Measurements of Ion Current from a Corona-needle Charger Using a Faraday Cup Electrometer
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110 Chiang Mai J. Sci. 2009; 36(1)
Chiang Mai J. Sci. 2009; 36(1) : 110-119
www.science.cmu.ac.th/journal-science/josci.html
Contributed Paper
Measurements of Ion Current from a Corona-needle
Charger Using a Faraday Cup Electrometer
Panich Intra* [a], and Nakorn Tippayawong [b]
[a] College of Integrated Science and Technology, Rajamangala University of Technology Lanna,
Chiang Mai 50300, Thailand.
[b] Department of Mechanical Engineering, Faculty of Engineering, Chiang Mai University,
Chiang Mai 50200, Thailand.
*Author for correspondence; e-mail: panich_intra@yahoo.com, panich.intra@hotmail.com
Received: 19 September 2008
Accepted: 17 December 2008.
ABSTRACT
Corona-needle charger is widely used to impose a known net charge distribution on
the aerosol particles for the electrical mobility particle sizer. However, the corona discharge
and charging processes in the corona-needle charger at different operating conditions is not
well understood. In the present paper, measurement of ion current from a corona-needle
charger using a Faraday cup electrometer was performed in order to optimize the corona-
needle charger with respect to maximization of the ion number concentration. It was shown
that the corona onset increased with increasing air flow rate. At higher air flow rate, the ion
current and concentration were found to be relatively high for the same corona voltage. The
highest ion current in the Faraday cup electrometer was found to be about 6.4 × 10-10, and
6.29 × 10-10 A, corresponding to the ion number concentration of about 2.98 × 1013, and 2.93
× 1013 ions/m3 occurring at the corona voltage of 2.9 and 3.7 kV for positive and negative
coronas, and air flow rate at 8.0 L/min, respectively.
Keywords: corona discharge, faraday cup, electrometer, ion current.
1. INTRODUCTION
One of the most common techniques to undergo electrical breakdown when the
produce high ion concentrations is corona electric field strength is high. For the case of
discharge, defined as the low energy electrical wire and tube, the only place this breakdown
discharge with non-thermal ionization that can occur is in a very thin layer on the wire
takes place in the vicinity of an electrode of surface. In this corona region, energy is highly
sufficiently low radius of curvature in a intense to knock electron from gas molecules
medium the pressure of which is close to creating positive ions and free electrons.
atmospheric [1]. Corona discharge is produced There have been numerous studies in
by a non-uniform electrostatic field such as corona discharge phenomena in the past [2]
that between a needle and plate or a concentric which are widely used in many industrial
wire and a tube. Air and other gases can applications such as electrostatic coating andChiang Mai J. Sci. 2009; 36(1) 111
precipitation [3 – 5]. Electrostatic charging of operating air flow rates and corona voltages.
fine particles by the DC corona dischargers is The research is aimed at the optimization of
also commonly employed in determining the corona-needle charger with respect to
particle size distribution by electrical mobility maximization of the ion concentration. A
technique both corona-wire and corona- detailed description of the operating principle
needle chargers [6, 7]. Corona-needle charger of the Faraday cup electrometer was also
is among the most commonly used to presented.
generate ions in aerosol diffusion chargers for
the particle sizing instruments. Knowledge of 2. EXPERIMENTAL APPARATUS
the mechanism of the corona discharge 2.1 Corona-needle Charger
attainable in a given charger is needed. The Figure 1 shows the schematic diagram
reason is that one has to know how particle of the corona-needle charger used in this study.
