DESIGN OF AN IMPROVED ION CHAMBER FOR THE SNS. Page: 2 of 8
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SIGNAL RISE TIME CONSIDERATIONS
With a 1 msec pulse width, signal risetime is important in the SNS. Electrons are
collected in a few microseconds with either bias polarity, but the heavier ions take
much longer moving to the inner electrode. At 2000 V the ion collection time would
be close to 700 psec. If the losses were constant the signal would continue to rise over
the 1 msec Linac pulse as the slow ions arrived, followed by a tail for another 700
psec after the pulse. While the electron signal would allow rapid beam abort for large
fast losses, the waveform during the pulse would require unfolding.
Electron and ion charges are generated equally, but will produce equal voltages
only for a parallel plate geometry. For cylindrical geometry, using the relative
capacitance of a line charge to the 2 electrodes, Shafer4 showed that the fraction of
current due to electrons at the inner electrode (anode) is:
<F2>z h2x2Ln(x)-(x" -I)
<F2>= = ,wherex=alb (1)
2zf RdR 2(x2 -l)Ln(x)
For the FNAL ion chambers, a = 0.75" and b= 0.125" and <F2>= 0.749 . That is,
the fast electron signal will be 3 times the slow ion current with the conventional
polarity and inverse for the reverse-bias polarity. This was observed in tests in the
RHIC transfer line. Clearly, the FNAL chambers would be unsuitable for SNS unless
the conventional bias polarity could be used.
The positive ion transit time is also a concern. It is given by:
t = (2)
d = Effective electrode separation [cm] for cylindrical geometry5
d = (a2 -b2) 2
po = Ion mobility at STP [ cm2/(V-sec)] P0 = Atmospheric pressure
V = Applied Voltage [V] P = Working pressure
The transit time can be reduced by decreasing the electrode gap but with increased
gradient corona may occur. A design was proposed6 in which the radii were made
closer together and the chamber lengthened to keep the same volume. For the original
outer electrode radius, a = 0 .75" and inner radius b= 0.5", at 3 kV bias, the ion
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WITKOVER,R.L. & GASSNER,D. DESIGN OF AN IMPROVED ION CHAMBER FOR THE SNS., article, May 6, 2002; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc733367/m1/2/: accessed November 12, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.