Biomedical Engineering Reference
In-Depth Information
Cathode
Trigger
Anode
Cooling water
Magnetic filter duct
FIGURE 19.3
Schematic diagram of cathodic arc plasma source with a magnetic fi lter duct.
Presheath
Sheath
~ few
De
n
e
= n
i
= n
o
n
e
= n
i
n
i
n
e
0
s
x
FIGURE 19.4
Qualitative behavior of plasma sheath and presheath in contact with a wall. (From Lieberman,
M.A. and Lichtenberg, A.J.,
Principles of Plasma Discharges and Materials Processing
, Wiley, New York,
1994. With permission.)
energy between electric and magnetic forms. In low-pressure cold plasma (electron temperature
ion temperature), the electron motion dominates the behavior of the waves as the mass of electron
is much smaller than that of the ion. Thus, the plasma can be confi ned and enhanced by bounding
the electron motions.
A metal probe inserted in a discharge and biased positively or negatively to draw electron or ion
current is one of the most useful tools for diagnosing a plasma. A conducting probe smaller than the
particle mean free path placed directly in the plasma is called a Langmuir probe [10]. The plasma
electron density, electron temperature, and plasma potential can be measured from the current ver-
sus voltage characteristics of this probe. In this way, the energy of the ions striking the cathode can
be inferred without the more diffi cult direct measurement.
Another technique to detect the plasma concentration is to use emission spectroscopy as most
plasmas emit light in the infrared and ultraviolet ranges. A simple analytical technique is to meas-
ure the intensity of these emissions versus wavelength with the aid of a spectrophotometer. Using
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