Biomedical Engineering Reference
In-Depth Information
difference
U
SO
−
U
SI
. The settings corresponding to the maximum in Fig.
22a
are
optimum.
2. By varying
U
IFO
the focus point of ion beam is varied. Again the maximum ion
current against
U
IFO
is the optimum. The voltage is varied between 10 and 120 V
with the optimum typically between 30 and 75 V. Figure
22b
shows a typical
result for the ion focus variation.
To perform these not time critical optimization steps corresponding routines are
implemented into the software of the present setup. However, in order to reduce the
power-on and response times, as it is advantageous for a number of operating modes,
this procedure will be embedded into the firmware in the future.
6.4
HV-Source
A MCP is used in the PIMMS as a secondary electron multiplier (see Sect.
3.7
). The
electron current measured after MCP compared to the initial ion current is amplified
by a factor of 10-1,000. The secondary electron emission coefficient is an averaged
number of secondary electrons emitted after each impact. This number depends on
the initial energies of the electrons or ions and so on the voltage applied to the MCP.
The amplification factor of a MCP configuration is expressed as:
(
)
()
n
Fc
=
U
se
HV
()
U
c
is the voltage and material dependent secondary electron emission
coef fi cient and
n
the average number of impact stages. A noise level of 1 V
p-p
for
U
HV
= 2,000 V (0.05 %) generates a noise on the amplified signal in order of the
signal magnitude. Thus, the HV-source should be as noiseless as possible. Higher
MCP-voltages (>3 kV), as commonly used in standard mass spectrometers, require
chamber pressures <0.1 Pa, higher pressures, however, restrict allowable voltages to
<2,000 V.
Currently, a commercial HV-Source is used (Spellman MPS3N 10/24). A minia-
turized, integrated, and noise reduced source is presently developed. It is based on
the EMCO GPMT module, a commercially available HV-source designed to drive
photo multiplier tubes.
se
HV
6.5
Micro-Plasma Support
To ignite the microplasma the pressure of the inert gas in the plasma chamber is
temporally increased to allow a controlled arc discharge according to the Paschen
law. With the RF-power applied between the center electrode and the chamber walls
