  In its normal mode of operation the Argonne Wakefield Accelerator Facility
uses a high charge (10-100 nC), short pulse (3-5 psec) drive bunch to excite
high-gradient accelerating fields in various slow-wave structures. To generate
this bunch, we designed a 1.5 cell, L-band, rf photocathode gun with an
emittance compensating solenoid to give optimal performance at high-charge; it
has recently completed commissioning. More recently, we have begun to
investigate the possibility of using this gun in a high-brightness, low-charge
operating mode, with charge equal to approximately 1 nC, for high-precision
measurements of wakefields. Two related measurements are reported on in this
paper: (1) measurements of the transverse beam envelope are compared to
predictions from the beam dynamics code PARMELA; and (2) investigations into
the use of a modified 3-screen emittance measurement method that uses a beam
envelope model that includes both space-charge and emittance effects. Both
measurements were made for the 1 nC, 8 MeV beam in the drift region directly
following the rf photocathode gun.
