# In-situ Electron cyclotron resonance (ECR) plasma potential determination using an emissive probe\*

H. J. You^\#^, Y. Liu, F. W. Meyer, ORNL, Oak Ridge, TN 37831-6372 USA

Abstract

Plasma potential measurements by use of a Langmuir probe (LP) and an
emissive probe (EP) were compared in the ORNL CAPRICE electron cyclotron
resonance (ECR) ion source. It is shown that, for normal ECR ion source
operating conditions, the large population of hot electrons may make the
emissive floating point method fail, and may cause the values deduced
using the LP method to be in error by more than 10%. In addition, the
gas mixing effect was studied by comparison of *in-situ* probe
measurements and measurements of the extracted ion beam charge state
distribution (CSD). An explanation of the effect in terms of a change in
plasma potential and hot electron temperature is proposed.

## Introduction

Recently Langmuir probe diagnostics have been attempted for the first
time in ECR ion sources \[1\]. Motivation for these attempts was an
independent determination of the internal ECR plasma parameters and
their dynamics. In general, plasma parameters such as electron density,
temperature, and plasma potential can be obtained from LP measurements
if they are properly carried out. In an ideal (i.e., unmagnetized,
collisionless, stationary, and purely Maxwellian) plasma, LP data can be
easily analyzed to provide precise values of the plasma potential
(V~s~). However, due to the geometrically complex, magnetized, and
non-Maxwellian nature of ECR plasmas, and its large population of fast
electrons, determination of V~s~ from LP data can be problematic\[2\],
and should be confirmed using another diagnostic such as the EP.

In this article LP plasma potential determinations were experimentally
checked by using an EP. Having in this manner determined the magnitude
of possible uncertainties of the deduced potentials, the gas mixing
effect \[3\] was reinvestigated by comparison of probe data and
extracted beam CSD's.

## Experimental setup

In order to operate a plasma probe (LP or EP) successfully, the probe
must be small in comparison to the plasma length scale in order not to
perturb the global state of the plasma, and at the same time be able to
withstand the heat load from the plasma without damage. In the case of
an ECR plasma, it is difficult to satisfy these requirements because of
its small plasma length scale and the high heat flux from its large
population of hot electrons.

In the present measurements both requirements were satisfied by proper
placement of the probe holder, positioning of the probe into the edge
region of the ECR plasma, and limitation of the injected microwave power
to values sufficiently low to avoid self-emission of the probe. The
probe was inserted in a location where the flux tube intercepted by the
probe has no direct connection either to the ECR zone where the
electrons are heated, or to the extraction region where high energy
backstreaming electrons may be present (see Figure 1). During the
present measurements, careful shielding of the probe leads in the
extraction vacuum chamber assured that operation of the *in-situ* probe
when the source was operated at high voltage resulted in no detectable
perturbation of the extracted beam currents.

The probe could be operated in both LP and EP modes, and was formed from
0.058 mm tungsten wire, which extended toward the source axis from two
small alumina tubes, forming a small loop of approximately 3 mm length.
The electron and ion Larmor radii at the probe position were estimated
to be of the order of 0.01 mm and 0.5 mm, respectively. Consequently,
the plasma electrons are magnetized while the ions are unmagnetized.
Further, the probe operates in the collisional nonlocal regime for
electrons and the collisionless thick sheath regime for ions.

The probe design, high voltage isolation and data acquisition via a
wireless connection, and automated data analysis are described in
greater detail elsewhere \[4\].

## Results

### Plasma Potential measurements

The plasma potential determined from LP data, is usually taken in the
ideal case as the maximum value of the first derivative I′(V~p~) of the
probe current with respect to the probe bias (V~p~). In reality, the
plasma state, the probe analysis operating regime, and a range of other
effects can result in deviations from this ideal case. The most reliable
value in principle is found by fitting to the appropriate theoretical
model \[2\]. However, such an approach is not amenable to real-time
measurements. In the present measurements, since real-time monitoring of
the plasma potential is a central focus, and since relative changes, not
absolute values, in the plasma potential are of interest, the peak value
of I′(V~p~) is assumed to give the plasma potential.

In order to delineate better the conditions under which this assumption
holds, and the magnitude of the error that results when it fails,
measurements were performed with the probe operated in the EP mode at a
source pressure of 4E-7 Torr, a microwave power level of 28 W, and a
high confining axial magnetic field, which are all conditions favorable
for the generation of highly charged ions, and thus hot electrons. The
result is shown in Figure 2, which displays both I-V and I′(V~p~) curves
at different filament heating currents. The plasma potential was
determined by monitoring the I′(V~p~) maximum as a function of heating
current and extrapolating the result to zero electron emission, as
illustrated by the solid line in the figure. This value, denoted by
V~se~, is expected to be the most accurate \[5,6\]. Two observations are
noted. First, the floating potential, V~f~ , is significantly different
from V~se~ even under maximum achievable electron emission conditions.
Second, the I′(V~p~) maximum in LP mode, denoted by V~sc~ and the dotted
line in the figure, differs from V~se~ in this case by 10%. For other
plasma conditions, differences of up to 30% were observed. These
features suggest that for typical ECR source conditions, the presence of
fast electrons can lead to erroneous plasma potential values in both the
emissive floating potential method and in the LP approach.

![](media/image2.wmf){width="3.252083333333333in"
height="2.5104166666666665in"}

Figure 2: Plasma potential measurement by an EP.

