Vacuum Systems Operation EiC Training and Refresher Course 26 th April 2007
Contents 1. Introduction 2. Pumps and pumping on JET Primary Volumes Primary Pumps Secondary Pumps Vacuum Diagnostics Vessel Conditioning (covered in separate talk) The interspace system 3. JET normal conditions Plasma operations Fuelling (covered in a separate talk) 4. JET abnormal conditions Loss of vacuum and leaks
Introduction JET is the largest magnetically confined fusion device in the world (and the only one capable of full tritium operations) 200 m 3 vacuum vessel Carbon tiles on plasma facing components (high surface area!) Large area Carbon divertor region in vessel 100 s Kms of lip welds ~120 ports with windows/diagnostics/ feed-throughs to the outside world Water cooling in vessel components Still manage to pump to better the 1 x 10-7 mbar!
Introduction Reliable plasma operations require that the vessel conditions be good (subjective) More over ; the vessel conditions must be controlled so that : Vast majority of the plasma is fuel Chosen gases can be injected as required for experiments Impurities kept to minimum possible level (better than 10-9 mbar) To avoid density limit disruptions To ensure that machine performance is repeatable - Extremely important
Introduction The overall air all leak rate on JET is estimated to be ~1 x 10-4 mbarls -1 BUT, adsorbed impurities are readily released from the carbon sponge by radiation and particle induced de-sorption. In JET, and other divertor machines, the during a shot the plasma can come into contact with the plasma facing components. Therefore we want: Clean well conditioned walls (low Z materials) High pumping speed to remove impurities Pumps that can also deal with exhaust, i.e. Helium Poor conditioning of the walls can lead to unreliable/ poor breakdown of the plasma and density limit disruptions wasting operational time!
Introduction When the plasma collapses (disrupts) the electric current generated in the plasma can interact with the magnetic field generated from the confinement coils. This interaction results in a force During Disruptions Forces can be: > 400 Tonnes for 10 ms Vertical >200 Tonnes for 10 ms Horizontal Can cause a movement of the Vacuum Vessel up to 7 mm with acceleration of 3g So the vacuum vessel is restrained by large brakes!
Introduction
Pumps and pumping on JET : Primary Volumes 1. Main Vacuum Vessel 200 m 3 Nominal but 187 m 3 Actual due to internal components. Surface Area Huge c 10 6 m 2 Carbon Tiles, Heat Shields. 100 s windows and ports. All welded construction in 8 octants. 2. 2 Neutral Injector Boxes at 50 m 3 each with water cooled shielding and bending magnets. 3. 2 Rotary Valves at 5 m 3 each with copper water cooled internal shielding. 4. 1 Lower Hybrid Launcher at 10 m 3 volume with 48 waveguides. 5. 1 Pellet Centrifuge Injector at 5 m 3 volume. 6. 2 Pumping Chambers at 5 m 3 each
Pumps and pumping on JET : Primary Volumes
Pumps and pumping on JET : Primary Pumps Pumps used as primary evacuation devices at JET fall into two categories. Those which retain the gasses they pump and those which exhaust to the secondary pumps either continuously or in batches. In the first category are Ion Pumps Non Evaporatable Getter (NEG) Pumps Used mainly for pumping small volumes with low gas load In the second category are Turbo-molecular Pumps Cryo-pumps Roots Used for pumping the larger volumes coupled with high gas load.
Pumps and pumping on JET : Primary Pumps Ion & NEG Ion Pumps are used in a few places at JET (e.g. IVIS) where little outgassing is experienced and the system is not often disturbed. They are placed away from the Torus magnetic field Unfortunately their mode of operation is such that any Tritium is retained in the pump and is hard to get rid of. NEG pump by chemically sorbing active gasses on to their elements and are used in closed volumes such as RF vacuum transmission lines where the main outgassing is Hydrogen. They must be regenerated by heating to high temperature at intervals to keep their pumping efficiency high.
