thermal response and very very low calibration drifts (almost negligible) when exposed to magnetic fields and ionizing radiation. Details of this sensor is discussed in following sections:

TT821:

TT821 is CERNOX TM (CX) temperature sensor manufactured by M/s Lakeshore. Sensor Model No: XCX-1050-SD-30 and is referred as Short Thermometer. This sensor is used to monitor magnets internal temperature and same is mounted inside the magnet on the SS nonmagnetic collars. For proper mounting of the sensor, a support block made out of PCB (developed by CERN) is used. CERNOX sensor is push fit mounted in a groove provided in this short thermometer block. The 4 lead wires of the CERNOX sensor are soldered to the 4 soldering points provided on the block. This block is then screwed on the SS collar inside magnet and a polymide foil is sandwiched between thermometer and mounting surface to avoid electrical damage of the sensor in case the surface is under high electrical potential. (5).

There is one drawback of this sensor.i.e. these sensors are not directly interchangeable. Because each individual sensor has it’s own specific calibration curve, it is strictly forbidden to interchange thermometers. Each thermometer has got its own fit and coefficients and same is available in MTF for the corresponding magnets.

The resistance of this temperature sensor is measured by four wire technique in order to get rid of lead wire resistances. Therefore, a 4 –wire twisted thermometer cable is soldered to the thermometer. To minimalise heat flow from ambient environment to the sensor by conduction of electrical leads, thin wires are used ( silver plated copper wires of AWG30 with polyimide insulation with ρ(300K) of 0.32 Ω/m is used). Stress on these thin wires is avoided by more robust extension wires, which are mechanically fixed (i.e. by a knot) close to the connector. Extension wires are also silver plated copper wires of AWG24 with polyolefine insulation and ρ(300K) of 0.07Ω/m.

Fig#4: installation of TT821 (Cernox) inside magnet- wiring methodology.

What is CERNOX™ (CX) ? (8)

The “CERNOX™ (CX)” temperature-sensing element is used in the magnet assembly (TT821). “CERNOX™ (CX)” (short for Ceramic Nitride-Oxide) is a thin film resistance temperature sensor commercialized by M/s Lakeshore Cryotronics,Inc. The sensor is fabricated from zirconium reactively sputtered in a nitrogen-oxygen atmosphere. The resulting thin film is comprised of conducting zirconium nitride embedded within a zirconium oxide non conducting matrix. This material has a negative temperature coefficient of resistance making it useful as a temperature sensor. To tailor the sensor to a given temperature range the ratio of conducting to non conducting material is varied. The main advantage is a single device can be fabricated for use from below 0.3 K to 420 K. Cernox temperature sensors also possess many attributes desirable in a temperature sensor including high sensitivity, excellent short-term and long-term stability, small physical size, fast thermal response and small calibration shifts when exposed to magnetic fields or ionizing radiation. It should be noted that each fabricated sensor has a typical characteristic polynomial curve. The typical resistance values are around 45000 Ohms at 1.6K and around 60 Ohms at Room temperature (300K).

Each sensor that is used in magnet is independently and individually calibrated in a lab. and its coefficients (fit) are made available for future reference. Coefficients are normally stored in MTF in components folder under the corresponding magnet in which this particular sensor is going to be used. Typical curve fit equation for a Cernox sensor is:

T = 10 ^∑{A(i) x [1/log10I]^i }, where i = 0,…..,9 and A(i) are coefficients.

R is resistance in Ohms and T is temperature in K.

e.g.: For a Cernox thermometer calibrated in a lab following are the typical observations:

Thermometer CX_LS_X09273.

Range: 1.615725 to 290.5813K.

(Table# 6) Coefficients:

A(0) / A(1) / A(2) / A(3) / A(4) / A(5) / A(6) / A(7) / A(8) / A(9)
41.44669 / -1192.59 / 14621.59 / -101618 / 443679.2 / -1262277 / 2342678 / -2739332 / 1834192 / -536570

Fig# 5: Resistance as a function of temperature for CERNOX family of temperature sensors:

Stability: Stability is one of the most important characteristics for a temperature sensor. Short term stability for Cernox sensors is tested during manufacturing and is found to be better than ± 3 mK repeatability at 4.2 k upon repeated thermal cycling. Long term stability data is available in terms of “ Mean deviation from original calibration after 5.8 years as a function of temperature for 39 Cernox sensors chosen. At temperature of 1.4 K, the mean deviation was + 0.05 milliKelvin and at 4.2K temperature the mean deviation observed was -0.17K.

For checking the effects of radiation effects on these Cernox sensors (Prior to their selection for LHC Project), experts and researchers at CERN has thoroughly investigated the radiation tolerance of Cernox temperature sensors. In one study, 66 Cernox sensors at 1.8 K were irradiated with neutrons to total fluences of 3 x 1014 n/cm2 to 1 x 1015 n/cm2. The mean calibration shift at 1.8K was +1mK. No signs of thermal annealing were observed. In a second experiment, cernox temperature sensors from two different lots were irradiated at 4.2 K by a neutron source to a total fluence of normally 1 x 1015 n/cm2. Sensors within each lot behaved in a similar manner. The first set had the lower sensitivity and showed a continually decreasing resistance throughout the irradiation with an equivalent temperature variation of 4mK. At 4.2K. The resistance of the second set initially increased slightly and then decreased showing an equivalent temperature variation of about 2.5mK. These data evidence the cernox’s insensitivity to radiation.