7T Superconducting Magnet 3

He Insert Manual 

Table of Contents: 

I. Introduction: ............................................................................................................. 2 

II. Sample Loading..................................................................................................... 2 

III. Insert Preparation................................................................................................. 2 

IV. Insert Loading ....................................................................................................... 3 

V. Cooling to 4K............................................................................................................. 3 

VI. Cooling to Base Temperature .............................................................................. 4 

VII. Temperature Control............................................................................................ 5 

VIII. Sample/Insert Unloading.................................................................................. 5 

IX. Drawings & Pictures: ........................................................................................... 7 
I. Introduction: 

This is a user manual to assist in the operation of the 3He insert for the 7T-VF 

magnet. The insert has a temperature range of 300mK to 300K, however, it is most useful 

at temperatures less than 50K. To run, the insert must be immersed in liquid helium 

above the 1K pot inlet (approximately 35% on the level meter). Other than that the only 

requirements are a rough pump and a temperature controller (found on the associated 

electronics rack; See Figure 4). While operation of the insert is fairly simple there are 

multiple modes of operation depending on the temperature range. These different modes 

can make things difficult, especially when trying to move smoothly from one to another. 

The following information should help you, but it is not intended to replace the advice of 

an experienced user. For assistance with this piece of equipment contact Evan Fitzgerald 

(Rm. A123, x6657) or Dan Dender (Rm. A118, x6225). 

II. Sample Loading 

a. Before making any preparations be sure that the sample can/holder 

will fit into the insert sample space. Consult the sample mounting 

webpage for the current dimensions. It is recommended that samples be 

loaded into cans with an atmosphere of helium gas to conduct heat since 

the insert relies on conduction of heat through the sample mount below 

4K. 

b. Once the sample is loaded into a can it can be mounted onto the insert. 

There are two options for mounts: 1. The original mounting surface of 

four #4-40 screws on a 1.06” bolt circle. 2. The standard NCNR mount of 

four #10-32 screws on a 1.75” bolt circle. Note that the standard NCNR 

mount requires the use of a 1/8” thick adapter, which decreases the 

available sample space. 

c. Once the sample is mounted on the insert the length should be adjusted so 

the center of the sample is in the center of the beam. The length from the 

bottom of the insert mounting flange to the beam center is 873mm. Any 

crystal axes or other information useful for sample alignment should be 

transferred to the top of the insert above the mounting flange. 

III. Insert Preparation 

a. The 1K pot line should be checked for proper flow before proceeding, 

especially if the insert was recently cold. To do this, pressurize the line by 

connecting helium gas to the 1K pot pumping valve on the top of the insert 

(See Figure 2). Cover what is normally the inlet, the small hole on the 

side of insert just above the taper seal, with alcohol and look for a small 

bubble of gas every few seconds to verify flow. If flow can not be verified 

contact a sample environment team member before proceeding. If flow is 

vverified close the pumping valve and disconnect the gas line. 

b. Clean the two mating taper surfaces with alcohol and then cover each in a 

liberal, but uniform, layer of grease. Connect a turbopump to the inserts 

vacuum valve (a KF 16-25 adapter will be necessary) and open the valve 

(See Figure 2). Place the vacuum can carefully over the sample and gently 

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mate the taper seals. Start the pump while holding the vacuum can and 

slowly rotate the can to spread the grease as the can pulls up on the taper. 

When the can is snug stop turning it and let it pump out for at least 15 

minutes at which time the pressure should be less than 1x10-4 Torr. At this 

time close the vacuum valve and remove the turbo pump and KF adapter. 

c. Remove the alcohol from around the 1K pot inlet with a small towel if 

there is any still present and then clean out the 1K pot using the following 

procedure. Connect a rough pump with a gas inlet to the 1K pot pumping 

valve and pump/purge the 1K pot with helium gas at least three times 

before leaving it pressurized with the gas still flowing. The 1K pot 

should now remain pressurized with helium gas until the insert is 

loaded and in an all helium environment. 

IV. Insert Loading 

a. If the magnet is above 4K (liquid helium temperature) open the magnet’s 

exhaust valve and begin filling liquid helium through the magnet’s fill 

port. Do not pressurize the storage dewar and move on to step “b” once 

the exhaust is calm, but steady. If there is already liquid helium in the 

magnet move on to step “b” without starting a helium fill. 

b. If you are filling the liquid helium lower the pressure by exhausting the 

storage dewar almost completely. If not, open the magnet’s exhaust valve. 

