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Electric discharge during electrosurgery

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Electric discharge utilized for electrosurgery is studied by means of a recently developed method for the diagnostics of small-size atmospheric plasma objects based on Rayleigh scattering of microwaves on the plasma volume. Evolution of the plasma parameters in the near-electrode sheaths and in the positive column is measured and analyzed. It is found that the electrosurgical system produces a glow discharge of alternating current with strongly contracted positive column with current densities reaching 10(3) A/cm(2). The plasma electron density and electrical conductivities in the channel were found be 10(16) cm(-3) and (1-2) Ohm(-1)cm(-1), respectively. The discharge interrupts every instance when the discharge-driving AC voltage crosses zero and re-ignites again every next half-wave at the moment when the instant voltage exceeds the breakdown threshold.
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Electric discharge during electrosurgery
Alexey Shashurin
1
, David Scott
1
, Taisen Zhuang
2
, Jerome Canady
2
, Isak I. Beilis
3
& Michael Keidar
1
1
Department of Mechanical and Aerospace Engineering, School of Engineering and Applied Science, The George Washington
University, Washington, DC 20052, USA,
2
Jerome Canady Research Institute for Advanced Biological and Technological Sciences,
6930 Carroll Avenue, Suite 300, Takoma Park,MD 20912,
3
School of Electrical Engineering, Tel Aviv University, Ramat Aviv 69978.
Electric discharge utilized for electrosurgery is studied by means of a recently developed method for the
diagnostics of small-size atmospheric plasma objects based on Rayleigh scattering of microwaves on the
plasma volume. Evolution of the plasma parameters in the near-electrode sheaths and in the positive column
is measured and analyzed. It is found that the electrosurgical system produces a glow discharge of alternating
current with strongly contracted positive column with current densities reaching 10
3
A/cm
2
. The plasma
electron density and electrical conductivities in the channel were found be 10
16
cm
23
and (1-2) Ohm
21
cm
21
,
respectively. The discharge interrupts every instance when the discharge-driving AC voltage crosses zero
and re-ignites again every next half-wave at the moment when the instant voltage exceeds the breakdown
threshold.
E
lectrosurgery has been utilized for cutting and coagulating tissue for about 90 years
1,2
. Electrosurgical
coagulation has improved treatment of many gastrointestinal diseases such as radiation proctitis,
Barrett’s esophagus, gastric antral vascular ectasia, and arteriovenous malformations
3–10
. Additionally, it
may decrease postoperative swelling and inter-operative blood loss for other areas of the human body such as
knee joint replacement.
Electrosurgical Argon Plasma Coagulation (APC) utilizes plasma produced by the ionization of a few milli-
meter diameter argon flow exhausting into ambient air from the electrosurgical hand-piece. The intensity of
treatment and the effect induced in the living tissue strongly depends on the plasma properties
11
. Such atmo-
spheric pressure microplasmas are difficult to study using conventional diagnostics. Microwave interferometry
operating in the GHz frequency range fails due to the small size of the plasma compared to the microwave
wavelength causing diffraction and unsufficient phase change. Electrostatic probes introduce very strong per-
turbations once inserted into the plasmas, and are associated with difficulties of interpretation at strongly-
collisional atmospheric conditions
12,13
.
Recently a few methods for the measurement of atmospheric pressure plasma parameters were proposed.
This includes various spectroscopic techniques, namely passive optical emission spectroscopy, laser-induced
fluorescence spectroscopy, diode laser absorption spectroscopy and Rayleigh, Thomson and Raman scattering
of laser radiation on microplasmas
14–21
. These techniques are characterized by good spatial resolution (down to
10–50 mm) and minimal detectable values of plasma electron density of ,10
13
cm
23
, but require great stability of
the discharge since they are based on averaging over large number of discharge events.
An alternative approach capable of detecting low plasma densities (down to 10
11
–10
12
cm
23
) at atmospheric
pressure is Rayleigh scattering of microwave radiation on microplasmas
22
. The concept of the method was first
proposed theoretically by Shneider
23
, and then implemented experimentally in studies of laser-induced avalanche
ionization in air, resonance-enhanced multi-photon ionization in argon and non-thermal atmospheric plasma
jets (widely utilized recently for biomedical applications)
22,24–28
. The method consists of measurements of radi-
ation scattered from microplasmas irradiated by microwaves in the GHz frequency range. In the Rayleigh regime,
the electric field amplitude of the scattered wave is proportional to total number of electrons in the microplasmas
and thus, the plasma electron density can be determined if the plasma volume is known
23
.
