  Injected beam energy and energy spread are critical parameters affecting the
performance of our rapid cycling synchrotron (RCS). A real-time energy
monitoring system is being installed to examine the H- beam out of the Intense
Pulsed Neutron Source (IPNS) 50 MeV linac. The 200 MHz Alvarez linac serves as
the injector for the 450 MeV IPNS RCS. The linac provides an 80 ms macropulse
of approximately 3x1012 H- ions 30 times per second for coasting-beam injection
into the RCS. The RCS delivers protons to a heavy-metal spallation neutron
target for material science studies. Using a number of strip-line beam position
monitors (BPMs) distributed along the 50 MeV transport line from the linac to
the RCS, fast signals from the strip lines are digitized and transferred to a
computer which performs an FFT. Corrections for cable attenuation and
oscilloscope bandwidth are made in the frequency domain. Rectangular pulse
train phasing (RPTP) is imposed on the spectra prior to obtaining the inverse
transform (IFFT). After the IFFT, the reconstructed time-domain signal is
analyzed for pulse width as it progresses along the transport line.
Time-of-flight measurements of the BPM signals provide beam energy. Finally,
using the 3-size measurement technique, the longitudinal emittance and energy
spread of the beam are determined.
