Gatling Gun Vacuum System Development and Status

Gatling Gun Vacuum System Development and Status

Report to the Gatling Gun Review CommitteeBy

John SkaritkaJune, 28, 2012

1Gatling gun review, June 28th-29th, 2012

Gatling gun review, June 28th-29th, 2012

Outline

• Vacuum requirements of Gatling gun system components(<10^-12 Torr )

• Large XHV chamber development and lessons learned,

• Gun chamber NEG pumping system estimates

• Comparison with other systems in Japan and China

• Cathode shroud and capacity Anode assembly

• SAES collaboration and NEG pump development

• Reliability of magnetic manipulator systems

• Cathode protection mechanisms and vacuum challenges

• Summary

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Estimation of the Gatling Gun Operational Lifetime• Best reported Charge Life time: 1000C* = 110h beam at 2.5mA per cathode.• Best reported QE: 1% = 400mW for 2.5mA• Laser power level increases as QE fall with time to maintain constant beam current.

*“Charge and fluence lifetime measurements of a dc high voltage GaAs photo gun at high average current”PHYSICAL REVIEW SPECIAL TOPICS - ACCELERATORS AND BEAMS 14, 043501 (2011)J. Grames, R. Suleiman, P. A. Adderley, J. Clark, J. Hansknecht, D. Machie, M. Poelker, and M. L. Stutzman

To be considered practical the minimum “Cathode Life Time” for a two gun polarized e-RHIC source must exceed ( ~770 C) or about 84 hours per cathode, so the exchange of cathodes can be integrated into the weekly RHIC run schedule. Ample evidence exists To reliably achieve this gun, storage, and transport chamber vacuum levels must be maintained at 10^-12 Torr or less.


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Out gas rate effects cathode Lifetime

Charge life time is defined as the amount of charge in C extracted from the photocathode before the quantum efficiency falls to 1/e of the initial value. The above two curves are the performance of two chambers at Jefferson Lab, The LL chamber is NEG coated and has a significantly lower out gassing rate then the BP chamber at < 10^-12 TL/s-cm2, thought ultimate vacuum is approximately identical.


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Extreme Vacuum Development

•To study the reliability and performance of Large XHV chambers with vacuum levels of <10^-12 Torr that are required for the Grand Central and Gun vessels a test chamber with 16.5in.CFs was built and tested.•BNL obtained the Vat Lab 3BG vacuum (super) gauge has <10-13 Torr resolution.•Based on this development manufacturing practices for large vessels have been improved and implemented by industry.

XHV 16.5”Tee test Chamber 3BG Super Vacuum Gauge


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Lessons learned, large chamber and XHV vacuum procedure

• For the Gatling Gun concept chambers and flanges larger than what was previously considered feasible for XHV service were evaluated and a XHV procedure was developed:

• Chamber walls and flanges are vacuum fired prior to welding.• After welding and leak tight assembly the chamber is pumped and baked

until a clean RGA spectrum is obtained and all Non H2 peaks are in the noise of the RGA’s most sensitive scale.

• NEG pumps are then activated until the chamber pressure is falls to XHV levels.

• Large flanges are sensitive to mechanical fit and thermal excursions but if done correctly standard 16.5” CF flange can be used in XHV systems.

• All metal VAT valves can be used but only the sealing surface side must face the XHV chamber.


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Large XHV chamber development and outgassing rate estimate

With a single NEG pump the Grand Central chamber vacuum level reached about 8x10^-12 Torr.The NEG pumping speed for H2 was estimated to be between 1000 to less than 2000 l/s. The chamber is 1 meter across with 6 x 16.5” and 8 x 8”flanges. It has an internal surface area of approximately 35750 cm2

, This yields an outgassing rate of between: 2.2 to 4 x10^-13 TL/s-cm2


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Gatling Gun Chamber Manufacturing

•Materials: 316 L stainless.•Finish: Electro-polish•All XHV thermal processed-•Pre-fab: vacuum bake at 950C for 12 hours at 1E-5Torr. Quick cool to 400C in 15 min. or faster. Slow cool from 400C to room temp.•Post fab: Air bake at 400C for 36 hours, slow cool to room temperature. •Based on this procedure manufacturer estimates the outgassing rate shall be approximately 1x10^-14 Torr-L/s-cm^2. •Given past history a conservative estimate for outgassing rate to size the NEG pumps would be 2x10^-13 Torr-L/s-cm^2


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Gun vessel NEG pump sizing

Cathode Exchange Chamber~20,000 cm^2Target Vacuum 1x10^-12 TorrMin NEG pump capacity required4000 l/s

Main Gun Chamber~24,000 cm^2Target Vacuum 6x10^-13 TorrMin NEG pump capacity required8000 l/s

Cathode Anode interstitial areaTarget Vacuum <5x10^-13 Torr NEG pump capacity >2000 l/s

Transition and Combiner Chamber~13,000 cm^2 + (beam line)Target Vacuum 1x10^-11 Torr6000 l/s NEG capacity oversized due to potential up streaming gas Load

Gas load from down beam diagnostics & and collector sections

This totals a minimum of about 20,000 L/s of NEG in the Gun assembly.This amount of NEG is comparable with other international projects as seen on the next 2 slides


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The Japanese photo cathode Gun project. In Japan the smaller gun vessel uses about 16000 L/s of NEG pumping cartridges.


