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       #  Drift Chamber specific information on high voltage supply  #
       #  and safe operation                                         #
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The Drift Chamber consists of two arc-shaped detectors for the East and West 
Arms, respectively, of the PHENIX central detector. Each Arm is a titanium 
frame, filled with drift chamber modules. Each frame is logically segmented 
into 20 `keystones'. 

Every keystone contains 6 wire module types: X1, U1, V1, X2, U2, V2. X wires 
are oriented parallel to the beam axis, while U and V wires form stereo angles 
of 4.5 degrees with respect to X  (see figures `Drift Chamber wire arrangement 
I, II').

Each module comprises 4 anode (sense) and 4 cathode wire nets. Cathode nets 
consist only of cathode wires. Anode nets, besides anode (sense) wires, 
contain also wires of other types: 2 Backdrift, 1 Guard, 1 Potential (field) 
and 4 Cathode (termination) wires. Every keystone comprises 4 x 40 sense wires 
(split in halfs for readout on either side of the detector) with its 
surrounding field wires, resulting in 3200 sense wires and a total of ~ 28 800 
wires for one full detector. 

The high voltages for XUV1 wire nets are supplied along one side of the drift 
chamber frame, while XUV2 wires are supplied from the other side. Cables from 
the high voltage supplies are connected to patch panels on the detector frame, 
from where the lines are distributed to the keystones. RC filter cards in the 
electronics compartment of every keystone multiplex the incoming high voltage 
lines to the wire nets. 

Every keystone needs to be supplied with four voltages for every set of wire 
nets;  X nets are supplied independently, while U and V nets are commonly 
supplied:  
              X1  (Cathode, Potential, Gate, Backdrift)
              UV1 (Cathode, Potential, Gate, Backdrift)
              X2  (Cathode, Potential, Gate, Backdrift)
              UV2 (Cathode, Potential, Gate, Backdrift)

The total number of external high voltage channels is therefore 320 for 
every drift chamber and 640 altogether. Internally, the high voltage 
supplies require additional channels. The total number of channels as 
controlled in the experiment is thus 350 + 350 = 700 .
  
In an operating gas mixture of Argon-Ethane 50:50 the required drift fields 
are obtained with:

              U_Cathode_XUV1   = ~4700 V
              U_Potential_XUV1 = ~2600 V
              U_Gate_XUV1      = ~1600 V
              U_Backdrift_XUV1 = ~ 900 V       

              U_Cathode_XUV2   = ~4800 V
              U_Potential_XUV2 = ~2600 V
              U_Gate_XUV2      = ~1600 V
              U_Backdrift_XUV2 = ~ 900 V       

However, individual tuning of every voltage is required for optimum detector 
performance.

Safe operation of the drift chamber requires that the maximum current per 
supply line never exceeds 100 uA. During normal operation the current is of 
the order of a few hundred nA and up to a few uA at most. 
The high voltage supply and control system needs to be designed to 
automatically detect abnormally high current and switch off the voltage 
generation for the lines affected (`channel trip'). 
Due to the close distances of Potential, Gate and Backdrift wires inside a 
drift cell  (see figure `Schematic layout of a drift cell'), it is essential 
that all three supplies shut down commonly in case of a channel trip there, 
to prevent from dangerously high fields during absence of one of the 
potentials. The high voltage control system is required to continuously 
monitor the detector status for such a situation and to automatically issue 
the required actions with as little delay as possible.

Such safety-off and trip-recovery procedures have been implemented for the 
Drift Chamber operation in:  

  a)  a PC-based high voltage control system that was used during the 
      Drift Chamber assembly and commissioning 

  b)  the high voltage control system within the PHENIX online computing 
      environment (via the Alarm Handler and MEDM Control Panels).