Pumps and pumping on JET : Primary Pumps Turbo Pumps Running continuously, exhausting via ML1 and ML2 to AGHS Four 2000 ls -1 turbo pumps on the Torus de-coupled mechanically from the Torus to avoid damage during disruptions. One 2000 ls -1 turbo pump on each Rotary High Vacuum Valve. One 2000 ls -1 turbo pump on each NIB Box. Most diagnostic vacuum systems have 500 ls -1 turbo pumps Turbo-molecular pumps used on the Torus or NIBs must be Tritium Compatible i.e. all metal, low amounts of oil grease etc
Pumps and pumping on JET : Primary Pumps Turbo Pumps
Pumps and pumping on JET : Primary Pumps Cryopumps 1. Cryopumps are used at JET to pump large amounts of gases such as Deuterium and Hydrogen. They collect gas in batches 2. Cryopumps which have large cold panels to condense and freeze gasln 2 and LHe cooling required 3. Regenerate by warming up the LHe / LN 2 cooled panels Inventory limited There are six cryopumps in total on JET comprising: 6,000,000 ls -1 in each NIB 100,000 ls -1 in the Torus Divertor Region - two separate pumps 50,000 ls -1 in the Lower Hybrid system 10,000 ls -1 in the Pellet Centrifuge
Pumps and pumping on JET : Primary Pumps Cryopumps Cold panels in a domestic freezer At 273K water vapour is collected as frost JET cryopanels At -269 C (5K) everything else collects as frost (except helium pumped by turbos) Need to be defrosted (regeneration) Strict inventory limit - pumping explosive gases
Pumps and pumping on JET : Primary Pumps Cryopumps : Regenerations 1. Generally LHe panels on NIBs, PD and LH are regenerated weekly with LH regenerated onto PD 2. Generally LN2 panel only regenerated for vessel warm up or loss of cryogen Do not want to release condensed water and spoil vessel conditions! LHe panels are regenerated before the allowed gas inventory is exceeded to avoid explosion hazard. e.g. Maximum inventory on NIB LHe panels 300 bar litres.
Pumps and pumping on JET : Primary Pumps Roots Currently there are no Roots pumps working as primary pumps on JET To be installed at Part of the HFPI project are two roots pumps in the torus hall 1. 7000 m 3 /hr under octant 2 2. 1000 m 3 /hr to pump Very high field side track on Torus
Pumps and pumping on JET : Primary Pumps Helium pumping 1. Helium is not pumped by the LHe Cryopanels, Argon condenses on the LHe panel and traps Helium at JET we have an Argon frosting system for this. 2. Helium is pumped by the turbo pumps.
Pumps and pumping on JET : Secondary Pumps AGHS The AGHS complex consists of the following major subsystems :- 1 Cryo-genic Forevacuum System (CF) 2 Mechanical Forevacuum System (MF) 3 Exhaust De-tritiation System (EDS) 4 Cryo-distillation System (CD) 5 Impurity Processing (IP) 6 Intermediate Storage System (IS) 7 Product Storage System (PS) 8 Gas Introdustion System (GI) This system now deals with all the exhaust gas from various sources on JET such as cryo-regenerations, TMP exhaust and diagnostic exhaust. No gas goes direct to atmosphere
Pumps and pumping on JET : Secondary Pumps diagnostic crown The diagnostic crown, like ML1 & 2, is of all welded construction. Has a volume of ~1.5m 3 provided a N 2 purged exhaust root for the many diagnostic backing pumps on JET. Kept at ~900 mbar with a 160 m 3 hr -1 Screw pump. See : MOM/0420/001- Description of the Diagnostic Exhaust Crown (DEC) MOGDOC 3.23 - Loss of Diagnostic Exhaust Crown Pumping
Pumps and pumping on JET : Secondary Pumps diagnostic crown
Vacuum Diagnostics : Gauges There are 4 main types of Vacuum Gauges installed on the JET machine. 1. Baratrons. These are capacitance gauges They are true reading gauges which are independent of gas type. Pressure range from 1000 mbar to 10-3 mbar 2. Pirani. These are thermal conductivity gauges. They are gas dependent but are calibrated as nitrogen equivalent. Pressure range from 100 mbar to 10-3 mbar. 3. Penning. Cold cathode ion gauges. Also dependent on gas. Calibrated for Hydrogen species. Pressure range from 10-3 to 10-9 mbar. 4. Ion. Hot cathode ion gauges. Also gas dependent and are calibrated as nitrogen equivalent. Pressure range from 10-4 to 10-10 mbar.
Vacuum Diagnostics : Gauges Pressure at inlet causes deflection of diaphragm changing capacitance Baratron capacitance manometers are usually stand-alone transducers typically requiring a ±15 volt power supply and deliver a 0-10 volt pressure signal that is directly proportional to pressure.
