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Vacuum system Components


Vacuum System Overview

Commissioning of the Vacuum System

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 the area is 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.

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 at 150 Bar using a CGA170 connector and once a small volume of gas has been released from the bottle into the CGA170 adapter by opening the bottle valve, the bottle valve will then be closed and the Sanmin NV-01F-N 1/8" NPT 6000 psi needle valve (Sanmin Import and Export Co.,Ltd., 2026) opened to allow the gas to flow into a 5 litre storage vessel (shown below middle), then both the needle valve and the VAT 01232-KA44-0001/0152 gate valve (VAT Vakuumventile AG, 2012) (shown below one from the right) 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 one from the left). The deuterium system is isolated from the vacuum system using a VAT 12036-PA06 manual ISO-F 63 aluminium gate valve (VAT Vakuumventile AG, 1999), (VAT Vakuumventile AG, 2000), (VAT Vakuumventile AG, 2003) (shown below right)



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 Ltd (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.
Humphrey. (2021). Humphrey 310/410 Series Inline Solenoid Valves
Ideal Vacuum (2023). Common Vacuum Fittings and Connections Selection & Assembly Guide
MDC Precision. (2019). MDC Precision Catalog - Imperial Section 2A
Sanmin Import and Export Co.,Ltd. (2026). Mini Female Thread Needle Valve
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
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 (1999). 12036-PA06 Vacuum gate valve Manual actuator.
VAT Vakuumventile AG (2000). VAT Vacuum Valves 2000.
VAT Vakuumventile AG (2003). VAT Installation, Operating, and Maintenance Instructions Series 12, DN 63 - 320 (2 1/2" - 12").
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

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