Requirements Document: M55/Path length/M56 Measurement System

AUTHOR/CARETAKER: G. A. Krafft
DATE:10/31/96
REV:1

The following document describes requirements for the system which provides measurements of M55, path length, and M56. The system must acquire and analyse data for three different experiments: 

M55:         Measure the M55 between the photo-cathode source
             and a detector located somewhere downstream of the
             injection merge point.

Path length: Measure the overall path length of the recirculation
             loop of the IRFEL.

M56:         Measure the M56 of the two recirculation arcs.

General Requirements

The systems shall be able to measure the beam in the standard
tune-up mode having 625 microamp, 18.7 MHz, and 250 microsecond
beam pulses.

Hardware installed on the beam-line should not produce unacceptable
degradation in beam quality.

The maximum beam current passing through the diagnostic devices
shall not exceed 5 mA CW.

For ease of use by operations, readings shall be independent of BEAM
position within the measuring device. Otherwise, much time is wasted
in performing the measurements.

Specific Requirements


M55 measurements

The M55 of the injector shall be measured throughout the injector, to a point somewhere downstream of the injector merge point. The method used is to modulate the photo-cathode laser phase, and simultaneously measure the time-of-arrival of the beam at the measurement location. The accuracy of the M55 determination should be 0.0083 (0.5 degrees over the full input modulation of 60 degrees). Individual readings from the time-of-arrival system shall be good to 200 fsec. 
Modulator

Modulation Amplitude                             +-45 degrees
Modulation Frequency                               60-75 Hz
Modulation Type                           triangle wave, sinusoidal
Modulation Accuracy                               0.6 degrees

The modulater should introduce no more than 0.1 degrees phase drift
over time scales of order one day when the modulator is off.


Phase detector

High frequency noise                               < 200 fsec
Low frequency drifts                           < 200 fsec in 20 sec
Long term stability                              no requirement
Time to calibrate                                  < 3 minutes

On demand measurement time                            20 sec
Time to remeasure for verification                    20 sec
Aquisition rate                                       60 Hz
Aquisition buffer                               2 times 2000 points


Display requirements: the following parameters should be
                      displayed and controlled

parameter                                          range

Modulation amplitude                             0 to +- 45 degrees
Modulation frequency                                60-75 Hz
Modulation state                                    on/off
Aquisition time                                    0-35 sec
Offset phase                                       0-10 V

All data acquired during a single measurement shall be displayed in a
2-d plot

Path length measurement

The path length around the recirculation loop of the IRFEL will be established with an overall accuracy of 60 micrometers to the nearest 0.5 RF wavelength. This translates to 0.1 degrees at 1497 MHz, or 200 fsec. The method used is to measure the RF power emitted from a device that is excited by both first pass and second pass beams. The amplitude of the signal excited in the cavity is proportional to (delta phi)^2/2, delta phi being the phase deviation from perfect cancelation phase. Therefore, conservatively, the relative amplitude noise should be at the few parts per million level, compared to the amplitude induced by a single pass beam.
Amplitude detector

High frequency noise                     < 10^-3 compared to CEBA amplitude
Low frequency drifts                < 10^-3 compared to CEBA amplitude in 20 sec
Long term stability                               no requirement

On demand measurement time                            20 sec
Time to remeasure for verification                    20 sec
Aquisition rate                                       60 Hz
Aquisition buffer                                   2000 points


Display requirements: the following parameters should be
                      displayed and controlled

parameter                                           range

Attenuation                                        0-10 V
Aquisition time                                    0-35 sec
Offset phase                                       0-10 V
Variable Average                                   1-2000
Phase detector output                                NA

M56 measurement

The M56 of the two recirculation arcs will be established with an overall accuracy of 5 cm. Given a permissable energy modulation of 2 times 10e-3, this requirement translates to a requirement of 300 fsec accuracy on measurements of time-of-arrival. 
High frequency noise                               < 200 fsec
Low frequency drifts                           < 200 fsec in 20 sec
Long term stability                              no requirement
Time to calibrate                                  < 3 minutes

Phase detector

On demand measurement time                            20 sec
Time to remeasure for verification                    20 sec
Aquisition rate                                       60 Hz
Aquisition buffer                               2 times 2000 points


Display requirements: the following parameters should be
                      displayed and controlled

parameter                                           range

Attenuation                                        0-10 V
Aquisition time                                    0-35 sec
Offset phase                                       0-10 V
Variable Average                                   1-2000
Phase detector output                                NA