MA09DescriptionManual

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Split Coil Cryomagnet System for Neutron Studies

1 Description of the System

1.1 General Description

The cryomagnetic system consists of a superconducting split coil magnet which is mounted inside a cryostat with vertical field axis. Neutron access to a sample positioned at the centre of the magnet is possible through the split of the magnet throughout 340 degrees. Two modes of operation of the magnet are possible; symmetric to 9 tesla at 2.2 Kelvin and asymmetric to 4 tesla at 4.2 Kelvin.

For a VTI section drawing see MA09Drawings.

Samples are introduced into the system on a sample holder from above. The temperature of the sample can be controlled within the temperature range of 1.5 to 300 K in the variable temperature sample insert (VTI). An additional feature of the variable temperature insert is a static exchange gas sample tube which can be inserted into the dynamic exchange gas sample space. Alternatively a separately acquired Heliox low temperature insert may be fitted into the VTI sample space.

The magnet is joined to a vacuum insulated low loss cryostat by two indium sealed flanges. The cryostat incorporates the magnet quench protection circuit and the magnet and cryogenic services.

The magnet has a pair of superconducting switches to allow it to be operated at a constant field with no external supply of current. A lambda point refrigerator is fitted to allow magnet operation at 2.2 Kelvin.

The drawing of the system accompanying this manual should be consulted in conjunction with the description given in the following pages.

Ancillary equipment accompanying this system, namely:

  • magnet power supply
  • helium level meter and probe
are described in separate manuals.

1.2 The Cryostat

The cryostat is an Oxford Instruments low loss type cryostat. It is of a vacuum insulated, all metal construction and features a full length liquid nitrogen cooled radiation shield. The materials used in the construction are as follows:

Outer vacuum case (OVC)aluminum alloy body and tails stainless steel top plate
Liquid nitrogen reservoiraluminum alloy
Liquid helium reservoirstainless steel
Spacersglass fibre

The liquid nitrogen reservoir and liquid helium reservoir are of all welded construction. The outer vacuum case (OVC) has a welded body, but has an "O" ring seals to allow access to the internal parts of the cryostat and to assist the installation of the Variable Temperature Insert (VTI).

The VTI and its radiation shield are removable from the top of the cryostat. No access is available to the magnet as it is welded into the helium reservoir. Glass fibre spacers are used to support and maintain position of the complete assembly, while minimising heat influx. Multilayer superinsulation is used on both the helium reservoir and the radiation shield to minimize radiate heating.

All services to the helium reservoir are mounted at the top end of the three service necks and on the cryostat top plate.

These services comprise :

  • Transfer tube entry port
  • Magnet current lead coaxial terminal (two sets)
  • Helium level probe
  • Electrical access to the magnet switch heaters and temperature sensors
  • Helium reservoir exhaust ports (connected to each other with a recovery manifold) VTI sample space needle valve and electrical access to heater and temp sensor (for clearing blockages)
  • Lambda point refrigerator needle valve and pumping line
  • Electrical access to VTI sample space heat exchanger temp sensor and heater Please refer to the wiring general arrangement drawing AGO 1351.

The siphon entry port has an associated cone located within the cryostat to ensure correct alignment. A tube runs from the cone to the lower chamber of the magnet, and ensures that helium filling is from the bottom of the system. Al1 ports should be sealed with the plugs provided when not in use.

Electrical access to the magnet is provided in the form of two coaxial magnet terminal posts; one post for symmetric energisation (on the long services neck with the VTI needle valve), the other for asymmetric energisation (on the short services neck with the syphon entry). Four ten pin connectors are fitted to the cryostat which have wiring for the persistent mode switch heaters, the VTI, helium level probe, and carbon resistor temperature sensors on the magnet.

The remaining cryostat services are the evacuation/vent valve mounted on the cryostat top plate, nitrogen fill/vent ports; one of which is fitted with a one way valve to prevent total blockage if air is sucked back into the nitrogen jacket through the other ports. The cryostat is fitted with a pressure relief valve which would operate in the event of a cryogen leak into the vacuum space.

The exhaust ports from the service necks are manifolded together for connection to a helium recovery line. The manifold incorporates a safety valve to relieve pressure in the event of a magnet quench into a blocked recovery line.

A ten pin seal on the cryostat top plate carries wiring to the VTI needle valve heater and sensor.

1.3 The Superconducting Magnet

The magnet is composed of two sets of superconducting coils. They are built onto a stainless steel structural formers separated by aluminium rings to resist the large attraction forces between the coil blocks. The field and bore axes of the magnet are vertical.

For a coil section drawing see MA09Drawings.

The vertical bore is occupied by the variable temperature insert into which samples can be loaded. Transverse access for neutrons is provided through 340 degrees through the magnet, VTI and cryostat; the remaining 20 degrees is used for cryogenic services. Please refer to the drawings accompanying this manual for the position. At the beam entry point holes are drilled through the magnet former to allow maximum neutron transmission.

Two modes of energisation are possible; symmetric which energizes all the coils to generate flux densities up to 8 tesla at 4.2 Kelvin and 9 tesla at 2.2 Kelvin. Also asymmetric which energises selected coils to generate flux densities up to 4 tesla at 4.2 tesla.

Niobium Titanium superconductor is used for the magnet coils. The wire is of a multifilamentary, copper stabilized design. All sections are constructed to the MAGNABOND system to form a matrix that is both physically and cryogenically stable under the considerable Lorentz forces generated during operation. All the constituent sections of the magnet are connected to allow series energisation.