charging depends on the ion concentration The corona-needle charger geometrical
flowing through the charging zone, charging configuration is similar to the charger used by
time, and the electric field inside it. The issue Hernandez-Sierra et al. [9], Alonso et al. [10]
of corona discharge in the corona-needle and Intra and Tippayawong [11]. However,
charger has not been extensively studied in differences between the present charger and
existing literature. Most papers concern about existing chargers are aerosol inlet geometry
characteristics of corona discharge in wire to which was modified to ensure uniform particle
cylinder geometry. Only a few of them focus distribution across the annular aerosol entrance
on the corona discharge in the needle to nozzle to charging zone. This charger consists
[8 – 11]. essentially of a coaxial needle electrode placed
In the present paper, the ion current and along the axis of a cylindrical tube with
number concentration of ions from the tapered end. The needle electrode is made of
corona-needle charger were measured using a stainless steel rod, 6 mm in diameter ending
a Faraday cup electrometer at different in a sharp tip. The angle of the needle cone is
Figure 1. Schematic diagram of the corona-needle charger.112 Chiang Mai J. Sci. 2009; 36(1)
about 10o and the tip radius is about 50 μm, and a Teflon insulator. To completely shield
as estimated under a microscope. The outer the HEPA filter collecting the air ions, outer
electrode is made of a stainless steel tube, 30 housing is made of a stainless steel, HEPA
mm in diameter and 15 mm in length with filter was equipped with a fine collection metal
conical shape. The orifice diameter is about grid, and was electrically isolated from the
3.5 mm. The distance between the needle outer housing and ground with Teflon stand
electrode and the cone apex is 1.75 mm. The (a volume resistivity exceeding 1018 Ωcm). The
corona-needle electrode head is connected to HEPA filter was used in this work, because
an adjustable DC high voltage supply, while the collection efficiency for small air ions was
the outer electrode is grounded. The corona very high. The Faraday cup plays a role to
discharge generates ions which move rapidly prevent electric noise for measuring low
in the strong corona discharge field towards electric signal current (in pA range) from
the outer electrode wall. accumulated charge of air ions on an internal
HEPA filter inside the Faraday cup corres-
2.2 Faraday Cup ponding to the total number concentration
The schematic diagram of the Faraday of the ions. If the object of measurement is
cup electrometer is shown in Figure 2. It not shielded completely, noise which is 1000
consists of an outer housing, a High Efficiency times of resolutions is expected. To transfer
Particulate Air (HEPA) filter, a filter holder, charges gathered at the HEPA filter to an
Figure 2. Schematic diagram of the Faraday cup.Chiang Mai J. Sci. 2009; 36(1) 113
electrometer circuit that is outside the Faraday flowing through a resistor is measured. The
cup, BNC connector is connected to HEPA circuit adopted two cascaded negative
filter. Because material of HEPA filter is feedback amplifiers. The extra component in
conductive such as glass fiber, charges collected this circuit is primarily for fine offset voltage
in the filter can move to the electrometer adjustment and input/output protection. A
through the low noise cable and BNC 12V DC power supply capable of
connector without delay. In the case of providing 100 mA is required. The feedback
existing electrometer air ions flow is curved capacitor and RC low-pass filter were used
at 90o while air is drifted from sampling probe to reduce high-frequency noise and to prevent
to the filter. It can become the cause of charge oscillations of the amplifier output [12]. In
loss. To solve this problem airflow into order to avoid expensive construction,
Faraday cup is straightened without changing commercially-available low-cost monolithic
the direction of the flow and loss the charge. operational amplifiers were used. The
commercially-available operational amplifiers
2.3 Electrometer Circuit used in this circuit is the LMC662, which was
An electrometer circuit is used to measure designed for low current measurement
the electric signal current, which are typically and featured ultra-low input bias current
in the range 1 pA to 1 nA, from the Faraday (2 fA maximum) and low offset voltage drift
cup. The schematic presentation of an (1.3 μV/oC) [13]. This circuit gives an output
electrometer circuit design for air ions voltage of 10 mV per 1 pA of input signal
detection system is shown in Figure 3. This current. The electrometer circuit was calibrated
circuit is a simple current-to-voltage converter, with a current injection circuit, high-impedance
where the voltage drop caused by a current current source [12]. The performance of the
Figure 3. Schematic diagram of the sensitive electrometer circuit.114 Chiang Mai J. Sci. 2009; 36(1)
electrometer circuit used in this work was also Leybold Didactic model 521721, was used
evaluated and compared with a commercial to maintain the positive and negative corona
electrometer, Keithley model 6517A, and voltages difference in the charger, generally in
good agreement was found from the the range between 1.0 – 5.0 kV. An air sample
comparison [14]. was first filtered through a HEPA filter, and
was then drawn into the charger. The ions
3. EXPERIMENTAL SYSTEM AND PROCEDURE produced inside the charger are then entered
The schematic diagram of the experi- the Faraday cup. In the Faraday cup, the ions
mental system for measurement of ion were removed from the air stream by the
number concentration from the corona-needle filter and the resulting ion current flow was
charger is shown in Figure 4. It consists of a measured with the electrometer. It should be
corona-needle charger, a Faraday cup electro- noted that the ion current was measured by
meter, a flow system, and a data acquisition the electrometer corresponding to the ion
and processing system. In our experiments, number concentration at the charger outlet.