Next unscrew the three nuts holding the clear lid on top of the magnet and 

remove it right before carefully placing the insert into the hole. Slowly 

lower the insert into the magnet and mount it onto the three screws. Pay 

attention to the location of the dark angle in relation to the sample axes, if 

necessary. With no liquid helium present the insert can be completely 

lowered in less than a minute. With liquid helium present it will take 

longer and the vapor will be thick and extremely cold. Low temperature 

gloves (and safety glasses as with any cryogen fill) are essential in either 

case. Be careful not to stress the rubber gasket when it is frozen as it will 

easily fracture. 

c. Once the insert is completely loaded secure it in place with the nuts on the 

three screws. Warm the rubber gasket with a heat gun as necessary before 

tightening the nuts. Increase the flow of liquid helium back to normal 

levels or begin a fill when the exhaust has died down to a reasonable level. 

d. To efficiently cool the insert, and sample, exchange gas must be added to 

its vacuum space. To add the smallest amount possible simply purge the 

open fitting of the insert’s vacuum valve with helium gas then quickly 

cover the fitting with your thumb and briefly open then close the valve. 

This takes some confidence and a suitably large thumb, but does work 

well. This will be followed by a sudden increase in helium vapor being 

exhausted. Continue filling the liquid helium until the magnet is 

completely full. 

V. Cooling to 4K 

a. With the insert loaded connect the temperature controller cable labeled 

“

3

He Insert” to monitor the sensors. There are three sensors: one on the 

1K pot (sensor C, Silicon Diode, 1.4 – 325K), one near the 1K pot on the 

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exchange gas sorb (sensor B, Ruthenium Oxide, 0.3 – 75K) that also 

functions as the exchange gas sorb heater (Analog Out 1) and one on the 

3

He pot which is the sample sensor (sensor A, Cernox, 0.05 – 300K). 

There is also a 3

He sorb heater (Analog Out 2) and a sample heater (Heater 

Out) 

b. If there is exchange gas in the sample space all three sensors should be 

cooling at approximately the same rate, but may be somewhat separated 

(see section IV-d on how to add exchange gas). To allow the system to 

cool most efficiently turn on the exchange gas heater by setting Analog 

Output 1 on the temperature controller to 100% ( See Figure 5) and 

turning the manual switch to “exchange gas heater” (See Figure 4). This 

will warm the exchange gas sorb and allow the sample to continue to be 

cooled by the helium reservoir via the exchange gas. Without using the 

exchange gas heater the sample temperature will cool very slowly below 

about 30K. 

c. When the sample has maintained 4K for a few minutes the exchange gas 

heater can be turned off by setting Analog Output 1 to 0% ( See Figure 5) 

or turning the manual switch to “temperature sensor” (See Figure 4). 

VI. Cooling to Base Temperature 

a. The sample temperature should be at 4K before starting this procedure. 

See section V “Cooling to 4K” if the sample is warmer. 

b. Verify that the exchange gas heater is turned off as described in section Vc. 

c. Connect a high capacity rough pump to the 1K pot pumping valve on the 

top of the insert and pump out the line. Open the 1K pot pumping valve 

and monitor sensors B and C to verify that they quickly drop to 1.4K. If 

their temperatures do not drop contact a member of the sample 

environment team. This pot uses an impedance to control the flow of 

liquid helium into it so there is no needle valve to adjust. 

d. Note: This system uses a “single shot” method that will sustain base 

temperature for a maximum of 36 hours before needing to repeat the 

following steps. This time can be drastically shortened if heat is 

added to maintain a temperature above base or by various other 

factors. 

e. Once the 1K pot is cold turn on the 3He sorb heater by setting Analog 

Output 2 to 100% (See Figure 5). This will drive the 3He gas out of the 

sorb to be cooled by the 1K pot. After ten minutes or so the sample 

temperature should slowly descend to around 1.4K. 

f. When the sample drops below 2K the 3He is condensing and collecting in 

the 3He pot. The longer you want to stay below 4K the longer you should 

allow the 3He to condense. One hour of condensing has been shown to 

give about 24 hours at base temperature and two hours should give the 

maximum amount of time, 36 hours. 

g. After condensing for an appropriate amount of time turn off the 3He sorb 

heater by setting Analog Output 2 to 0% (See Figure 5). In approximately 

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half an hour the temperature should reach a base temperature around 

300mK. 