The type of discharge and the plasma parameters utilized during electrosurgical coagulation has not been
extensively studied, even though this type of plasma source probably has the widest practical application in
medicine thus far. In this work, we utilize an electrosurgical system produced by US Medical Innovations LLC,
and present for the first time, measurements of plasma discharge parameters and discuss processes developing in
the positive column and in the near-electrode sheath.
Experiment. Electrosurgical system. The experiments were conducted using the electrosurgical system SS-200E/
Argon 2 in combination with the electrosurgical Canady Vieira Hybrid Plasma scalpel by US Medical Innovations
LLC shown in Fig. 1 (see Ref. [29] for histological studies). The Canady Vieira Hybrid Plasma scalpel is comprised
OPEN
SUBJECT AREAS:
PLASMA PHYSICS
BIOMEDICAL
ENGINEERING
Received
29 October 2014
Accepted
9 March 2015
Published
16 April 2015
Correspondence and
requests for materials
should be addressed to
A.S. (shashur@gwu.
edu) or M.K. (keidar@
gwu.edu)
SCIENTIFIC REPORTS | 4 : 9946 | DOI: 10.1038/srep09946 1
of a flexible hose ending with a hand-piece
29
. The hose delivers argon
flow and electrical power produced by the electrosurgical generator
to the hand-piece. The hand-piece is nearly a cylindrical hollow
volume with a high-voltage tungsten electrode of about 1 mm in
diameter installed on the axis and surrounded by the argon flow.
The hand-piece is equipped with a 2.4 mm opening at its distal end
through which the argon flow exits into the ambient air. The
electrode was protruded about 5 mm from the distal end in the
experiments. The electrosurgical unit (ESU) was utilized in a range
of power settings from 15 to 60 Watt and 3 LPM argon flow rate.
Inorganic replica of the living tissue. The power produced by the ESU
is delivered into the biological tissue by means of an argon plasma
column producing an electrically conductive channel between the
high voltage electrode of the electrosurgical hand-piece and the tis-
sue. A photograph of the SS-200E/Argon 2 in operation in Argon
Plasma Coagulation (APC) mode with a fresh chicken liver sample is
shown in Fig. 2(a). Application of the ESU to the tissue sample
produces instant tissue burning, accompanied with smoke produc-
tion at the treated point [see Fig. 2(a)]. Additionally, the plasma
column attachment to the tissue is non-steady and characterized
by fluctuations and motion of the plasma column around the tissue.
To avoid problems with the non-reproducibility of the measure-
ments, due to non-steady motion of the plasma column over the
tissue, the experiments were conducted using an inorganic replica
of the tissue made of two metal electrodes separated by water gap [see
insert in Fig. 2 (b)]. A photograph of the ESU in operation in the APC
mode with the inorganic replica is shown in Fig. 2 (b). It was observed
that the discharge with the inorganic replica sample produced quasi-
steady, immovable diffusive attachment to the replica electrode tip as
shown Fig. 2 (b). All measurements were conducted at a separation
distance between the replica electrode tip and the electrosurgical
hand-piece tip of about d54.5 mm.
The water gap size in the replica sample was adjusted to obtain
current and voltage waveforms similar to that of the chicken liver
tissue samples. The waveforms of the voltage produced by the ESU
(U
tot
) and the discharge current (I
d
) were similar for both samples
(peak voltage of about 2 kV and peak current ,1 A for 60 W and
3-3.5 LPM) as shown in Fig. 2.
The details of the electrical schematics are shown in Fig. 2. The
electrical circuit was as follows: ESU (Accessory port) R electrosurgical
scalpel R tissue on patient pad/replica R ESU (Patient port). Note, for
experiments conducted with biological samples tissue was placed on the
standard patient pad, while inorganic replica experiment s were made
with no patient pad (see schematic in Fig. 2). The Patient port of the ESU
was grounded in all experiments as shown in Fig. 2.
Microwave scattering diagnostics. The schematics of the Rayleigh
Microwave System (RMS) are presented in Fig. 3. Linearly polarized
microwave radiation (10.58 GHz) was scattered on the collinearly-
oriented plasma channel and then the scattered signal was measured.
Microwaves were irradiated and detected using two horns shown in
Fig. 3. The detection of the scattered signal was accomplished using a
homodyne I/Q Mixer, providing in-phase (I) and quadrature (Q)
outputs. The total amplitude of the scattered microwave signal was
determined by: U
MW
~
ffiffiffiffiffiffiffiffiffiffiffiffiffi
I
2
zQ
2
p
. The amplifiers and the mixer used
in the microwave system operated in a linear mode for the entire
Figure 1
|
Photograph of electrosurgical system SS-200E/Argon 2 with
connected electrosurgical Canady Vieira Hybrid Plasma Scalpel by US
Medical Innovations, LLC.
