L1S modulator performancemeasurement

Tao Tang

Outline

PurposeMeasurement setupMeasured signalTrigger sourceResultsFuture work

Purpose

Better understand L1S jitter problemImprove L1S modulator stabilityCross validation with other measurementsCollect data for thyratron tuning procedureCollect data for automated thyratron tuningCollect data for thyratron performance

Measurement setup

Oscilloscope:• Tektronix DPO 4054B, 2.5GSps, 500MHz BW, vertical

resolution (Hi Res) 11 bits (~980ppm)• Typical measurement setup: 100k point, full BW, ~3

measurement per secondVoltage offset:

• Preample Instruments 1855 Differential Amplifier• 100MHz BW, 50 Ohm, 7ppm resolution

Trigger generation:• SRS DG535 delay generator

Signal measured

PFN signal from Ross divider• Connected to thyratron anode• Buffered by new HV regulation board outside thyratron

cabinentBV signal from MKSU

• Measured at klystron tank with capacitive divider• Buffered by MKSU

Trigger signal from RF system and modulator• Increase pulse width to 5ms for better visualization by DG535

Signal measured - PFN

Source

Signal conditioning

Channel 1: Buffered PFN voltageà Scope (1MOhm)Channel 4: Buffered PFN voltageà Diff-ampà Scope (50Ohm)Channel 3: Buffered BVà Scope (1 MOhm)Channel 2: Buffered BVà Diff-ampà Scope (50 Ohm)Ferrite on each line to reject common mode signalRaw data without scaling (scaling factor needed)

Trigger

Clock base: AC line zero crossing (120Hz)Modulator trigger:

• Always 120 Hz• Acceleration trigger, stand-by trigger (6us before beam)

RF trigger/accelerator trigger• Variable rate• Fixed delay relative to clock base

Charging waveform

Measurement conditions• Infinite persistence• Long time scale (10min)• Low sample rate• 1 hour difference in screen time and real time

Charging waveform

Two-state charging due to AC phase• 120Hz trigger vs 60Hz AC line• Two charging states with opposite resonant peak polarity• Switching between two states due to operation condition

(frequency change)Possibly additional switching noise due to SCR voltageregulation in AIP stations

• SCR voltage regulation board working at 60° spacing(2.78ms)

• 6 SCR formed 6 poles

Large scale view (PFN charging)

CH1: PFN, CH2: Reconditioned acceleration trigger, CH3: BV, CH4: PFN w. offset

SCRswitching?

PFN charging

CH1: PFN, CH2: BV w. offset, CH3: BV, CH4: PFN w. offset

Very close to60° spacing(2.78ms)

Switching between two charging states

Charge state #1 Charge state #2

CH1: PFN, CH2: Reconditioned acceleration trigger, CH3: BV, CH4: PFN w. offset

Charge state change due to beam rate?

CH1: PFN, CH2: Reconditioned acceleration trigger, CH3: BV, CH4: PFN w. offset

Noaccelerationtrigger

PFN waveform shape change

CH1: PFN, CH2: Reconditioned acceleration trigger, CH3: BV, CH4: PFN w. offset

Thyratron instability

Thyratron instability• Self breakdown

- High gas pressure / reservoir voltage- Impact next a few pulses, much lower BV (beam drop out)- Can not start modulator

• Random recovery- Low gas pressure / reservoir voltage- Impact almost every pulse- High PFN instability

Random recovery

CH1: PFN, CH2: BV w. offset, CH3: BV, CH4: PFN w. offset

Worse PFNStability

Self breakdown

CH1: PFN, CH2: Thyratron grid, CH3: BV, CH4: PFN w. offset

Thyratron selfbreakdownNo grid trigger

Data acquisition system

Using Tektronix scope on sun-ray network• Can not measure PFN and BV at the same time• Jumping between two different configurations every 5 minute• Statistics for about 900 data points with a 200-point window

Save screen capture at regulate interval• Infinite persistence

Take measurements at regulate interval• Mean, minimum, maximum, standard deviation, sample count

Matlab script running on lcls-srv01• Daily cleaning routing problem• Lcls-srv02 for long term running• Matlab 2012a pause memory leak/inaccurate timing

Modulator stability

Data with less than 800 samples are ignored

Waveform with different

12/3 4:47 34ppm 12/3 5:18 170ppm

L1S phase

L1S low spike count

PFN voltage

Data with less than 800 samples are ignored

Beam voltage

Data with less than 800 samples are ignored

Sample count

Future work

Understand PFN stability measurementTest station?