the Faraday cup is connected directly to the The output signal from the electrometer circuit
charger outlet via a very short connecting pipe. is in the range of 0 to +10V. It is then sent to
The air flow was regulated and controlled by the ADAM-4017 analog input module, which
means of a mass flow meter and controller is a 16-bit, 8 channel analog input module,
with a vacuum pump, typically in the range controlled and data sampled by an external
between 3.0 – 8.0 L/min. A commercial personal computer via RS-485 to RS-232
adjustable DC high voltage power supply, a converter interface. Software running on an
Figure 4. Schematic diagram of the experimental system for measurement of dc ion current
from the corona-needle charger.Chiang Mai J. Sci. 2009; 36(1) 115
external computer was developed, based on needle electrode was measured directly with
Microsoft Visual Basic programming for all the micro-ampmeter via the outer electrode
data processing. The software is able to display of the charger. Figure 5 shows the current-
the ion current and number concentration. voltage characteristics in the charging zone of
The ion current measurements were translated the charger. In this charger, the corona onset
into ion number concentrations given the total was found to be about 2.4 kV, and 2.0 kV
air flow rate through the charger. Thus, the for positive and negative coronas, respectively.
total number concentration of the ion at the Increase in corona voltage produced a
charger outlet, Ni , can be calculated from the monotonic increase in charging current. It was
expression [11] shown that the spark-over phenomena
occurred for both positive and negative
(1) corona voltages larger than about 4.2 kV.
Above these values, the current was found to
where Ii is the ion current at the charger outlet, exhibit a fluctuation in an uncontrollable
e is the elementary charge (1.6 × 10-19 C), and manner and no measurement could be made.
Qa is the aerosol flow rate. Generally, the currents for negative ions were
slightly higher than those for positive ions. This
4. RESULTS AND DISCUSSION was expected because negative ions have
4.1 Current-voltage Characteristics of the higher electrical mobility than positive ions
Charger ( Z i+ = 1.15 × 10-4 m2/V s, Z i− = 1.425 ×
The charging current from the corona- 10-4 m2/V s, based on the work of Reischl
Figure 5. Current-voltage characteristics in the charging zone of the corona-needle charger.116 Chiang Mai J. Sci. 2009; 36(1)
et al. [15]). Thus, it was more likely to impact with the same corona voltage. This is because
and deposit on the outer electrode wall of the ions can be more easily drawn off the
the charger. The ion concentration in the charger by faster flowing air. In case of
charging zone, N i , of the charger was positive corona, the ion current and concen-
approximately proportional to the charging tration appeared to depend on applied
current. Thus, the high ion concentration in voltage only within a narrow voltage interval.
the charging zone of a charger is desirable For larger voltages, ion current and
for high particle charging efficiency. The ion concentration of positive corona became
concentration in the charging zone can be practically constant, independent of the
estimated from the relation applied voltage. Meanwhile, ion current and
concentration of negative corona slightly
(2) increases with increasing applied voltage.