Figure 2
|
Photograph of the ESU in operation in Argon Plasma
Coagulation mode, electrical schematics and current/voltage waveforms
of the discharge for ESU power 60 W and flow rate - 3-3.5 LPM:
(a) chicken liver sample, (b) inorganic replica. Similar voltage and current
waveforms indicate good agreement of the electrical properties of tissue
sample and inorganic replica.
Figure 3
|
Schematics of the RMS system utilized in this work.
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SCIENTIFIC REPORTS | 4 : 9946 | DOI: 10.1038/srep09946 2
range of the scattered signal amplitudes, thereby ensuring that the
output signal U
MW
is proportional to the electric field amplitude of
scattered radiation E
s
at the detection horn location: U
MW
~E
S
.
The absolute value of the plasma electron density, conductivity
and electric field in the plasma channel were determined from
Eq. (4)–(6) according to the methodology developed in Methods.
A digital oscilloscope was used to simultaneously record signals for
the microwave system, along with the electrical parameters of the
discharge. In addition, the plasma column was simultaneously
photographed using an Intensified Charge-Coupled Device
(ICCD) camera: Andor USB iStar.
Results
Fig. 4 presents the waveforms of the total voltage produced by the
ESU (U
tot
) as well as the discharge current (I
d
). One can see that
the ESU produces a series of high voltage bursts, having a peak
amplitude of about 1 kV and a repetition frequency of about
60 kHz which are filled with sinusoidal oscillations at about
600 kHz. Two stages can be distinguished on the operation cycle,
namely the active and inactive stage as indicated on the Fig. 4 by the
darker and brighter bars, respectively. The active stage (t,0-4 ms) is
characterized by the presence of the non-zero discharge current
spikes (I
d
peak values up to 250-300 mA) and a bright plasma column
oscillating between the discharge electrodes as shown in the typical
discharge image in Fig. 4 (the image is taken at t50 ms). The inactive
stage (t,4–16 ms), is characterized by an absence of the discharge
(I
d
50) and a dark inter-electrode gap.
The active discharge stage is shown in more details in Fig. 5. It was
observed that a bright discharge column develops between the elec-
trodes in the volume occupied by argon gas and oscillates overlap-
ping (covering) left and right discharge electrodes by turns (see e.g.
image T1 in Fig. 5 where plasma column covers the tip of the right
electrode; image T3 same for the left electrode). One can see that the
plasma column always overlaps the electrode that is negative at this
particular moment (instant cathode) - see images T1, T3 and T4 in
Fig. 5. Time intervals in vicinity of the voltage zeroes (U
tot
,0) were
associated with the presence of the plasma column strictly between
the electrodes without any electrode overlapping - see image T2 in
Fig. 5.
The total AC voltage produced by the ESU (U
tot
) represents the
sum of the discharge voltage (U
d
) and the voltage drop on the replica/
tissue sample (U
sample
): U
tot
5 U
d
1U
sample
. U
d
was determined by
subtracting the voltage measured at the surgical probe from that
measured at the replica electrode. Waveforms of the discharge volt-
age and current are shown in Fig. 6. One can see that breakdown
occurred at about 1.2 kV and afterwards the discharge voltage was
generally in the range ,300–400 V.
The diameter and length of the interelectrode plasma channel
and the area of its attachment to the discharge electrodes were
determined from processing of the instant discharge photographs
taken with an ICCD camera (see Fig. 5) and summarized in
Table 1. The length of the plasma column coincided with the size
of the interelectrode gap (plasma column length ,4.5 mm). The
diameter of the discharge column was ,0.3 mm and slightly
increased with ESU input power. Area of the attachment to the
instant cathode was measured from the clearly observed illuminated
part of the cathode surface at the moment of maximum discharge
current (see images T1 and T2 of Fig. 5). For the replica electrode, this
corresponded with the moment I
d
peaked during the 1st positive
Figure 4
|
Total AC voltage (
U
tot
) and discharge current (
I
d
) produced by
the ESU system for an input power of 15 Watts and an argon flow rate of
3 LPM. The discharge-driving voltage represents a sequence of the high
frequency, high voltage pulses. The operation cycle has two stages, namely
the active (t,0–4 ms, darker bar) and inactive stage (t,4–16 ms, brighter
bar). Typical photographs of each stage are presented for time moments
t 5 0 and 9.8 ms respectively (exposure time 20 ns).
Figure 5
|
Temporal evolution of discharge voltage (
U
d
) and discharge
current (
I
d
) of the electrosurgical system SS-200E/Argon 2 for
P
515W
and flow5 3 LPM. Photographs of the interelectrode gap at different
moments of time T1-T4 are shown on the right.
