SLAC Timing System Overview

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SLAC Timing System Overview

• Mike Stanek
• SLAC Accelerator Dept.
• Operations Group
• 16-Sep-2003

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SLAC Aerial View
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SLAC Timing System Specifications

• SLC Design
• (1982)
• Resolution lt10 nSec
• - Jitter lt1 nSec
• - RF distribution

PEP II (1998) - Resolution 2.1 nSec -
Jitter 20 pSec
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Synchronization Requirements

• AC Line Voltage
• (2 zero crossings x 3 phase 60 hz 360 hz)
• Damping Ring RF bucket
• (8.5 MHz revolution harmonic)
• Linac Main Drive Line
• (476 MHz)
• PEP RF buckets
• (3492 buckets, 136 KHz rev. harmonic)

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Devices

• Klystron Modulator thyratrons
• Pulsed kicker magnets
• Beam diagnostics (BPM, Gated ADCs, Toroids)
• For devices requiring lt 10 nsec precision,
  specialized hardware has been developed
• TGAS (gun triggers)
• Vernier Delay Unit 100 psec (Damping Ring
  kickers)

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Fiducials (Master Clock pulses)

• Linac at 360 Hz
• one double amplitude 476 MHz cycle inserted on
  Main Drive Line RF distribution
• PEP-II
• 360 Hz injection fiducial synced to Linac
• 136 KHz ring fiducial from PEP Master
  Oscillator (also locked to Linac M.O.)
• One (two) double period cycle

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Waveforms 476 MHz with FIDUCIALS
LINAC
PEP - 2
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Trigger Generation, simplified
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Sector 0 Trigger Generation, oversimplified
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Trigger Generation, simplified
1
5
2
4
3
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Linac Sector Timing

• 476 MHz fiducial coupled from MDL
• Divide-by-four (? 119 MHz)
• 8.4 nsec ticks (SLC specs)
• Convert double amplitude fiducial to missing
  cycle of 119 MHz.
• FIDucial Output chassis (FIDO)

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LINAC Timing Typical Sector
Divide by 4 ? 8.4 nsec ticks
CAMAC
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LINAC Rack, Front
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LINAC Rack, Rear
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MDL Coupled Output
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Waveforms 119 MHz and NIM Pulse
FIDO Output
PDU Output
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NLC Test Accelerator

• Uses Linac Fiducials
• Distributed to NLCTA via fiber optic link, from
  the downstream end of the Linac to the Research
  Yard.

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Fiber Optic Xmtr
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Fiber Optic Link
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NLCTA Triggers
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PEP-II fiducial generation

• PEP-II uses two types of fiducials, generated in
  Region 8
• 360 Hz, synced to Linac fiducials for injection
• 136 KHz, synced to PEP Master Oscillator for
  stored beam diagnostics and control
• PEP Master Oscillator is (usually) locked to
  Linac Master Oscillator.

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PEP II Timing Generation
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PEP-II timing distribution

• PEP has 2 RF distribution cables
• One WITH the fiducial superimposed
• One WITHOUT fiducial (used by the RF systems)
• In each PEP IR hall, the Timing distribution
  signal is compared and phase locked to the RF
  only signal.

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PEP 2 Timing Distribution
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Timing Phase Lock Distribution B.D.
Graphic by E.L.Cisneros
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TPLD SCP Diagnostics 2
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Instructions for each Fiducial

• Master Pattern Generator (microprocessor)
• uses SLCnet cable to send 126 bits (x3) every
  fiducial pulse to remote micros
• PNET band of SLCnet
• Pipelined bits for the next 3 fiducials
• MSB a.k.a. Beamcode or PP (pulsed pattern)

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• Each bit has designated purpose, e.g.
• BPM data acquisition for specific application
• No beam (gun trigger suppress)
• Use this pulse for e- feedback data
• HER injection
• Fire the HER Injection tune-up dump kicker

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MPG multi-tasking

• Linac Klystron maximum rate is 120 Hz (only uses
  1/3 of available fiducials)
• Other 240 fiducials can be labeled for other
  programs (NLCTA, Gun Test lab)

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MPG bit patterns determined by inputs from

• Machine Protection Systems
• Algorithm processor micros
• Direct inputs from older hardware
• BaBar stop injection
• Operator requests
• PEP injection system (BIC)
• which bucket to fill, and how much charge
• shared memory

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What happens at a remote micro?