The magnet is equipped with two superconducting switches, one for each of the two modes of possible energisation. The superconducting switches consist of a length of superconducting wire non-inductively wound with a wire electrical heater. The superconducting switch has the length of superconductor wire in parallel with the magnet sections to be energised. The superconducting wire is made resistive by raising its temperature using the heater. The switch is then in its open state and current due to a voltage across the magnet terminals will flow in the superconducting magnet windings in preference to the resistive switch element. The switch is in its closed state when the heater is turned off and the switch element becomes superconducting again.

The process of establishing persistent mode operation of the magnet consists of energising the magnet to give the required field with BOTH switches in the open state. The switches can then be closed and the current flowing through the leads reduced to zero, leaving the magnet in its previously energised state. The current flowing in the magnet windings remains constant as the magnet lead current is reduced, the current flowing in the closed switch then being the difference between the magnet and lead currents.

Protection resistors are provided in series with diodes for all magnet sections, restricting the development of potentially high voltages in the event of a

magnet quench (rapid conversion from the superconducting to the normal resistive state). The resistors also dissipate some of the energy stored in the magnet, thereby reducing the energy dissipation within the magnet windings. The resistors are mounted on plates attached to the magnet in the helium bath of the cryostat. Connections between the magnet and the cryostat are made by electrical connectors and solder joints at indium sealed flange level just above the magnet.

Barrier diodes are used in the protection circuit so that under limited voltage conditions, e.g. energisation, all the current passes through the magnet. Under quench conditions, the barrier voltage is exceeded and the protection circuit shunts a proportion of the current away from the magnet windings.

1.4 The Variable Temperature Insert

The variable temperature insert ( VTI ) is of the dynamic exchange gas type, but has an optional center tube that can be fitted to convert it to a static exchange gas type. The sample space region is made from aluminum to allow low impedance to the neutron beam.

The VTI operates over the temperature range 1.5K - 300K, using dynamic flow of helium liquid or gas through a heat exchanger below the sample position. The temperature range can divided into two regimes each requiring a slightly different mode of operation.

a) 1. 5K - 4.2K

Temperatures between 1.5K and 4.2K may be obtained by allowing liquid helium to collect in the heat exchangers and reducing its vapor pressure by over pumping. The heat exchangers can be filled with helium from the main reservoir using the VTI needle valve situated on one of the service necks. In this case, temperature control can best be achieved using the pressure controller/monastic device to maintain a referenced pressure over the liquid.

b) 4.2K - 300K mode

Temperatures between 4.2K and 300K may be obtained by balancing the cooling power due to liquid helium flow into the heat exchangers, against the power input, due to a non-inductive heater on the heat exchanger. Temperature control is achieved by setting the helium flow rate, using the VTI needle valve and gas flow pumping system, in conjunction with the use of a temperature controller connected to the heater/temperature sensor mounted on the sample space heat exchanger.

A radiation shield surrounds the sample cell, the shield being connected directly to the nitrogen cooled radiation shield. The sample cell and radiation shields share the same vacuum as the cryostat OVC.

The sample is introduced into the sample space from the top of the VTI, mounted on a sample holder. The sample holder ideally should include a temperature sensor and preferably a small non-inductive heater to give close temperature control of the sample and to optimize thermal settling time.

Electrical access to the VTI heat exchanger heater and temperature sensors is made using the ten pin seal on the VTI top fitting, and a ten pin seal on the cryostat top plate to the needle valve heater and thermometer (blockage clearing).

1.5 Lambda Point Refrigerator

MA09DescriptionManual/LambdaPumpingCircuit.gif

The Lambda point refrigerator, shown schematically above, allows the temperature of the magnet reservoir to be reduced to 2.2K. The normal method of producing sub cooled helium would be to pump on the helium exhaust line, thereby reducing the vapor pressure of the helium and therefore its temperature. This has the disadvantage that all access to the helium reservoir is prevented, and all access ports have to be vacuum tight rather than merely gas tight, in order to prevent ice formation and contamination of the helium recovery system. Consequently it is not possible to refill the cryostat whilst operating at reduced temperatures.

The Lambda point refrigerator cools the helium around the magnet to 2.2K by conduction and convection, without any reduction of pressure in the reservoir. It consists of a needle valve fed from the main helium bath, and connected via a cooling loop to an exhaust port that exits the cryostat through the main bath. By pumping against the needle valve a cooling effect is produced. The loop then cools the surrounding helium by conduction. The cooler and therefore denser helium sinks to the bottom of the cryostat and is replaced by liquid 4.2K. The cooling continues until the Lambda point (2.17K) is reached, when a layer of superfluid helium accumulates at the bottom of the cryostat. The superfluid has an extremely high thermal conductivity and therefore contains no temperature gradients. The result is that there are two distinct helium phases (the normal and superfluid) with a temperature gradient only in the normal phase. Further cooling simply causes the above boundary to move further up the helium bath.

In order to monitor the temperature of the helium, carbon temperature sensors are provided above and below the magnet. Temperature is measured using an electronics multimeter on as high a resistance range as possible to avoid self heating of the carbon sensors.


Attachments:  LambdaPumpingCircuit.gif 
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This page last changed on 16-Feb-2005 14:00:41 UTC by MarkusZolliker.