Speaker
Description
Klystron modulators are key elements in free electron lasers. They provide high-voltage
pulses to bias klystron tubes with energies of several hundred joules.
Amplitude variations directly affect the gain and phase of amplified RF pulses and therefore
the accelerating fields created by RF cavities.
For machines such as the SwissFEL (Swiss Free Electron Laser), the required HV pulse
stability must be better than 15 ppm (parts per million).
Stability is calculated from 100 pulses up to 100 Hz as the relative standard deviation
of gated averages around the pulse's flat-top region, where the RF pulse is amplified.
Measuring such small variations often uses pulse offsetting technique and magnifying
the flat-top region for better quantisation resolution. This requires low-noise
analog electronics like summing amplifiers and clippers with adequate bandwidth and
settling time. Such a measurement setup usually uses an external differential
amplifier and a high-performance oscilloscope with statistical analysis to measure stability.
Our approach uses a RedPitaya STEMlab 125-14 board connected
to a custom signal conditioner board developed at PSI (Paul Scherrer Institut) to monitor stability
in klystron modulators. It captures both pulse current and voltage in real time,
and analyzes only the voltage for stability metrics. The concise design fits easily
into modulator cabinets for ongoing, precise performance checks.
Our system called the PMU (Pulse Measurement Unit) achieves a resolution limit of 5-6 ppm
at a 1 microsecond measurement gate, exploiting 67% of the ADC full-scale range.
We present the complete system and report on our initial
operational experience.
| Minioral | Yes |
|---|---|
| IEEE Member | No |
| Are you a student? | No |