High precision vacuum measurement system based on Bayard-Alpert ionization gauge

System overview for LHC, system performance, first operational measurements and noise immunity (July 2009 – February 2010)

G. Peñacoba 07/04/2011

G. Peñacoba 2

Nude UHV Ionization Gauge

• Down to 10 -4 mbar

Pirani Gauges

• Down to 10 -10 mbar

Penning Gauges

e-

e-

I+

Filament

GridCollector

10 V 5 A

4 mA1 pA

4/7/2011

• Down to 10 -12 mbar

Bayard-Alpert Gauges

G. Peñacoba 3

ProfibusCommunication Electrometer

Filament-gridControl

• Gauge controller

Volotek VGC1000

Cable multi-conductor, single shielding

Cable Triaxial(coaxial with double shielding)

4/7/2011

G. Peñacoba 4

• Point 1

Layout in LHC

200 m

4/7/2011

~ 20 / LHC point

G. Peñacoba

• PVSS• Oscilloscope

Noise figures

Electrometerhacked foroscilloscopeaccess

4/7/2011 5

4 hours 3 days

10 s

5 V

Gauge 5R3

10 s

1.5 V

Gauge 6L8Gauge 7R1

100 ms

2.5 V

Gauge 7R1

10 s

3.5 V

G. Peñacoba 6

Machine side

Rack side

•Hacked electrometer

Measures with stand-alone Volotek

Gauge 6L8Gauge 6L8

4/7/2011

Original

Gauge 6L8

- No Low Freq.- Stable reading

10 s

G. Peñacoba 7

Machine side

Rack side

•Hacked electrometer

Measures with precision current source

Double shielded Triax

Gauge 5R3

4/7/2011

10 s

Gauge 5R3

Original

The contact of shielding and GND brings in the noise

G. Peñacoba 8

•Noise basics•Ground current loop through shielding

Working hypothesis

Gauge 5R3(shield connected)

Gauge 5R3(shield disconnected)

4/7/2011

Disconnecting the shielding reduces the noise

10 s

10 s

5 V

1.5 V

G. Peñacoba 9

Gauge 7R1

Gauge 7R1

•Capacitor box on gauge

Shielding decoupling

Machine side

Rack sideGauge 5R3

Gauge 5R3

4/7/2011

The capacitor boxeliminates the noise

10 s5 V

10 s

0.5 V

0.5 V

10 s

10 s

3 V

10

Machine side

Rack side

•Capacitor box on current source

Shielding decoupling

1

2

Gauge 5R3Gauge 7R1

Gauge 5R3Gauge 7R1

4/7/2011 G. Peñacoba

Contact to GND: Position 1

Contact to GND:Position 2

5 V

10 s

10 s

0.5 V

3 V

0.5 V

Capacitor box within GND loopensures clean signal

G. Peñacoba 11

•Triax internal shielding discontinuity

Cabling non-conformity

Machine side

4/7/2011

Floating internal shielding

G. Peñacoba 12

• The dominant source of the signal degradation has been identified.

• The discontinuity of the internal shielding in some of the cables makes the acquisition system more vulnerable to noise pick-up.

• A possible solution implying a minor modification of the cabling infrastructure has been successfully tested.

• Systematic tests and detailed analysis are required for further conclusions.

Conclusions

4/7/2011

TO BE CONTINUED …