Characterisation and performances of a glow discharge as a source of electrons for portable mass spectrometer

Achouak Chalkha1, Aurika Janulyte1, Yves Zerega1, Boris Brkić2, Stephen Taylor2 and Jacques Andre1 1Aix-Marseille Université; 2 University of Liverpool

1. Overview:

Purpose:

GDES as source of electrons for EI ionisation in portable mass spectrometer (MS)

Determination of optimal conditions to operate DC glow discharge electron source (GDES) cell, with a continuous inlet

flow from atmospheric air

Low power consumption.

Maximize number of electrons available for further ionization.

Determination of the kinetic energy distribution of electron beam.

Methods:

Experimental GDES cell in vacuum chamber.

Discharge sustaining power supply and polarisation power supply.

Current measurement.

Electrodes modelling of GDES with collection plate and simulation of electron trajectories by CPO program (boundary

element method).

Results:

Characteristic of glow discharge : current, voltage.

Paschen curve.

Define the operating points in order to achieve current stability and lower power consumptions.

Electron energy distribution at GDES exit for ionisation.

2. Introduction:

A portable MS requires very low electrical and throughput consumptions

and operates at higher pressure 10-4-10-5 torr .

Problem of EI filament: short lifetime and high power consumption (5W for 5µA emission current) with efficiency 1

µA/W.

Solution: Replace the filament by a GDES.

Advantage: longer lifetime, less power consumption (0,5 W for 5µA emission current), better efficiency ca 10 µA/W, a

cold cathode (reducing out-gassing phenomena) [1,2,3].

3. Methods:

DC GDES in vacuum chamber at 10-4 -10-6 Torr (pM).

Pressure in GDES (pGD) varies according to capillary length (L) estimated by Hagen-Poiseuille equation.

Continuous inlet flow through capillary

Plasma sustaining by power supply applied between anode and cathode

R limits the current of the discharge in instability mode and is used only for I-V characteristic

R’ limits the current of the undesired discharges in instability mode

For different capillary lengths

1. I-V characteristic:

o Increase UGD voltage and measure IC and IA currents.

2. Paschen experience:

o Increase UGD and measure IA.

o As from one voltage value the current increase suddenly, it is breakdown voltage.

3. Power consumption PGD:

o Increase UGD and measure IC , IA and IP.

o Calculate PGD =>

4. Distribution of electron kinetic energy

o Varying Upp at UGD =430V and measure IC , IA and IP.

5. Conclusions and prospects:

Last experiments:

Observation of current instability versus time

similar to capacity discharge curve

effect of insulating material to the quantity of electrons at exit of anode

Currently investigations of the current discharge stability time:

Current stability versus in time

Modelling of secondary electron between anode and plate to quantify

Simulation of secondary electron with CPO for coupling GDES-MS

.( )GD AC AP V I Ip

Fig. 4: Power of discharge sustaining PAC (▲: 16.5cm and ●:

20cm) and discharge voltage (Δ: 16.5cm and ○: 20cm) vs Ip

Fig. 5: IA (Δ: 16.5 cm, ○: 20cm) and Ip (▲: 16.5cm,

●: 20cm) and – electrons simulated vs UPP.

Simulation steps:

For one value of potential applied to the plate

-> Uniform initial kinetic energy distribution of

electrons Ek,e, ranging between 0 and 20eV

-> Count the number of electrons hitting the

plate

Same simulations are repeat for different values

of Upp (V)

Operation parameters :

Initial Electron number: 1050 for one Upp value

Anode: 0 V

Electron plate: -100 to +100 V

Geometrical :

Identical to the experimental setup

'C

A

P

I Id Ibp I d

I Id Isg

I Ibp Isg

Some 400V for few µA

Acknowledgements:

The research leading to these results has received funding from the European Community's Seventh

Framework Programme managed by REA Research Executive Agency ([FP7/2007-2013]) under grant

agreement #285045.

L (cm) pGd (torr)

7,1 1,4

9,8 1

16,5 0,6

20 0,5

33 0,3

50 0,2

,max

( )PP

PP

Ek

U kqU

I f E du

4. Results:

A. GDES characteristics

Experimental Paschen curve confirm the theoretical GD regime for atmospheric air

Stable regime is achieved for higher pressure

Capillary lengths chosen 16.5 and 20cm:

lower sustaining voltage (Paschen curve)

stable discharge regime (V/I characteristics)

B. Power consumption / Efficiency

C. Distribution of Electron Kinetic energy (Eke)

Theoretical:

When the plate potential is repulsive, electrons having initial kinetic energy higher than the potential plate hit

the plate => inducing electronic current

When the plate current attains 0 A, the potential value represent the maximal value of Ek,e

Measured plate current is deduced from kinetic energy distribution:

Simulation results is take account an uniform distribution from 0 to 20eV (red curve)

Experimental results show curve with 2 slopes:

more complex kinetic energy distribution than uniform

hypothesis: other phenomenon secondary electron energy emission

between anode and plate

Balance of current is expressed with secondary electron :

Fig. 3: V/I characteristics: (●) 7.1

cm, (◊) 9.8 cm, (○) 16.5 cm, (Δ) 20

cm, (×) 33 cm and (►) 50 cm.

Fig. 2: Paschen curve according to capillary

length: (●) 7.1 cm, (◊) 9.8 cm, (○) 16.5 cm,

(Δ) 20 cm, (×) 33 cm and (►) 50 cm

References:

[1] R. A. Dugdale, J. T. Maskrey, S. D. Ford, P. R. Harmer, and R. E. Lee, Journal of Materials Science 4 (1969) 323-

335.

[2] J. R. Roth, Industrial Plasma Engineering: Principles, Institute of Physics Publishing, 1995.

[3] A. Bogaerts, E. Neyts, R. Gijbels, and J. van der Mullen, Spectrochimica Acta Part B: Atomic Spectroscopy 57

(2002) 609-658.