Turning up high voltage with air in chamber
We typically operate with 2000 - 2200 V with chamber gas to be in
the proportional gain region and have efficient signal / track detection.
It is useful to initially and briefly turn the chambers up to operating
voltage with air in the chamber.
Since air has many electronegative gas components, there is almost no gain
or efficiency when high voltage is applied to STP air.
This test determines that the system is mechanically intact / can hold
operating voltage.
The leakage current with air in the chamber is due to discharges through
the capacitors and, e.g., across the surfaces of circuit boards.
It should be no more than a few nA per power supply.
If it is higher, there is probably some problem with the chamber;
call in an expert.
The important issue for turning up high voltage is the high leakage current
associated with increasing voltage.
The capacitance in the system is dominated by the two 1500 pF HV caps for each
straw.
The capacitance of a 1 m straw can be calculated, using linear
charge density lambda = 2 pi
epsilon0 V / ln(ro/ri) and C = Q/V,
to be about 9 pF.
Similarly, the capacitance of a meter long HV cable is around 40 pF.
Both of these are negligible compared to the HV capacitors on the
circuit boards.
Each high voltage supply provides HV for about 200 - 250 straws.
The capacitance of this number of straws is about 600 - 750 nF.
Thus, increasing voltage at 10 V per second leads to a leakage current
of 6 - 7.5 microAmps.
Voltage should not be turned up much faster than this.
The RC time constant for charging a straw, given by the 1 Mohm resistor
coupling the HV bus to each straw and the two 1500 pF capacitors, is
tau = RC = 3 ms.
With about 250 straws powered by each supply, the time constant for voltage
to stabilize is of order 1 s.
In practice, you should probably wait ~1 minute for leakage current to settle.
Turning on high voltage with chamber gas
High voltage required depends on the gas mix used, geometry, etc, and
is typically 2000 - 2200 V for FPP-type straws.
For more information, see writeups on the
straws and on the
high voltage cards .
A discussion of
leakage current
through the capacitors plus due to cosmic rays shows that it should be
at the few nA level per power supply.
Leakage current is initially higher when the chamber is first turned on
with chamber gas after being constructed, and sometimes when it is turned
on after being off for significant periods of time.
This results from obscure chemistry and contaminents in the gas / on the inside
surfaces of the chamber.
Optimal training is generally believed to result from slowly increasing the
voltage, and allowing the leakage current to drop to close to ~0, until
operating voltage has been reached.
Time scale for this process can be a few days, and the procedure is recommended
at starts of run cycles.
Please send any comments on this page to Ronald Gilman, gilman@ruthep.rutgers.edu .
Revised May 6, 1996 Norma Lucero