Vacuum Diagnostics : Gauges Pirani thermal conductivity gauge. Thermal conductivity of filament proportional to gas pressure. Thermionic and cold cathode (Penning) ionisation type. Energetic electrons ionise gas. Collected current proportional to gas pressure
Vacuum Diagnostics Useful Gauges T0n pressure measurement (n=turbo pump number) VC/TTn-PEN<PRS:001 VC/TTn-PIR<PRS:001
Vacuum Diagnostics Useful Gauges
Vacuum Diagnostics Useful Gauges VC/BARTRN-1H<PRS Baratron on Octant 1 (also 5H) torus total pressure. Displayed on Vacuum overview mimic VC/DECP-PRS<P1 Baratron Pressure measurement of Diagnostic crown. Displayed on Diagnostic crown mimic VC/DCP-P1<PRS Pirani measurement of Divertor coil case pressure. Displayed on PD case pumping mimic
Vacuum Diagnostics Useful Gauges
Vacuum Diagnostics RGA 4 Main Residual Gas Analysers on JET (quadrupoles) 1. QS1 Torus main spectrometer 2. QS2 Torus backup spectrometer. Both QS1 & 2 can sample torus atmosphere through valve GA1. Useful for GDC and vacuum analysis at high pressure 3. QS3 NIB 8 Spectrometer 4. QS4 NIB 4 Spectrometer Can be used as qualititative tools. Periodic calibration of RGA required for quantitative analysis.
Vacuum Diagnostics RGA The RGA is an ionisation gauge with mass filtering. A combination of DC and RF (~few MHz) applied to rods allowing only species with specific mass/charge to pass to detector.
Vacuum Diagnostics RGA Large air leak on JET Mass 28 : Nitrogen 100% Mass 32 : Oxygen 20% Mass 40 : Argon 1% Carbon gettering will suppress oxygen peak when vessel is above ~ 80 C. Beryllium also getters oxygen
Vacuum Diagnostics RGA Mass 20 indicating Neon in chamber probably from interspace system
Vacuum Diagnostics RGA Mass 4 D 2 Typical JET spectrum D 2 operations Hydrocarbon groups C1 C2 C3 Many experts make for subjective interpretation of spectra
Vessel Conditioning There are a number of systems on JET used to condition the vessel to improve the quality of vacuum. There include : 1. Glow Discharge Cleaning. 2. Beryllium evaporation. These systems are covered in a separate talk by L worth
The interspace system Pressure gauge Items at high risk of vacuum failure are protected by the interspace system. The system protects the following : Air Neon filled interspace (500 mbar) Vacuum Windows 114 Bellows 24 Internal volumes 30 Feedthroughs 48 Sundry Others 10 Be. Evaporators 4 Total 330 Windows
The interspace system If a leak is suspected (from RGA) a visual inspection of the interspace system is carried out by Vacuum Group
JET normal conditions Plasma operations During a deuterium operational day there may be up to 50 pulses with : Turbo pumps run continuously Gas injection to the Torus - various locations Torus at 200 C continuously Cryopanels typically regenerated once a week LH regenerated onto pumped divertor GDC and Be evaporation typically once per week
JET normal conditions Plasma operations In tritium operations, you might expect 15 or 20 pulses in a day. Turbo pumps run continuously Gas injection to the Torus - various locations. Tritium through GIM 15 Torus at 200 C continuously Gas inventory strictly controlled and accounted for Cryopanels regenerated every day Full tritium accountancy every day GDC and Be evaporation as required Some diagnostic systems isolated
JET normal conditions Fuelling Fuelling Gases:- Hydrogen, Deuterium, 4 Helium,Tritium Covered in separate talk by L Worth Impurities for:- Diagnostics Control of plasma edge RF coupling Wall deposition experiments 3 Helium, 13 CH 4, Argon, Xenon, Krypton, Nitrogen Silane
Loss of Vacuum incidents on JET What is loss of Vacuum? Could be defined as a rise in pressure to the point where operations are not possible. i.e. By ingress of air, water, cryogen's, SF6, neon or any other gas or fluid which has an adverse effect on Torus or NIB pressure. Tentative maximum ingress rate allowable is 5 x 10-4 mbar.ls -1 before halt called to operations. This is very subjective!