• Interrupt the micro
• Translate 126 bit word ? 8 bit word
• Broadcast to CAMAC crates
• PDU (Programmable Delay Unit)
• Combines Analog (119 MHz fiducial) with Digital
  (beamcode info)
• 16 programmable countdown channels
• Generate trigger on CAMAC upper backplane

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PDU flexibility

• Each channel generates output trigger
• Adjustable over 2.7 msec range (8.4 nsec steps)
• Or using (n-1) or (n-2) fiducial with pipeline
  info, that range can be shifted up to 5.4 msec
  early
• On specific combinations of PNET bits
• On demand for BPM data acquisition
• (YY mode)
• On every fiducial, independent of PNET bits
• On a subset of fiducials (multiples of 10 hz)

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PDU SLC Type
Sixteen channels per PDU One channel shown
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LINAC Timed Crate
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PEP micro CAMAC

• PEP PDU
• Direct 476 MHz fiducial input (2.1 nsec ticks)
• Programmable for
• Injection mode or Ring mode (136 KHz)
• HER or LER
• Continuous or Fixed length Pulse Train

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• The combination of Micro code
• (both MPG and remote micros)
• and PDU (and VDU) modules
• flexibility
• in generating triggers.

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An important feature Synchronous data collection

• BPM data can be acquired for a single e- pulse as
  it travels through the accelerator.
• Correlation of Buffered BPM data (labeled with
  Pulse ID) can be used to make difficult
  measurements and diagnose accelerator
  instabilities.
• Feedback systems can be Cascaded, to prevent
  overcorrection of errant trajectories.

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PEP RF bucket synchronization

• Generate a specially timed Linac Fiducial for
  each PEP bucket (3492)
• i.e. shift the entire Linac timing
• Beam to be injected is already stored in the
  Damping Ring (8.3-16.6 msec store time)
• Delaying the fiducial by n Damping Ring turns,
  we can hit 25 of the PEP buckets

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How to figure out n ?

• TPEP3492 b476 TDR56 b476 (4997)b476
  (427)b476
• TPEP(873/14)TDR
• 14 TPEP 873 TDR
• By changing the number of stored DR turns we
  inject into 873 different PEP buckets.
• (1/4 of total 3492)

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• If the desired bucket number (D) is known, MPG
  can calculate number of DR turns (R) to wait.
• R (D686) mod 873

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How is the shifted fiducial created?

• PEP Trigger Generator (PTG) module in an injector
  CAMAC crate
• Input bits from MPG (over PNET) tell it how many
  DR turns to delay.
• Line locked gate from SLC MTG module (873 TDR
  wide)
• 476 MHz input (-gt generates PEP bucket clock,
  period 873 TDR)
• Outputs fiducial pulse to SLC MTG for
  distribution on MDL.

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What about the other ¾ of PEP buckets?

• While beam is stored in DR, shift Linac Master
  oscillator -1, or 1, or 2 buckets of 476 MHz.
• 720 degree (476 MHz) Pulsed phase shifter in
  series with the Linac Master Oscillator.
  Programmed to shift phase based on special PNET
  bits from MPG.
• DR extracted beam is locked to Linac RF ramp
  phase slow enough to keep the beam stable

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Check to see if it worked

• Compare Linac fiducial with a PEP bucket 0
  fiducial. (both signals are in MCC)
• Difference should be predicted by MPG.
• Time Difference Counter (TDC) circuit in injector
  PTG module, and in MPG crate.
• MPG synchronization feedback.
• MPG can correct for changes gt 1 PEP bucket.
• Can disable injection to PEP if not stable.

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Some Common Timing Problems

• RF signal degradation and noise
• Poor cable connections
• Sensitivity to temperature extremes
• Bucket jumps in PEP timing system
• One leg of the TDC comparison shifts wrt the
  other.
• Often the beam has not shifted just the
  measurement, but feedback still applies a
  correction.

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References

• Thanks to Duane Thompson (retired) of ESD
• (I borrowed many of his Power Point
  slides)
• SLAC-PUB-3508 A New Timing System for the
  Stanford Linear Collider
•  
• SLAC-PUB-3476 The Design of a Semi-custom
  Integrated Circuit for the
• SLAC SLC Timing Control System
•  
• PEP-II Injection Timing and Controls
•  
• SLAC-PUB-4231 Timing Stabilization for the SLC
  Electron Source
•  
• SLAC-PUB-4906 Timing and RF Synchronization for
  Filling PEP/SPEAR with The SLC Damping Rings
•  
• IEEE Trns A Programmable Delay Unit
  Incorporating a Semi- Nucl. Sci. Custom
  Integrated (Circuit PDU Write-up)
• NS-34, No. 5, 2112 (1985)

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• SLAC-CN-144 Pulse-to-Pulse Control of the LINAC
  with the new Controls System 
• NSS 1984 A Vernier Delay Unit, W.B. Pierce
•  
• BD-135-730-NN SLC Timing System (update in
  progress, Oct 2001)
•  
• SLACSpeak http//www.slac.stanford.edu/spires/slac
  speak/
•  
• Principles Of OPeration
• http//www.slac.stanford.edu/grp/cd/soft/wwwman/p
  oop.html
• Basic Users Guide
• http//www.slac.stanford.edu/grp/cd/soft/wwwman/b
  ug.www/
• SLC Hardware Manual
• http//www.slac.stanford.edu/grp/cd/soft/wwwman/ha
  rd.www/