Rapid Ultralow Pressure Manometry by Light Absorption Spectroscopy

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Antti Meriläinen1, Ivan Kassamakov1,2, Kenneth Österberg1,2 and Edward Hæggström1

1) Department of Physics, University of Helsinki

2) Helsinki Institute of Physics

Rapid Ultralow Pressure Manometry by Light

Absorption Spectroscopy

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Motivation

• CLIC main beam vacuum requirements from beam dynamics: H20, CO, CO2 partial pressures < 3*109 mbar

• The RF causes outgassing in CLIC accelerator structures mainly to two mechanism: breakdown and dark current

• Expected pressure rise from simulations: several orders of magnitude on 100 ns time scale. To be verified experimentally.

• Aim: develop method to measure partial pressure changes of ~108 mbar on ~100 ns time scale at a RF test stand (”Xbox”). For the moment focusing on absorption spectroscopy and Cu.

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Test setup

Quartz crystal microbalance (QCM) as reference

l = 5 ± 0.5 cm

R = 25 mm

RQCM = 6.5 mm

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Photo multiplier tube (PMT)


Simulated Visualization

Light Source

QCM

CU source

PMT

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Setup inside vacuum

QCM

Cu vapour source:Cu wire around W wire

1 cm

1 cm

1 mm

l = 5 ± 0.5 cm

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Results

Measurement 1 Measurement 2

Sample rate: 100 ks/s6 kHz low pass filter for PMT1 Hz low pass filter for QCM

Change in intensity ~5%≈ 2 * 1011 absorbances≈ 10-4 mbar

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Calibration (Relate intensity change to number of atoms)

Cu-source

QCM → ΔM → N1

Light beam ΔI

• atoms was measured by QCM

- Geometrical correction gives

- Calibration gives

N2

∫∆ 𝐼 ⅆ𝑡∫∆𝑀 ⅆ𝑡


Progresst – dP – P Diagram

Target

Measurement 1 & 2

Faster PMT

1 ng/s = 1013 Cu atoms/s ≈ 5∙10-5 mbar 4.2.2014 8 of 10


Achieved:

Measured few ng/s Cu at 10-5 mbar base pressure

Challenges:

Faster response time

Absolute calibration

Interpretation of measurement:

Can one atom have multiple absorptions?

Conclusion

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Thank You!

Questions and Comments

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Light Source

Setup & Cross section dimension

QCM

Cu source

RLight = 25 mm

RQCM = 6.5 mm

l = 5 ± 0.5 cm

Photo multiplier tube (PMT)

4.2.2014


Absorption spectroscopy

- 324.8 nm and 327.4 nm wavelengths are strongly absorbed in copper vapor

- Light source must be stable- Copper hollow cathode lamp

is suitable low cost light source

- Light intensity changes is measured with Photomultiplier tube (PMT), bandwidth 20 kHz

4.2.2014


Beer-Lamberts law(Light absorbtion in copper vapour)

Cross section for 324.8 nm and 327.4 nm wavelengths

Emitted intensity of 324.8 nm and 327.4 nm wavelengths

length of light beam in Cu vapornumber of absorbing Cu atoms

Current operation area (Signal > |Noise| )I = 90% - 10% lN = 4.9 * 1011 - 1.2 * 1013 Absorbances/meter

Operation point sensitivity: 9 * 1011 Absorbances/meter

4.2.2014

Rapid Ultralow Pressure Manometry by Light Absorption Spectroscopy

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