Figure 6
|
Discharge voltage (
U
d
) and discharge current (
I
d
) for 60 Watts
and argon flow rate 3 LPM.
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SCIENTIFIC REPORTS | 4 : 9946 | DOI: 10.1038/srep09946 3
half-wave and occurred at the surgical probe when I
d
peaked during
the 1st negative half-wave.
Temporal evolution of the average plasma electron density and
electric field in the interelectrode column measured using RMS sys-
tem is shown in Fig. 7. One can see that the peak values of the electric
field in the plasma channel and plasma electron density reached
about 350 V/cm and 8.10
15
cm
23
respectively, at an ESU input power
of 15 Watts. Dependence of the maximum plasma electron density
with ESU power is shown in Fig. 8. n
e
increased with ESU power
reaching about (0.9-1)
?10
16
cm
23
for 60 Watts.
Discussions
Temporal behavior. Let us discuss the evolution of the plasma
parameters in the positive column and in the near-electrode
sheath. The experimental data indicates that the discharge was not
continuous during the active stage. Instead, it was interrupted every
time the discharge-driving voltage crossed zero (U
tot
,0) and was
re-ignited on the next voltage half-wave (see Fig. 5 showing that
moments when the voltage crosses zero were accompanied by sub-
microsecond, quiescent intervals when I
d
remained at ,0).
In order to analyze the nature of the discharge interruptions,
it is important to discuss plasma decay rates. When discharge
driving voltage is changing polarity and the cathode switches to
another electrode, the sheath formed near the ‘‘old’’ cathode starts
to decay. Photographing of the discharge demonstrates an almost
instantaneous termination of the cathode sheath when the discharge
driving voltage crosses zero (compare images T1 and T2 in Fig. 5),
which can be explained by fast recombination of plasma at the elec-
trode surface
30
. Instant sheath termination causes the discharge cur-
rent to remain low in the beginning of every next half-wave, until the
discharge-driving voltage reaches about V
br
,300 V and breakdown
occurs again. The breakdown restores the cathode sheath near the
‘‘new’’ cathode and causes an observed peak in the discharge current
(see Fig. 5)
30,31
. The discharge lasts until the discharge driving voltage
changes sign again and the process is repeated. When the amplitude
of U
tot
oscillations decreases below 300 V (starting at the 6
th
half-
cycle in Fig. 4), no further breakdown is possible and this indicates
the start of the inactive stage, which lasts for about t
i
,12 ms (brighter
bar in Fig. 4) when the plasma is decaying.
The positive column also experiences significant decay during the
discharge interruptions as one can see from the direct measurements
of the interelectrode plasma electron density shown in Fig. 7.
Both estimations (using typical electron-ion recombination coef-
ficient ,10
27
cm
3
/s)
30
and experiments indicate that n
e
decays to
,10
14
cm
23
during the discharge interruption lasting about a fraction
of a microsecond, when discharge driving voltage is changing sign.
This remaining plasma causes a glow in the gap shown in photo-
graphs in Fig. 5 during the discharge interruptions in contrast with
almost instant termination of the glow around the cathode. It is
interesting to note that, in contrast with plasma electron density,
the conductivity of the plasma channel does not reduce significantly
during the discharge interruptions (see Fig. 7). This is related to the
simultaneous decrease of electron collisional frequency around zero
U
tot
when the electric field in the column is crossing the zero
(see Fig. 7).
Therefore, the discharge produced by the electrosurgical system
SS-200/Argon 2 represents an AC discharge as opposed to an RF
discharge, where near electrode plasma sheaths are permanent and
do not decay significantly over the period of oscillation
30,31
. The
discharge is governed by an instant value of the applied AC voltage
and the amount of plasma remaining after the last breakdown event.
Table 1
|
Cross-sectional areas of the positive column and areas of attachment to the instant cathodes. The attachment areas to the instant
cathodes were measured at moment of the corresponding discharge current peaks (for the cathode at replica electrode I
d
peak at 1
st
positive half-wave and for the cathode at the surgical probe - I
d
peak at 1
st
negative half-wave). ESU argon flow rate was set to 3 LPM.
P515W P540W P560W
Cross-sectional area of positive column, cm
2
7.10
24
10
23
10
23
Area of attachment to the replica electrode, cm
2
1.6.10
22
6.3.10
22
10
21
Area of attachment to the surgical probe, cm
2
2.5.10
22
6.6.10
22
8.10
22
Figure 7
|
Temporal evolutions of electric field, collisional frequency,
plasma conductivity and plasma electron density in the positive column
produced by electrosurgical system SS-200E/Argon 2 at
P
515 W and
argon flow53 LPM.