The reason for this may be due to greater
where Zi is the electrical mobility of ions, E degree of ion loss. It was evident that when
is the electric field, and A is the inner surface the applied voltage increased, the charging
area of the outer electrode of the charger. current and electric field strength in the
This charging current increased with the electric charging zone were found to increase. More
field, hence applied voltage. ions have tendency to be electrostatically lost
in the charging zone of the charger. The ion
4.2 Ion Current and Concentration at the loss inside the charger due to electrostatic loss
Charger Outlet is defined as the ratio of the ion number
Figures 6 and 7 show the variations in concentration at the charger outlet, Nout, over
the ion current and concentration of the the number concentration of ions inside the
charger outlet with corona voltage at different charger, Nin. The ion penetration, P, through
operating air flow rates for both positive and the charger can be estimated by Deutsch-
negative coronas. The resultant ion current and Anderson equation as [16]
concentration of both positive and negative
coronas were evaluated for 3.0, 5.0, and 8.0 (3)
L/min and 1.0 – 5.0 kV. The obtained results
were expected for the effects of aerosol
flow and corona voltage. As seen in Figure 6, Form Eq. (3) it can be calculated that the ion
the negative corona onset (i.e. negative ion penetration was getting smaller with increasing
generation) appeared at about 2.0, 2.1, and electric field strength as a function of the
2.3 kV for air flow rates of 3.0, 5.0, and 8.0 corona voltage. It is commonly known that
L/min, respectively, while the positive corona the ion current and concentration for positive
onset was observed at about 2.0, 2.3, and 2.5 corona of the charger was slightly higher than
kV for air flow rates of 3.0, 5.0, 8.0 L/min, for negative corona. The highest ion current
respectively. For corona voltage less than 2.0 in the Faraday cup was found to be about
kV, the ion current was low. In this range, 6.4 × 10-10, and 6.29 × 10-10 A, corresponding
corona discharge was not present. It can be to the ion number concentration of about
seen that the corona onset increase with 2.98 × 1013, and 2.93 × 1013 ions/m3 occurring
increasing air flow rate. For both cases, at at the corona voltage of 2.9, and 3.7 kV for
higher air flow rates, the ion current and positive and negative coronas, and air flow
concentration were found to be relatively high rate at 8.0 L/min, respectively.Chiang Mai J. Sci. 2009; 36(1) 117
(a) Positive corona
(b) Negative corona
Figure 6. Variation in ion current with corona voltage at the charger outlet.118 Chiang Mai J. Sci. 2009; 36(1)
(a) Positive corona
(b) Negative corona
Figure 7. Variation in ion number concentration with corona voltage at the charger outlet.Chiang Mai J. Sci. 2009; 36(1) 119
5. CONCLUDING REMARKS [5] Tan X., and Shang J.K., Electric Field-
In this paper, the Faraday cup electro- Induced Intersections of 90o Domain
meter was used to measure the DC ion Walls in Tetragonal Ferroelectric Crystals,
current from the corona-needle charger in Chiang Mai J. Sci., 2005; 32(3): 245 - 252.
order to study the corona discharge inside it. [6] Intra P., and Tippayawong N., An
Overview of Aerosol Particle Sensors for
A semi-empirical method based on current
Size Distribution Measurement, Mj. Int.
measurements was used to determine the total J. Sci. Tech., 2007; 1(2): 120 - 136.
ion concentration at the outlet of the charger. [7] Intra P., and Tippayawong N., An
It was found that the corona onset increased Overview of Differential Mobility
with increasing air flow rate. At higher air flow Analyzers for Size Classification of
rate, the ion current and concentration were Nanometer-Sized Aerosol Particles,
found to be relatively high for the same Songklanakarin J. Sci. Technol., 2008; 30(2):
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Sci. Instrum., 1961; 32(12): 1351-1355.
be in higher concentration than the positive [9] Hernandez-Sierra A., Alguacil F.J., and
corona. The highest ion current in the Faraday Alonso M., Unipolar Charging of
cup electrometer was found at the air flow Nanometer Aerosol Particle in a Corona
rate of 8.0 L/min about 6.4 × 10-10, and 6.29 Ionizer, J. Aerosol Sci., 2003; 34: 733 - 745.
× 10-10 A, for positive and negative coronas, [10] Alonso M., Martin M.I., and Alguacil
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[11] Intra P., and Tippayawong N., Corona
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ACKNOWLEDGEMENT Electrical Engineering Conference, Pattaya,
Financial support from the Thailand Thailand, 2006; 9 - 10 November.
Research Fund (TRF) is gratefully acknow- [12] Intra P., and Tippayawong N., An Ultra-
ledged. Low Current Meter for Aerosol
Detection, CMU. J. Nat. Sci., 2007; 6(2):
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