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Roughing: (Leybold TW300 + EcoDry M15)×2Titanium , vacuum firing, bakeout 190 /100h℃Ceramic tube ， bakeout 190 /100h℃Ulvac bakable cryopump （ 2000L/s ）Saes C400-2 NEG×48 (>18000L/s)500L/s Ion pumpLeybold IE514 extractor gauge （ Reliable to 1.0E-10Pa ）

March.19-21,2012, IHEP DC-Gun Workshop

In China the smaller gun vessel uses about 18000 L/s of NEG pumping cartridges.

Gun body achieve EHV （＜ 1.0E-9Pa ）

The Chinese photo cathode Gun project.


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Gun Material SelectionGun Cathode Shroud

• Material 316L Stainless Steel.

Photo micrograph of shroud stainless microstructure

The stainless ingot was cross forged after near net shape is machined it will be vacuum fired and then hand polished using J-Lab’s surface preparation procedure.


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Cathode cooling system and capacity • The Cathode shroud has a water cooling

loop that is brazed to an internal copper ring that is in contact with each cathode.

• The cooling loop can conservatively provide 100 watts of cooling per cathode.

• A steady state power estimate of about 2 watts deposited into the Cathode will result in a rise in temperature of less than 10 C on the cathode surface.

• A leaf spring spider secures and pre-loads the array of cathodes against the copper cooling ring.

• The Molybdenum cone of the cathode self aligns in the conical copper seat providing an extended contact surface that is immediately adjacent to the water cooling line.

• Alternative coolants, cold gas and even liquid nitrogen can be used if low temperature operation is desired.


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Material Selection for Gun Anode assembly

The anode assembly is composed of a vacuum fired 316L Stainless Steel 22” wire seal flange with an expected noted in the red circle. The shell that contain the focusing solenoids and immediately facing the cathode is made from titanium having an potential outgassing rate that is as noted below.

Conductance between chambers is about 1 l/s per orifice, SAES will supply a NEG strip to line the wall that will provide about 35 l/s pumping per orifice creating a net H2, CO pump between chambers


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Gun Material Selection, Transition section

Magnets are used to bend the electron beams into the combiner field.Field quality and cost requires this part to be Aluminum with an expected outgassing rate of about 7x10^-14 Torr-l/s-cm^2.Vacuum fired 316L to AL, 16.5” CF bimetal flange on back side and 10”CF Titanium bi-metal in front to interface to combiner.

SAES will construct an electrically activated array with up to 6000 l/s NEG pumping with temperature of the transition section not to exceed 250C


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SAES NEG pump development for the Gatling Gun

• We will be using these pumps in the acceptance testing of the Gun and Cathode preparation chambers later this year.

• The following views and performance data of the custom designs SAES Getters proposed for use that can maintain <10^-12 vacuum levels in the Gun using ST-122 Getter material.


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SAES NEG pump cartridge for Gun and Preparation chambers.

The Capaci-Torr cartridges are based on St172 sintered disks which are stacked and arranged in the cartridge structure in a way to optimize speed and capacity performances.

Inspired by the technical challenges of the Gatling Gun project, SAES Getters has been able to double the pumping capacity of their ST-172 NEG cartridges to up to 4000L/s


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SAES proposed getter for custom XHV NEG pumps


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SAES Getters proposed, Cathode Zone pump

Total available pumping surface = 104cm2x140 =14560 cm2 Total maximum pumping speed = 14560*0,2 = 2912 Activation efficiency = 100% Effective pumping speed (H2) = 2912 l/s The Cathode Zone pump to be thermally activated up about 450°C so the activation will be 100%.


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SAES Getters Transition Section Pump

Total available pumping surface = 260 cm2x140=36400 Total maximum pumping speed = 36400*0,2 l/s = 7280 l/sEstimated effective speed = 7280x0,5 = 3640 l/sThe Transition section pump can be activated at 250°C with an activation efficiency = 50% An auxiliary heater in the center of the pump could bring the temperature of majority of the area of the pump fins up to 400°C boosting the activation efficiency up to 80% or about 6000 l/s.

March.19-21,2012, IHEP DC-Gun Workshop

Reliability of magnetic manipulators The photocathode projects around the world rely heavenly upon magnetic manipulators. In this view a multi chamber structure for the Chinese Gun system magnetic manipulators play a critical role to move cathode substrates from one chamber to another.


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Reliability of Magnetic Manipulators

• From a leading manufacturer the life of a magnetic manipulator is about 10,000 to 15,000 full stroke cycles before maintenance is required. Failures are primarily the ware between bearing races and the inside wall of the manipulator tube.

• For the gun 10,000 full strokes of the main central manipulator will give an expected life of about 100 years.