Figure 8
|
Maximum electron density versus input power of the
electrosurgical system SS-200E/Argon 2 (flow53 LPM).
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SCIENTIFIC REPORTS | 4 : 9946 | DOI: 10.1038/srep09946 4
The discharge interrupts every time when the discharge voltage
changes sign due to the decay of the near-cathode sheath and
re-ignites on the next voltage half-wave in the vicinity of the ‘‘new
cathode", when voltage increases above the breakdown threshold and
a cathode sheath is formed near the ‘‘new cathode’’.
Positive column and near-cathode sheath. Let us now discuss some
parameters of the cathode sheath determined in this work. As shown
in Fig. 5, the discharge re-ignites at around V
br
,300 V, which
corresponds to a well-known minimum voltage required for
self-sustained DC discharges (minimum breakdown voltage of
Paschen curve)
30
. The current density at the instant cathode was in
the range of 10–20 A/cm
2
on the stainless steel replica electrode and
5-10 A/cm
2
on the tungsten surgical probe electrode as show in Fig. 9
(based on measured I
d
and area of attachment shown in Table 1).
These current densities are close to previously measured normal
current densities in glow discharges, indicating that nearly normal
cathode layer is established
30
. The depth of the cathode sheath can be
now estimated from Paschen curve minimum (using (p d)
min
<1cm
Torr for argon and thus d
c
[cm]51/p[Torr]) yielding cathode sheath
thickness d
c
<10 mm
30
.
It should be noted that higher breakdown voltage ,1 kV observed
during the first high voltage oscillation compared to that on the
following cycles when breakdown happens around Paschen’s curve
minimum ,300 V (see t,0 in Fig. 5 and Fig. 6) can be explained as
follows. This is caused by the fact that for the first oscillation the
entire gap has to breakdown, while on the following half-waves,
a well-conducting plasma channel remains after the discharge is
interrupted (see conductivity waveform in Fig. 7) and only the
near-cathode sheath needs to breakdown.
Visual observations indicate that attachment of the discharge
channel to the living tissue sample may differ from the quasi-steady,
diffusive attachment observed at the surgical probe. Instead, the
attachment of the plasma column to the tissue was non-steady,
accompanied with random motion over the tissue while the plasma
column at the point of contact with the tissue was contracted. It
should be noted that the scenario of the non-steady contracted
attachment can be mimicked with the replica at low ESU powers
and flow rates. Visual observations of the attachment to the replica
electrode indicate that a decrease of power and flow rate causes
sporadic transitions from quasi-steady, diffusive to non-steady, con-
tracted attachment. This transition is captured in Fig. 10 for an ESU
power of 20 W and an Ar flow rate 2.5 LPM. It was observed that
transition to the non-steady, constricted attachment was accompan-
ied by an abrupt increase of I
d
and a decrease of U
d
,100V which
indicates transition to arc.
Experiments indicate that current density and electrical conduct-
ivities in the positive column were up to 10
3
A/cm
2
and (1-2)
Ohm
21
cm
21
, respectively. These values of the current density sig-
nificantly exceed normal current densities of glow discharges, and
along with high measured plasma electron density in the channel
n
e
,10
16
cm
23
are typical for contracted (filamented) positive col-
umns
30
. Typically, plasmas at such conditions are nearly thermal
and therefore, gas temperatures of about several thousand K might
be expected in electrosurgical plasmas
30
. Note, since measured
plasma electron density is relatively high, other diagnostic techniques
such as Stark broadening and Thomson scattering can be potentially
utilized for testing of such plasmas.
Let us now discuss how utilization of the inorganic replica instead
of the living tissue affects the discharge. Experiments indicate that
discharge attachment was different in these cases, namely non-steady
attachment to the living tissue versus quasi-steady attachment to the
replica. This can be explained by the fact that application of the ESU
to the living tissue causes instant localized burning of the tissue
which leads to a change in its local physical properties (electrical
conductivity, thermal conductivity etc.) and causes a shift of the
discharge column to the new unburnt location. In other words, the
tissue can be considered as an electrode with constantly fluctuating
properties. Since tissue burning causes carbonization, it may be
hypothesized that averaged over time, the surface layer of the living
tissue in contact with the plasma would exhibit properties similar to
the electrode made of carbon. Also, since the effect of the cathode
material on the discharge characteristics (such as cathode fall, break-
down voltage, plasma properties in the positive column etc.) is fairly
weak
30
, it can be concluded that the properties of the discharges with
the inorganic replica and the living tissue are similar. Similarity of the
discharge current and voltage measured with the inorganic replica
and with the living tissue also supports this conclusion.
