	Stafford Fusion Lab
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Vacuum system

To achieve fusion a good vacuum is required, the following chart shows a range of tokamaks that have been used successfully and the vacuum they have achieved:

Medusa - (Garstka, 1997), Jet - (Pearce et al., 2001), NCST - (Huang et al., 2023), Novillo - (Valencia-Alvarado et al., 1983), HL-2A - (Zeng et al., 2005), KSTAR - (Kim et al., 2009).

As the pump down pressure achieved in the Medusa system (Garstka, 1997) of 2.7E-7 mbar (improved to 6.7E-8 mbar using a titanium getter) is in the molecular flow region (Santeler, 1966), the limit on the pumped flow rate will be the cross sectional area of the connection to the vacuum chamber which in the case of the Medusa vacuum chamber consisted of two 5.1cm diameter holes giving an area of 13.005Π cm² and for a vacuum chamber using an ISO63 connection with an internal diameter of 6.3cm has an area of 9.9225Π cm² which is only 23.7% less, so a similar vacuum should be achievable using a turbomolecular pump with an ISO63 inlet port.

Based upon this the target vacuum will be around 5E-8 mbar, which should be achievable using viton seals (Ideal Vacuum, 2023) and an Edwards EXT75DX turbomolecular vacuum pump (Edwards Ltd, 2007)) similar to the one shown below (left).

The turbomolecular vacuum pump requires a rotary backing pump on the outlet and the V-i120SV (Value, 2009) pump meets the required criteria and is shown below (right):

The turbomolecular vacuum pump is connected to the vacuum chamber via an LGV-2500V-P 307003 MDC high vacuum pneumatic gate valve (MDC Precision, 2019) as shown below (left).

The electrical connector required to connect the gate valve to the control system is a Switchcraft connector (part number 3282-7SG-318) available from Mouser Electronics.

As pneumatic gate valves requires a compressed air source and the humphrey 410 solenoid valves (Humphrey, 2021) can be used with a maximum pressure of 8.5 bar, a 5 litre pressure vessel capable of withstanding 12.5 bar with a 10 bar pressure relief valve and a pressure gauge is used as an air reservoir as shown below (right)

To measure the vacuum pressure a SenTorr BA2C Vacuum gauge (Varian, 1995) will be used with a 0564-k2500-302 Varian hot cathode ion gauge (Agilent, 2002) which uses a 5 Way Mate N lock connector cable (Varian, 2006) and two L9090301 Varian ConvecTorr gauges (Agilent, 2021) with this pinout (Varian, 2004) and 9 pin D-Sub cable (Varian, 2006) as shown below:

The deuterium used will be from a lecture size bottle of gas and once a small volume of gas has been released from the bottle into a short section of pipe by opening the bottle valve, the bottle valve will then be closed and a needle valve at one end of the pipe opened to allow the gas to flow into a 5 litre storage vessel (shown below right), the needle valve will then be closed and the lecture bottle removed. From the 5 litre storage vessel the gas will be reduced to 1 mbar using a Fairchild Model 10 (10212J) regulator (Fairchild, 2007) as shown below (left), before being fed through 2 meters of 2mm internal diameter tube to a VAT 01228-KA44-0001/0569 gate valve (VAT Vakuumventile AG, 2012) (shown below middle).

An RS232 output connection is an optional extra on the SenTorr BA2C but was not installed on the equipment available so an RS232 interface board has been made based upon the L6439-301 PCB (Varian, 1997), the new circuit (Stafford Fusion Lab, 2026a) has been produced on veroboard (Stafford Fusion Lab, 2026b) and confirmed as fully functional using the communications protocols shown in the manual (Agilent, 2004), so when filling the vacuum chamber with deuterium the gate valve supplying the restricted flow of deuterium can be closed when a pre-defined pressure (around 0.24 mtorr) has been reached allowing for accurate repeatable configuration of the tokamak.

Ideally grade N5.0 deuterium would be used (99.999% purity), unfortunately the only deuterium that could be obtained was N2.8 (99.8% purity), however the isotope analysis showed that the major impurity was Hydrogen-Deuterium molecules (at 2009ppm), with a low level of H₂ molecules (at 1ppm) and N₂ molecules (at 0.6ppm) with everything else being below measurable limits:

O₂ (less than 0.2ppm),
CO (less than 0.2ppm),
CO₂ (less than 0.2ppm),
Water content (less than 0.5ppm),
total hydrocarbon content (less than 0.2ppm).

It was decided that this level of purity would still be acceptable because it is the heavier molecules which have a disproportionate effect on cooling of the plasma (Stacey, 2010) that need to be as few as possible and the isotope analysis showed that if the hydrogen-deuterium molecules (which will have a limited amount of plasma cooling) were not taken into account the purity would be better than the equivalent of N5.0 (10ppm of impurities).

The first test of the vacuum system has resulted in vacuum of 6.6E-5 mbar after running the system for 31 minutes (the target vacuum is 2.7E-7 mbar before using a titanium getter), the setup is shown below:

In the photo the sensor TC1 shows atmospheric pressure (1.0E3 mbar) and TC2 displays the minimum value that the sentorr gauge will read which is 1.3E-3 mbar (1.0E-3 torr).

The deuterium feed part of the system has been separated from the vacuum system using DN25 and DN16 blank flanges.