• A second smaller manipulator exchanges cathodes from a 20+ cathode tray to the spider array and back again moves about 40 cycles per cathode exchange.

• The Gun will require a maintenance period about every 5 years to exchanged this manipulator.

• The Grand central chamber might have many manipulators to produce and test about 1000 cathodes a year so an annual maintenance period will likely be required.

• Two chambers would allow planned maintenance to be performed every two years on each unit while reducing the risk for a single point failure.


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Protecting the cathode from damage mechanisms

Photocathode performance is sensitive to ion back bombardment and transmission losses.The Gatling gun system shall have number of effective safeguards to effectively isolate the cathode from back streaming particles.


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Cathode Protection(Depressed Collector)

The first line of defense is the Depressed Collector.The collector is an effective means to minimize the back streaming gasses and ions from the beam stop.A detailed presentation will be presented by Dr. Alexander Pekin later today. But sources still exist, with the beam diagnostics and steering errors.


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Cathode Protection(Combiner dog Leg)

Back streaming and trenched ions will have significantly lower random energies. The first bend magnet will act as a filter deflecting most ions from a path back to the cathode.

0 2 4 6 8 10 12 14

-3000

-2500

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0

500

1000

1500

2000

potential with bias

potential_all anode1000(v)

potential_0.05in sheet 1000(v)

potential_0.05in sheet 3000(v)

Distance(inch)

pote

ntial

(vol

t)

Approximate path of reflected ions

Cathode Protection (Ion shield)

A charged plate between the Anode and Transition sections forms an ion barrier , Holes in the plate allow laser light in and electron beams out with little effect but defects ions where NEG and Ion pumps in the transition section absorb them.

Up to a 1kV bias

Wall grounded


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Cathode Protection (Ion trap)

The field of the solenoid magnet located inside the anode assembly will deflect ions that may have passed through the ion shield into the NEG coated wall of the titanium tube between the Cathode and the transition chamber. The solenoid coil is constructed from high temperature resistant materials to survive NEG activation temperatures


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vacuum system components for grand central

The Grand Central chamber shall have a 2000L/s ion pump with TSP, 4000L/s of NEG pumping, two extractor gauges and a RGA,System built to maintain 10^-12 Torr scale vacuum during cathode manipulation and Cathode preparation in the tree assemblies

Each tree shall have a 35 l/s ion pump, a 200 L/s NEG pump a RGA and a extractor gauge .

Each manipulator will have port that could allow pumping to be introduced from either side of the magnet

Manipulators that are part of a load lock system will have a 35l/s ion pump and a 200 L/s NEG pump a extractor gauge and a RGA


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Vacuum systems and mechanisms for large scale preparation of cathodes for the future e-RHIC

• To reduce cost and improve system reliability a recent decision was made to decouple the cathode preparation system from the Gun system.

• Instead of preparing cathodes in the RHIC tunnel the grand central chamber will be located in building 905.

• A storage system that could hold 100’s of cathodes for a period of months will be developed to transport and load cathodes from the grand central system into two Gatling Guns.


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Bulk Cathode XHV Storage and transport system

The grand central chamber with magnetic manipulators and full compliment of 4 trees

To reduce programmatic risk and dependent upon cathode through put there will likely to be a minimum of two grand centrals chambers and a minimum of three cathode storage systems.


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Cathode Storage system Concept

• Each Cathode train will be constructed from material with extremely low outgassing rates • An array of tubes made from low out gassing material are NEG coated will be located in a central

cylindrical chamber. Manipulators will extract completed trains ~2+ cathodes per train from Grand Central and into the storage system. once injected into the storage chamber low conductance effectively isolating the cathodes. Special load locks and valves will be developed to inspect and load cathodes into the Gatling guns in the RHIC tunnel with out degrading cathode performance.


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•A Pair of Gatling guns and the cathode storage system as it may eventually appear in the 2 o’clock hall in the RHIC tunnel


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Additional Vacuum Challenges

• Valves have significant problems, there is evidence that simply passing a cathode through an open gate valve may substantially degrade it’s performance.

• Establish the reliable operation of the photocathode preparation trees and provide an effective isolation between the tree and grand central vacuums.

• Develop a reliable cathode storage system that preserves, transports, inspects and transfers cathodes between preparation and Guns chambers with negligible degradation.

• Set up vacuum lab and find skilled staff for the Gatling gun laboratory in building 905.


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Summary• Cathode damage mechanisms and the necessity for the best possible XHV chamber

properties was established.

• Large XHV chamber development and lessons learned were discussed.

• Gun chamber target vacuums and estimated NEG pumping speeds were presented along with comparison of systems in Japan and China.

• The Cathode Shroud , Cathode cooling system and the Anode assembly was presented. • Developmental efforts at SAES Getters, recommended NEG pump materials and designs

for XHV systems were presented.

• Reliability of magnetic manipulator systems was discussed

• Cathode protection concepts of the Gatling gun and Vacuum challenges were presented.

• A path forward to achieve acceptable XHV chamber properties and in turn optimize polarized photocathode performance is proceeding.

Documents

Gatling Gun Vacuum System Development and Status