Conclusions
The discharge produced by the electrosurgical system SS-200E/
Argon 2 was studied experimentally. The plasma electron density
was measured in the range of (7.5-9.5)
?10
15
cm
23
for applied powers
of 15-60 Watts. The discharge can be classified as a glow discharge of
the alternating current with contracted positive column. The dis-
charge ignites every half-wave of the driving voltage when voltage
increases above the breakdown threshold and is interrupted at the
end of each half-wave when the voltage approaches zero.
Methods
Electron density measurements. A methodology for electron density measurement
by means of microwave scattering on elongated atmospheric pressure plasma
channels will be developed. It should be noted that this consideration differs from that
conducted by Shneider in Ref. [23] for short plasma channels when the restoring force
produced by the channel ends with non-compensated charges is significant. In
contrast, the case for when channels have large aspect ratios (when the contribution of
the channel ends is negligible) will be presented. The method described in this section
considers the general case of both free-standing plasma channels and those attached
to the discharge electrodes. Furthermore, it presents an approach to determine the
electric field and takes into account the depolarization effect and dependence of
collision frequency on the magnitude of the electric field, in contrast with simplified
description in Ref. [22].
Figure 9
|
Current densities at cathode and in positive column.
Figure 10
|
Discharge voltage and current waveforms indicating sporadic
transition between modes with steady and non-steady attachment to
the cathode on the positive half-wave (ESU power - 20 W, Ar flow
rate - 2.5LPM).
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SCIENTIFIC REPORTS | 4 : 9946 | DOI: 10.1038/srep09946 5
Let us consider the scattering of microwaves on the conductive or dielectric
channel having slender prolate shape when channel length (l) significantly exceeds
the diameter (d). An incident microwave radiation oriented along the channel excites
oscillations of the electric current along the channel. The amplitude of the microwave
electric field in the channel for the general case of scatterers with dielectric permit-
tivity e and conductivity s when channels are thin compared to skin depth
((d<
0:5
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
s½V
{1
cm
{1
p
mm for microwave frequency ,10.58 GHz use in this work) can be
written as:
E
MW
~
E
0
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
(1zk(e{1))
2
z(k
s
e
0
v
)
2
q
,
where k - depolarization factor governed by the channel geometry, E
0
- incident
microwave electric field at the channel location
32,33
. The depolarization factor k for the
channels with large aspect ratio m5l/d?1 is small: k<
1
m
2
ln (m)=1
34,35,36
.
For the plasma considered in this work, the skin depth d,0.5 mm (s,1 V
21
cm
21
according to experimental data) exceeds plasma channel diameter d,0.2 mm and
m,20 which yields depolarization factor ,7.10
23
. Thus, the electric field inside the
channel is close to E
0
. The amplitude of the resultant electrical current excited in the
plasma channel can be written as I5s E
0
S for a pure conductor and I5e
0
(e21) v E
0
S
for a pure dielectric. The distribution of the current along the channel can be con-
sidered nearly uniform if the distance to the radiating horn is large, therefore the front
of the incident wave is nearly flat on the channel length: r
r
?l
2
/l where l - wavelength
of microwave (channel is in a far zone).
Electrical current in the channel oscillates at the frequency of the incident micro-
wave field and therefore, it represents a radiating antenna. In the far-field, when the
distance from the detecting horn to the radiating channel r
d
?l
2
/l, the radiation
pattern coincides with radiation from a Herzt dipole with oscillating current I.The
overall power radiated by the channel averaged over the microwave period can be
written as follows: P~
p
3e
0
c
l
l

2
I
2
34
.
The output signal measured by the linear microwave detection system described in
the Experimental Details section is proportional to the electric field in the radiated
wave at the location of the detecting horn E
s
and thus: U
MW
?E
s
?
ffiffi
P
p
r
?
Il
lr
. For a
microwave system with fixed operational parameters (such frequency, horn locations
etc.), the dependence of U
MW
is governed by the following propertied of the scatterer
channel:
U
MW
?I
:
l: ð1Þ
Combining Eq. (1) with the above expressions for current excited in the channel it
can be found that the dependence of output RMS signal on parameters of the scat-
tering channel can be expressed as follows:
U
MW
~AsV {for conductor ð2Þ
U
MW
~A e
0
(e{1) v V {for dielectric ð3Þ
where A –proportionality coefficient and V - channel volume (V5 Sl).
The proportionality coefficient A is a property of the specific microwave system
(utilized elements, geometry, microwave power, etc.) while independent of scatterer
properties and it can be found using scatterers with known properties. Utilization of
prolate-shaped metal cylinders to determine A is problematic due to smallness of skin
layer depth in metals for microwave frequencies (, 1 mm for 10 GHz for copper).