VII. Temperature Control 

a. T < 1K: When starting from base temperature the sorb can be gently 

warmed (< 15%) to increase the sample temperature slightly (See Figure 

5). Alternately the sample heater can be used at a low power setting 

(25mW) to control at slightly warmer temperatures, however, this will 

shorten the time temperatures less than 1.8K can be maintained. 

b. 1K < T > 1.8K: The 3

He must be condensed and pumped on to work 

below 1.8K, but it is difficult to make it last for more than a few hours 

when working above 1K. To work in this temperature range use both the 

sample heater and the sorb heater. The sorb can be warmed with about 15 

– 30% of its heater power to slow the pumping of the 3

He and then the 

sample heater used at a low power to control the temperature of the 

sample (See Figure 5). 

c. 1.8K < T > 4.2K: In this range the cooling from the 1K pot is necessary, 

but pumping on liquid 3He is not. The sorb heater should be set at 30% or 

above to provide exchange gas in the 3

He circuit. The sample heater can 

then be used on a low setting (See Figure 5). 

d. 4.2K < T > 50K: At temperatures close to 4 K the same method as 

suggested in “c” should be employed. At higher temperatures the cooling 

from the 1K pot is not useful to cool the sample, but it us useful in keeping 

the exchange gas sorb cold. At this point the 3

He sorb temperature is not 

important, so its heater can be turned off. The sample will then be isolated 

in a vacuum and its temperature entirely dependant on the sample heater. 

e. 50K < T > 300K: Everything from the previous step applies here except 

that at somewhere around 50K is becomes necessary to pump out the 

exchange gas. The exchange gas sorb will eventually warm up from the 

heat applied to the sample and start releasing the helium gas. When this 

happens the sample will be thermally connected to the LHe reservoir, 

which will burn off a large amount of LHe. To prevent this, close off the 

1K pot pumping valve and connect a turbopump to the inserts vacuum 

valve and pump continuously while at high temperatures (See Figure 5). 

f. Cooling from High Temperatures: For temperatures above approximately 

50K it is necessary to add exchange gas (if it was pumped out) to cool the 

sample back down at a reasonable rate. If the exchange gas was not 

pumped out simply turn on the exchange gas heater by setting the manual 

switch to “exchange gas heater” and setting analog output 1 to 100%. A 

setting of 100% will cool the sample as quickly as possible, but a lower 

setting may burn off a bit less helium and offer a slower more controlled 

cool down (See Figure 5). 

VIII. Sample/Insert Unloading 

a. If you have any reason to suspect a leak of helium into the insert or the 

overpressure valve releases during this process immediately connect a 

rough pump to the insert’s vacuum space and contact a sample 

environment team member. Any liquid helium present will suddenly 

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expand to many times its initial volume and create a dangerous build up of 

pressure that will release at the weakest joint. 

b. Removing the insert is quick and simple. First disconnect the 1K pot 

pumping line and the sensor cable. Then remove the three nuts holding 

the insert down and pull the insert out. Be sure to have a pair of cryogenic 

gloves on and have the insert’s cart nearby. After setting the insert on its 

cart remember to screw it down. Place the clear lid back onto the magnet 

and screw it down. 

c. Use a heat gun to bring the outside of the insert to room temperature. If 

you have time let it sit for at least an hour to allow the inside to warm as 

well. If you need to open the insert quickly give it fifteen minutes after 

warming the outside then move to the next step. 

d. Connect helium gas set to 5 psi to the insert’s vacuum valve using a hose 

barb. Hold the vacuum space’s overpressure valve closed with a piece of 

tape. Hold the plastic bin under the insert so that its padding is an inch or 

two below the bottom of the insert and open the vacuum valve. Slowly 

increase the pressure until the can pops off into the bin. The pressure 

necessary to remove the can is normally around 10 psi. Then turn off the 

helium gas and carefully lower the can off of the insert. 

e. The two taper seal surfaces should be cleaned to remove the grease and 

the can stored in the tube on the insert cart. Keeping in mind any radiation 

hazards the sample can now be removed. 

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IX. Drawings & Pictures: 

Figure 1: Magnet and Inserts’ Outer Dimensions 

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