The total running time, excluding the time to get back to previously achieved vacuum pressure is shown below:

Atmospheric pressure --(31 minutes)-->6.6E-5mbar
6.6E-5mbar--(26 minutes)-->4.5E-5mbar
4.5E-5mbar--(21 minutes)-->1.3E-5mbar
1.3E-5mbar--(52 minutes)-->9.0E-6mbar
9.0E-6mbar--(171 minutes)-->2.3E-6mbar
2.3E-6mbar--(80 minutes)-->1.7E-6mbar
1.7E-6mbar--(432 minutes)-->1.0E-6mbar
1.0E-6mbar--(420 minutes)-->9.3E-7mbar
9.3E-7mbar--(420 minutes)-->7.0E-7mbar
7.0E-7mbar--(150 minutes)-->7.0E-7mbar
7.0E-7mbar--(390 minutes)-->6.5E-7mbar
6.5E-7mbar--(240 minutes)-->6.0E-7mbar

This gives a running total of the time to improve pressure readings of 40 hours 33 minutes (the last run was 14 hours 11 minutes long).

It has been decided that to speed up the desorption rate (Edwards Vacuum, 2024) the vacuum system will need to be heated, however the maximum inlet-flange temperature of the turbomolecular vacuum pump when cooled with water is 70 degrees centigrade (Edwards Ltd, 2007)), so up to but not including the vacuum adapter conected to the turbomolecular vacuum pump the system will be heated to around 100 degrees centegrade using nichrome wire in heat resistant fibreglass sleeve wrapped around each section with a thermostat (110 degrees Centegrade open/90 degrees Centegrade reset - close) controlling each of those sections. The section adjacent to the turbomolecular pump will be heated to a maximum of 70 degrees centegrade. A water cooling system has been added to cool the turbomolecular vacuum pump.

The Medusa system (Garstka, 1997) used a similar vacuum system to achieved a pump down vacuum pressure of 2.7E-7 mbar and then used a titanium getter (Clausing, 1961) to achieve the final pressure of 6.7E-8 mbar, so as long as a pressure of around 2.7E-7 mbar is achieved through pumping it is expected that the target pressure of around 5E-8 mbar should also be achievable.

References

Agilent (2002). 564 Ionization Gauge Tube Instruction Manual. Document No. 699905564.
Agilent (2004). RS-232 and RS-485 Options for the senTorr Guage Controller. Document No. 699908170.
Agilent (2021). ConvecTorr Gauge Tube Data Sheet.
Clausing, R. E. (1961). A Large-Scale Getter Pumping Experiment Using Vapor Deposited Titanium Films. Oak Ridge National Laboratory, U.S. Atomic Energy Commission
Edwards Ltd (2007). EXT Compoind Turbomolecular Pumps EXT75DX, EXT75iDX, EXT255DX AND EXT255iDX Instruction Manual.
Edwards Vacuum (2024). Application Note: Outgassing.
Fairchild (2007). Fairchild Model 10 Pneumatic Precision Regulator Installation, Operation and Maintenance Instructions.
Garstka, G (1997). Startup and Stability of a small Spherical Tokamak. Dissertation University of Wisconsin-Madison. UMI Number 9803422.
Huang, F. H. et al. (2023). Design of vacuum system for NCST spherical tokamak. AIP Advances 13, 085313 (2023); doi: 10.1063/5.0160942
Humphrey. (2021). Humphrey 310/410 Series Inline Solenoid Valves
Ideal Vacuum (2023). Common Vacuum Fittings and Connections Selection & Assembly Guide
Kim, S. T. et al. (2009). Baking results of KSTAR vacuum vessel. Transactions of the Korean Nuclear Society Spring Meeting, Jeju, Korea, May 22, 2009
MDC Precision. (2019). MDC Precision Catalog - Imperial Section 2A
Pearce, R. et al. (2001). Vacuum Pumping Developments on the JET Tokamak. EFDA-JET-CP(00)01/07.
Santeler, J.S. et al. (1966). Vacuum Technology and Space Simulation. NASA SP-105, Washington, D.C.
Stacey, W.M. (2010). Fusion: An Introduction to the Physics and Technology of Magnetic Confinement Fusion (2nd Ed.).Wiley-Vch. Weinheim.
Stafford Fusion Lab (2026a). Circuit designed by Stafford Fusion Lab based upon Varian L64390-301
Stafford Fusion Lab (2026b). Veroboard designed by Stafford Fusion Lab based upon Varian L64390-301
Valencia-Alvarado, R. (1983). Vacuum System of the Tokamak Novillo. Instituto Nacional de Investigaciones Nucleares, Laboratorio de Fisica de Palmas, Mexico.
Value (2009). Value iPump Vacuum Pump Operation manual
Varian (1995). senTorr Gauge Controller - Instruction manual
Varian (1997). Based upon photos of L64390-301 PCB.
Varian (2004). Wiring Diagram - All models
Varian (2006). Instructions for Varian Gauge Cable Connector Kits. Document No. 699908080.
VAT Vakuumventile AG (2012). VAT Installation, Operating & Maintenance Instructions Series 012, DN 16-50 (I.D. 5/8" - 2") Mini gate valve with pneumatic actuator double acting
Xeng, C. et al. (2005). Vacuum System HL-2A Tokamak Cao Zeng et al 2005, Plasma Sci. Technol. 7 2632. doi: 10.1088/1009-0630/7/1/007

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