Therefore, utilization of dielectric scatterers is preferable to determine A using
Eq. (3). The constant A was found to be 214 V V/cm
2
using dielectric scatterers
with known volume and dielectric permittivity similar to that described in details
in Ref. [22].
Let us first apply this method to determine the electric field E in the tested plasma
channel. Note: E is assumed here to be governed by the processes developing in the
tested plasmas, rather than the amplitude of the microwave electric field in the plasma
channel. This imposes a condition on the maximum microwave power level that can
be utilized for the diagnostic system to ensure that the entire microwave system is
non-invasive with respect to the processes developing in the tested plasmas.
The amplitude of the microwave field used in this work can be estimated to be
E
0
, 1 V/cm which is significantly lower than E caused by the discharge (see Fig. 7).
If the current flowing through the plasma channel is known (e.g. for the case of a
discharge initiated between two electrodes), the average electric field in the plasma
channel can be determined using Eq. (2) as follows:
E~
j
d
s
~
I
d
s:s
~
I
d
Al
U
MW
, ð4Þ
where j
d
discharge current density and I
d
–discharge current.
Now let us apply the developed methodology to determine the plasma conductivity
and electron density in the plasma channel. For many practical cases, the two
following conditions are satisfied for atmospheric plasmas. First, plasma ionization
degree is low enough so that electron collisions are governed by collisions with gas
particles (n
m
). Second, electron collisional frequency is significantly larger than
microwave frequency v, meaning that the plasma channel can be treated as
a conductor since the ratio of conductivity current
j
c
to the polarization current
j
p
excited in the plasma channel
j
c
j
p
~
s
e
0
ve{1jj
~
n
m
v
?1 ?1
31
. Both of these conditions
are satisfied for typical atmospheric plasmas (n
m
,10
12
compared with v,10
10
,
ionization degrees ,10
23
–10
24
)
30
. In this case, plasma conductivity can be found
from Eq (2):
s~
U
MW
AV
ð5Þ
Plasma electron density can be found from s and electron collision frequency n
m
:
30
n
e
~
sv
m
(E)
2:82 10
{4
ð6Þ
Generally speaking, electron collision frequency depends on the electric field in the
channel (E) and thus n
m
in Eq. (6) is not constant, but instead n
m
5 n
m
(E). This
dependence for argon utilized in this work is plotted in Fig. 11 (determined from data
in Ref. [30]). Note, static collisional frequencies shown in Fig. 11 can be utilized if the
electric field changes weakly on the time between the collisions (up to frequencies
of E oscillations ,10
10
GHz). Therefore, the electric field found from Eq. (4) has to be
used in combination with dependence n
m
5n
m
(E) in order to determine the plasma
electron density using Eq. (6).
It is important to make a few notes on the applicability of the methodology
developed here. First, the method can be applied for both free-standing plasma
channels, when radiating antenna size coincides with the size of the plasma channel
(such as laser-induced plasma)
25
, as well as for plasmas in contact with the discharge
electrodes, when radiating antenna is comprised of plasma column along with pieces
of adjacent electrodes. Indeed, since the excitation of the electric current in the
channel is driven by the amplitude of the incident wave, microwave scattering from
the stationary discharge electrodes contributes to the DC component of the output
signal U
MW
(similar to the contribution from other surroundings) which is filtered
out from the transient signal scattered from the plasma. Second, since it is typical that
near electrode sheaths, which form at the contact of the plasma channel with the
electrodes, be short compared to the plasma channel length, their contribution to the
resultant scattered signal is small. In this case, Eq. (4)-(6) is applicable for measure-
ments of average values of plasma electron density, electrical conductivity and electric
field in the positive column.
Summarizing the above spatially-averaged plasma electron density in small
atmospheric plasma objects can be determined using scattering of microwaves on the
plasmas. In the case of elongated plasma channels, the radiation has to be polarized
along the channel and the microwave frequency has to be chosen so that
f ½GH
Z
ƒ
2:5
s½V
{1
d½mm
2
to ensure that channel is thin compared to the skin layer depth.
This method measures the total number of electrons in the plasma volume and the
current setup has sensitivity down to ,10
11
electrons. The error of the plasma density
measurement is governed by the accuracy of the plasma volume determinations, and
it was 10-15% in current experiments. If the electrical current in the plasma channel is
measured independently, this method can be also utilized for the determination of the
electric field.
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Acknowledgments
This research was supported by US Medical Innovations. L.L.C. Authors would like to thank
Dr. M.N. Shneider for very fruitful discussions.
