The high voltage cards described here were designed by Stan Sherman of the Rutgers University Department of Physics & Astronomy electronics shop. Cards were constructed with outside vendors, and tested by the electronics shop.
To collect electrons ionized by a charged particle passing through the straw chamber, high voltage must be applied to the central wire in each straw. To do this, and to allow a testing system to be implemented, special high voltage boards were built for the FPP chambers.
To isolate high voltage channels from each other, each channel is connected to the common HV bus by a 1 Megaohm resistor. Note that if the channel shorts to ground, the 2000 V results in a 2 mA current and 4 W power dissipation, which would quickly ``blow'' the resistor. Protection is provided by the HV power supplies, which are current limited with a trip setting of about 10 microAmperes.
To make the signal cleaner for the straw readout board, the straws are also impedance matched to ground at the HV card to reduce signal reflections. This is done by placing a 1500 pF capacitor in series with a 360 ohm resistor, about the impedance of the straw. The capacitors are rated for 3000 V, but generally the chambers have not been tested above 2300 V. (However, we note that in the past, errors in understanding operation of Bertan power supplies led to a test of a previous nonFPP straw chamber plus capacitors to 5 kV. This was the maximum supply voltage, and no damage was found to have occurred.)
To allow for testing, a 20 kohm resistor connects a test pulse line to the ground side of the HV capacitor. A 1 V fast signal in the test line leads to an ~ 8 mV signal at the HV card pin. Connections and the ~ 3 m attenuation length of the straws leads to a signal of about half this amplitude for a typical straw at the readout board.
Leakage current arises from several effects. An obvious one is the gain in the gas, with cosmic rays or beam particles causing signals, and a leakage current. The area of chamber 3, for example, is about 3.3 m2, and the cosmic ray flux through a horizontal plane is about 200/m2s. At operating voltage, with a gain of about 105 and ~ 20 primary ionizations per track, this leads to a current of 200 * 3.3 * 105 * 20 / 6.8*1018 = 0.2 nA per plane. At 1 MHz rate per plane, however, the leakage current would be 300 nA.
In addition there can be leakage through the high voltage capacitors. Specs list a minimum resistance for the capacitors of 50,000 Mohms, leading to a maximum current per cap of 2000 V / 50,000 Mohm = 40 nA! In practice, we find with 2000 V on a power supply for ~ 250 straws filled with air, for which there is no gain, has a leakage current no more than about 5 nA. This implies that the resistance of the capacitors is typically higher than the spec by at least a factor of 4000.
Dirt, oil, grease, etc on the circuit boards or inside the straws can also lead to a leakage current. This should be independent of the gas mix, so it is important to turn on HV with air in the chamber briefly to confirm that HV can be obtained in a gas with no gain; thus one knows there are no surface discharge problems.
Typically, however, chambers when first turned on with a gas with gain do exhibit leakage currents sugnificantly in excess of the estimates above. Contaminants lead to more noise / additional discharges compared to the gain estimate above. The chamber should be gradually trained, by increasing voltage and letting leakage go to 0. Typically a few days should be sufficient to reach the performance numbers estimated above.
Revised May 6, 1996 Norma Lucero