Loss of Vacuum incidents on JET The following fluids have the potential for ingress into the vacuum vessel. 1. Air from a leak to atmosphere or an RF AVS bellows failure. 2. Water from failure of a cooling line. 3. Nitrogen either from vent gas or an internal cryogen leak. 4. Helium from an internal cryogen leak. 5. Neon from an interspace failure. 6. SF6 from a LH double window leak. 7. Galden from a leak in the divertor coil. Note that 6. and 7. are unlikely but would damage the catalysts in AGHS. Reconditioning of the vessel after loss of vacuum incident can take ~2 weeks
Loss of Vacuum incidents on JET Three levels of ingress for different pressure regimes are defined on JET they are, 1. Level 1 - Ingress up to 1 mbar.ls -1 2. Level 2 - Ingress from 1 mbar.ls -1 to 10 2 mbar.ls -1 3. Level 3 - Ingress above 10 2 mbar.ls -1
Loss of Vacuum incidents on JET Level 1 Scenario Ingress up to 1 mbar.ls -1 : Most Common on JET At maximum a pressure in the Torus of 10-4 mbar with turbomolecular pumps and 10-5 mbar with the cryopump operational. Expected scenario is : 1. Cryopump is loading at maximum rate of 3.6 bar.l/hour of ingress gas. 2. No increase in backing line pressure if cryopump on. 3. Backing line pressure 10-2 mbar with conventional pumps only. 3. Torus RGA operational. 4. Torus turbomolecular pumps operational.
Loss of Vacuum incidents on JET Level 1 Scenario Ingress up to 1 mbar.ls -1 : Actions The following actions are advisable. 1. Call out the vacuum on-call officer. 2. Inform AGHS of the rate and species of ingress. This may be ascertained from the RGA. 3. Follow the advice of AGHS on what to do next. They have to deal with the gas. 4. Prepare to possibly regenerate cryo-pumps. 5. Prepare to cool down vessel if necessary. 6. Get out the leak detector!
Loss of Vacuum incidents on JET Level 2 - Ingress from 1 mbar.ls -1 to 10 2 mbar.ls -1 At maximum a pressure in the Torus of 10-2 mbar with turbomolecular pumps and 10-3 mbar with the cryopump operational. Expected scenario is : 1. Cryopump is loading at maximum rate of 3.6 x 10 2 bar.lh -1 of ingress gas. 2. Backing line pressure 10-2 mbar with cryopump, 10-1 mbar without. 3. Torus RGA not operational normally but possible with T01 top valve shut and GA1 open. 4. Torus turbomolecular pumps operational but not recommended at this pressure.
Loss of Vacuum incidents on JET Level 2 - Ingress from 1 mbar.ls -1 to 10 2 mbar.ls -1 : Actions The following actions are advisable. 1. Call out the vacuum on-call officer. 2. Inform AGHS of the rate and species of ingress. This may be ascertained from the RGA if operational. 3. Follow the advice of AGHS on what to do next. They have to deal with the gas. 4. Prepare to regenerate cryo-pumps. 5. Prepare to cool down vessel. 6. Prepare to close down turbomlecular pumps if torus pressure riseover 10-1 mbar.
Loss of Vacuum incidents on JET Level 3 - Ingress above 10-2 mbar.ls-1 Out of control Damage limitation 1. Torus cryopumps will spontaneously regenerate. 2. NIB cryopumps will also regenerate if RHVV are open or have large seal leaks 3. All turbo molecular pumps will trip. 4. At 15 mbar, Draining and Refilling System will operate. 5. At 200 mbar the turbo bypass valves will open. This is a hard wired interlock and cannot be over-ridden. 6. At 250 mbar EDS will automatically route to pump ML1 and ML2. As the pressures in ML1 & 2 approach atmosphere non-return valves will open and EDS will remove up to 500m 3 hr -1. This will protect the vessel from ANY foreseeable event. 7. If atmospheric pressure were still to be exceeded MOST unlikely the Torus bursting disc is the final protection.
Loss of Vacuum incidents on JET Related documents MOGDOC 3.26 - Unplanned Torus Vacuum Pressure Rise MOGDOC17.06 - Operation of KS1 Beamline Torus Valve MOGDOC 9.11 - Vacuum Vessel Temperature Change Checklist MOGDOC 3.23 - Failure of the RF Double Window (DCF)
Finally On the whole, JET vacuum is the cause of little concern when JET is operational, though production of a decent vacuum can be difficult - especially during a restart. Most of the vacuum equipment runs smoothly and reliably. Dedicated team of crack vacuum experts and vacuum instrumentation control specialists are on call 24 hours a day, 365 days a year, to ensure that vacuum problems account for little loss of operational time.