Author contribuions
A.S. conceived and designed the experiments, performed research, analyzed the data and
wrote the manuscript. D.S performed research and analyzed the data. T.S. and I.I.B
discussed results of the study, provided assistance in manuscript writing. M.K. and J.C.
conceived the project, discussed results of the study, provided assistance in manuscript
writing.
Additional information
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Shashurin, A. et al. Electric discharge during electrosurgery.
Sci. Rep. 4, 9946; DOI:10.1038/srep09946 (2014).
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SCIENTIFIC REPORTS | 4 : 9946 | DOI: 10.1038/srep09946 7
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The use of a low energy, high peak intensity (>100 TW/cm{sup 2}) femtosecond laser pulse is investigated for guiding and control of a sub-microsecond high voltage discharge. Study of the laser induced plasma channel and measurements of the field required for breakdown in air and nitrogen at atmospheric pressure are presented. Direct imaging of the dynamics of the discharge breakdown shows effective laser guiding. The effectiveness of laser guiding is shown to be critically dependent on the laser focusing geometry, timing, and location relative to the electrodes.
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Argon plasma coagulation (APC) is a thermoablative technique increasingly being used in endoscopy. Since its introduction, the flexible APC probe has been employed by endoscopists throughout the world. APC has helped change the endoscopic management of many gastrointestinal (GI) diseases, including hemorrhagic proctitis, watermelon stomach, bleeding peptic ulcer, and colonic varices. Endoscopists and surgeons are creatively combining standard and new electrosurgical techniques with APC. For instance,. APC used in combination with piecemeal polypectomy, endoscopic mucosal resection, balloon dilatation for strictures, and plasma welding of bleeding vessels after sclerotherapy injection are among the recent innovative techniques reported. Other emerging innovations using APC that are being considered include endoscopic en bloc resection of mucosal and submucosal tumors of the GI tract, endoscopic mucosal resection supplemented with APC for high-grade dysplasia and early GI cancers, endoscopic repair of anastomotic strictures, and welding GI fistula tracts. As such, endoscopists require more efficient and cost-effective multifunctional thermoablative probes. This review discusses the development and the potential application of dual-mode plasma endoscopic probes in fulfilling these emerging needs.
Article
Plasma and injury properties produced by US Medical Innovations (USMI) elec­trosurgical systems were characterized using an explant pig’s liver samples. It was observed that plasma length, tissue temperature, and injury size increases with applied power increase. Transition from conventional to argon coagulation mode (<0.5 L/min) leads to redistribution of the discharge power over the larger tissue area causing abrupt decrease of injury depth and increase of eschar diameter. Flow rate is not a primary factor affecting the tissue temperature. The depth and diameter of injury was minimal for the case of hybrid argon plasma cut opera­tional mode.
Article
A series of instant photographs of a discharge produced by the electrosurgical system SS-200E/Argon-2 by U.S. Medical Innovations is presented and analyzed.
Book
Here is both a textbook for beginners and a handbook for specialists in plasma physics and gaseous electronics. The book contains much useful data: results of experiments and calculations, and reference data. It provides estimates of typical parameters and formulas in forms suitable for computations. Gas discharges of all important types are discussed: breakdown, glow, arc, spark and corona at radio frequency, microwave and optical frequences. The generation of plasma, and its application to high power gas lasers are treated in detail.
Article
This review focuses on one of the fundamental phenomena that occur upon application of sufficiently strong electric fields to gases, namely the formation and propagation of ionization waves–streamers. The dynamics of streamers is controlled by strongly nonlinear coupling, in localized streamer tip regions, between enhanced (due to charge separation) electric field and ionization and transport of charged species in the enhanced field. Streamers appear in nature (as initial stages of sparks and lightning, as huge structures—sprites above thunderclouds), and are also found in numerous technological applications of electrical discharges. Here we discuss the fundamental physics of the guided streamer-like structures—plasma bullets which are produced in cold atmospheric-pressure plasma jets. Plasma bullets are guided ionization waves moving in a thin column of a jet of plasma forming gases (e.g., He or Ar) expanding into ambient air. In contrast to streamers in a free (unbounded) space that propagate in a stochastic manner and often branch, guided ionization waves are repetitive and highly-reproducible and propagate along the same path—the jet axis. This property of guided streamers, in comparison with streamers in a free space, enables many advanced time-resolved experimental studies of ionization waves with nanosecond precision. In particular, experimental studies on manipulation of streamers by external electric fields and streamer interactions are critically examined. This review also introduces the basic theories and recent advances on the experimental and computational studies of guided streamers, in particular related to the propagation dynamics of ionization waves and the various parameters of relevance to plasma streamers. This knowledge is very useful to optimize the efficacy of applications of plasma streamer discharges in various fields ranging from health care and medicine to materials science and nanotechnology.