design and operation of existing zs

Design and operation of existing ZS


B.Balhan, J.Borburgh, B.Pinget

B.Balhan

SPS ZS Electrostatic Septum Upgrade Review on 20th February 2013


Outline• LSS2 ZS characteristics

– Layout , geometry, HV circuit,…• ZS behaviour observed during operation

– Spark rate, Sparks phenomena, Scrubbing • Multi cycling

– Software upgrade– Main HV & Itrap Generators time response

• ZSTF in LSS6• Conclusion

B.Balhan


ZS @ LSS2 characteristics• 5 ZS separated by pumping modules (pumps & BDI equipment) in the

same vacuum sector.• All 5 ZS powered in parallel by 1 HV power supply (-300kV).• Individual power supplies for each Itrap top/bot plate (- 10 kV).• Individual ZS gap adjustment for individual field setting.• Individual Anode positioning (radial /angular).• Automatic cathode angle adjustment to assure parallelism to Anode.• Global positioning system (ZS girder) to retract all 5 ZS for special

operation mode (MD).• Spark acquisition for Anode/Cathode (spark count and spark time).

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ZS layout

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• ZS1 & ZS2 , Invar Anode 60 µm wires.• ZS3, Invar Anode 100 µm wires.• ZS4 & ZS5 , stainless steel Anode 100 µm wires.• Pumping module (MP) with BDI equipment at

extremities and between ZS1 and ZS2.• Pumping module (MP) without BI equipment

between all other ZS.


ZS geometry/HV circuit

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-220 kV

-6 kV

-3 kV

R 1MΩ

R 1MΩ

R 500MΩ

R 500MΩ

R 100KΩ

I ANODE

Itrap box under ZS


ZS behaviour observed during operation

• Important Spark Rate → damage ZS, reduce lifetime

• High spark rate → Switch Off: ZS main voltage + Itrap power supplies

• Important Outgassing → Switch Off ZS, close sector valves

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Spark rates(4h) over 2012 run

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Spark rate remain low during 2012 run, few high spark rate also occur


Scrubbing Effect

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• To allow multi-cycling (LHC/Slow extraction), ZS girder should remain ‘In Beam’.

• A scrubbing / cleaning effect has been observed, reducing outgassing and spark rate.

→ Avoid as much as possible venting of the ZS vacuum sector to maintain effect of scrubbing / conditioning.


Different sparking phenomena

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I Sparking/Outgassing at injectionII Sparking/Outgassing during RampIII Sparking/Outgassing when beam disappear


Sparking / Outgassing at injection

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MD conducted in 2010 has shown that by applying a non-zero main gap field the vacuum activity can be reduced.

→ 100 kV seems to provide good results

Sparking / Outgassing during Ramp

B.Balhan Design and operation of existing ZS

• In case of Itrap sparking during ramp, Itrap voltage drops to zero and the vacuum rise provokes an interlock of the HV power supplies (both main as well as ion trap).

• To avoid this interlock (takes few cycles to restart), a beam Dump interlock has been added in case of spark during LHC cycle. Not used in 2012.

To reduce sparking during the ramp, Itrap voltages can be reduced during the LHC cycle to ~500 V (value confirmed by G. Rumolo in SUSG to be sufficient to suppress e-cloud in ZS geometry).


Sparking / Outgassing when beam disappears

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When LHC beam is extracted, significant outgassing of ZS5 is observed.To reduce this effect, the ZS gap has been increased to 28 mm to reduce the main field strength.


Main High Voltage ppm

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• In order to change voltage references cycle by cycle, a software upgrade has been deployed for Itraps as well as for the main generator.

• For the main generator a set of two current references has been built, the first one “Beam” with the typical reference of ~200µA and a second one “no beam” to recharge quickly the HV circuit with a current of max. ~2mA.

• To protect the ZS in case of spark during the recharging, an interlock must be implemented during LS1 to swap from reference “no Beam” to reference “Beam”.


Main HV circuit response time

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To calculate the fall time of the ZS main HV Generator, the equivalent RC circuit consists of:- a capacitor of ~70 nF (mainly the long coaxial cables) - a resistor of 600 MΩ (internal resistance of HV generator)

This RC circuit has a time constant of ~40 s, and to decrease from 220 kV to 180 kV ~8 s are needed.

170.00

175.00

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220.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

Actual

Add haefly divider

Add generator

By connecting in // to the generator a passive load (using HV divider or spare HV generator) it’s possible to ramp down in half the time ~4 s.

The ramp up time is limited by the current of the generator.Typically with 2mA, 10kV/300ms.


Itrap HV circuit response time

• To decrease voltage from 6 kV to 3 kV, about 3 s are needed• The ramp up time is limited by the current of the generator.

Typically with 300 µA, 1 kV/100 ms.

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ZSTF LSS6

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- ZS test facility already recommended by Multi-Cycling Study group back in 2005/2006.

- ZSTF constructed and in stalled in LSS6, since infrastructure from West extraction still available.

- ZSTF operational since 2011.- RF shields installed as from 2012.- MD’s conducted with ZSTF mainly in 2011.- Behaviour not identical to the

behaviour of ZS in LSS2.- Beam position w.r.t septum not

identical, since beam not bumped as in LSS2 and ZSTF installed as in ‘retracted’ position.

- Remains still very useful, especially for new Itrap box validation.


Conclusion• ZS still alive despite harsh beam conditions.• ZS outgassing strongly depends on beam parameters, in particular on

the bunch length.• Tests during operation in 2011-12 showed that the main circuit needs

to be powered at -100 kV to reduce the vacuum activity.• Cycling ZS generators can provide relief, but this is still a slow process

that may not allow full fast multi-cycling.• By spacing the SFTPRO sufficiently apart from the |LHC cycle in the

supercycle, time can be gained to permit decreasing the mains voltage. However, this may be more difficult without CNGS cycles in the future.

• Avoid as much as possible ZS vacuum sector venting to maintain effect of scrubbing/conditioning.

B.Balhan

design and operation of existing zs

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