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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).

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).

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. Edwards (2007). EXT Compoind Turbomolecular Pumps EXT75DX, EXT75iDX, EXT255DX AND EXT255iDX Instruction Manual
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.
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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