### The gas mixing effect

Some gas-mixing-effect studies were carried out as well. The
measurements used the LP method to obtain both the plasma potential,
V~s,~ and fast electron temperature, T~ef~, were focused on the
correlation of these plasma parameters with external Ar CSD's modified
by addition of helium and oxygen mix gases. Four different Ar plasmas
were investigated: a pure Ar plasma, an Ar/He plasma, and two Ar/O~2~
mixtures. In the first Ar/O~2~ mixture and the Ar/He mixture, the Ar
leak rate was kept at the rate determined for the pure Ar case to give
the maximum Ar^8+^ current, while the mix gas flow rate was adjusted to
further optimize the Ar^8+^ current. For the second Ar/O~2~ mixture,
both gas flow rates were optimized for maximum Ar^9+^ current. For all 4
mixtures, slight adjustments of rf power and axial magnetic field
strength were also made. After each optimization, a number of LP
measurements were made and recorded. The experimental parameters for
each mixture are summarized in Table 1, and the corresponding CSD's are
shown in Figure 3(c). It is important to recall that the rf power levels
used were limited by probe lifetime issues and do not represent values
for fully optimized external CSD's. It is noted that when closing the He
gas valve, the plasma parameters and CSD immediately returned to their
original values (pure Ar case), while a much longer time interval (\~0.5
hour) was required after closing the O~2~ mix gas, suggesting
significant surface sticking for this gas.

Table 1: Experimental conditions for gas mixing studies

  -------------- --------------- ---------------------- ------------------
                 rf Power\       Source Pressure\       V~s~\
                 (W)             (×E-7 Torr)            (V)

  Ar             30              1.8                    27±0.4

  Ar+He          30              2.0                    23±1.2

  Ar+O~2~ I      31              20.0                   18±1.5

  Ar+O~2~ II     34              12.0                   10±0.5
  -------------- --------------- ---------------------- ------------------

The LP I′(V~p~) curves are shown in Figure 3(a), and the peak positions,
assumed to correspond to the relative plasma potential, are summarized
in the final column of the Table 1. Prior to these measurements the
plasma potential dependence on the power, pressure, and axial magnetic
field was more extensively mapped in a pure Ar plasma. Typical trends
found were that the plasma potential increases by less than 5 V as the
source pressure goes from 2E-7 to 10E-7 Torr and as the rf power
increases from 10 to 50 W, and that there is only a weak dependence on
B-field. Therefore the small variations of the pressure, magnetic field,
and rf power in the gas mixing measurements are not believed to
contribute significantly to the observed differences of plasma
potential.

![](media/image3.wmf){width="6.466666666666667in"
height="4.482638888888889in"}In addition to the plasma potentials, fast
electron temperatures (T~ef~) are obtained from the LP data. Figure 3(b)
shows I-V curves (I~N~) drawn on a semi-logarithmic scale, normalized by
dividing by the electron saturation current taken at the maximum of
I′(V~p~). T~ef~ can be extracted by fitting only the linear part of the
curves (see values in parentheses in Figure 3(b)). This direct fitting
is possible only when the fast electron contribution dominates, and when
their temperature is sufficiently different from that of the cold
electrons, as is the case in the present plasma. Based on other
investigations in magnetized plasmas, this fitting approach can
overestimate the fast electron temperature by up to 30% \[2\]. It is
noted that, unlike the plasma potential, T~ef~ was found to be very
sensitive to small changes of plasma conditions, particularly the rf
power level.

It is known from probe theory \[7,8\] that in magnetized plasmas the
electron energy distribution is proportional to I′(V~p~). The long tails
extending to the left of the I′(V~p~) peaks in Figure 3(a) therefore
provide further evidence for the presence of significant populations of
fast electrons, which are obviously closely related to the generation of
the highly charged ions observed in the extracted CSD's.

Both the plasma potential and fast electron temperature variations with
gas mixing show a correlation with the changes of the CSD's shown in
Figure 3 (c). The observed \~30% variation in T~ef~ is ascribed mainly
to varying source conditions. From the much larger change (factor of
2.7) of the plasma potential found in going from the pure Ar to the
Ar+O~2~ II case, the earlier noted lack of sensitivity of V~s~ to source
conditions, and the greatly different CSD's for these two cases, it
would appear that a decrease of the plasma potential and the
corresponding increase in ion confinement time is the dominant mechanism
responsible for the gas mixing effect. Similar conclusions have been
reached by other groups \[9,10\].

## References

\[1\] L. Kenéz, et al., Nucl. Inst. Meth. B 187, 249 (2002).

\[2\] V. I. Demidov, et al., Rev. Sci. Instrum. 73, 3409 (2002), *and
references cited therein*.

\[3\] A. G. Drentje, Rev. Sci. Instrum. **63**, 2875(1992).

\[4\] http://cfadc.phy.ornl.gov/mirfhome/Default.html.

\[5\] N. Hershkowitz, IEEE Trans. Plasma Sci. 22, 11 (1994).

\[6\] J. R. Smith, et al., Rev. Sci. Instrum. 50, 210 (1979).

\[7\] V. I. Demidov, et al., Plasma Sources Sci. Technol. 6, 350 (1999).

\[8\] R. R. Arslanbekov, et al., Plasma Sources Sci. Technol. 3, 528
(1994).

\[9\] Z. Q. Xie and C. M. Lyneis, Rev. Sci. Instrum. 65, 2947 (1994).

\[10\] Y. Kato, et al., IEEE Proceedings of the 11^th^ Int. Conf. on Ion
Implantation Technology, Vol. 1, 1996, p. 418.