# Presentation: 265065

## Beam Instrumentation Development of an Abort Gap Monitor for the LHC

- J.-F. Beche, J. Byrd, _S. De Santis_, P. Denes,
- M. Placidi, A. Ratti, W. Turner, M. Zolotorev (LBNL)
*M. Facchini (CERN), R. Thurman-Keup (FNAL)*

***bnl - fnal- lbnl - slac***

_**US LHC Accelerator Research Program**_

- LARP Meeting - October 19th-21st, 2004

## What is the Abort Gap and Reasons for Monitoring It

-  3.3 s gap in the machine fill, corresponding to the raise time of the abort kicker.
-  Gap can populate by:
-  Injection errors (450 GeV, fast mechanism)
-  Diffusion (mainly 7 TeV, slow mechanism)
-  Debunching (mainly 7 TeV, slow mechanism)

- This may cause:
-  Quenching of SC magnets (Q4 after septum in IP6)
*also:*
* “**Radiation flashes” during ramp (unbunched beam)*

## LHC Specifications(C. Fischer, LHC-B-ES-0005)

-  Maximum allowable charge density:
- 6•106 p/100 ns @ 7 TeV
- 4•109 p/100 ns @ 450 GeV
-  Integration time:
- 100 ms (~1100 turns)

## Techniques for the AGM

## MCP-PMT for the AGM 

- Gate min. raise time: 1 ns
- <2.5 ns RF bucket spacing; _can gate out filled buckets_
- <<100 ns resolution
- Gate voltage: 10 V
- Low voltage switching required
- Max gain: 106
- < 10 dark counts/sec
- High S/N
- Max duty cycle: 1%
- Max gate length: 10 s

## Synchrotron light source for longitudinal diagnostics

-  5T SC undulator in IR4 (low energy)
-  Exit edge of D3 magnet (high energy)

***Extraction mirror***

***Available photon flux***

- (M.Facchini)

## MCP-PMT has the required sensitivity

- Conservative estimates (1% QE, 10% BW, etc.) point out that a MCP-PMT based AGM can easily detect the required charge densities at both injection and collision energy.

## Sensitivity Bench Tests

- Light

- 10 V Gate
- 100 ns

- HP8114A
- Pulser

- MCP
- PMT

- Hamamatsu C3360
- HV

- -3 kV

- LeCroy
- LC564A

**BP filter + diaphragm**

- Newport 1835C/818-UV
- optical power meter

- Light intensity is adjusted to simulate
- photon flux in the LHC

## Sensitivity Bench Tests Results

- Photon flux (140 ph/turn)
- equal to what expected in the LHC at 7 TeV with a 5% bandwidth at 500 nm.
- Flux at 450 GeV is 4 to 5 times lower.
- However: MCP-PMT gain can still be increased and bandwidth would be larger.

## AGM Beam Tests

- An MCP-PMT has been tested at the ALS (*dynamic range**, **photocatode saturation**, **noise properties*) and on the Tevatron (*unbunched beam*).
- The MCP-PMT will also be tested as a possible device for accelerator physics applications (bunch length, tails, etc.).

## Tests at the ALS

- Bunch spacing 2 ns

- Bunch length ~30 ps

- “Camshaft” pulse

**(LHC parameters)**

- 328 RF buckets
- 276+1 filled

- (2808/35640)

- (280-620 ps)

- (2.5 ns)

- (no camshaft)

## Experimental setup

- SROC

- Hamamatsu
- Streak Trigger Unit

- HP8114A
- Pulser

- MCP
- PMT

- Synchrotron Light

- Stanford DG535
- Delay

- 1.5 MHz

- ~100 kHz

- 10 V Gate

- Hamamatsu C3360
- HV

- -3 kV

- Tektronix
- TDS754D

## First experimental data

- Gate signal on

## ...and more recent data

- No need to average data, at least at the ALS.
- Future experiments at the ALS will carefully evaluate the available photon flux and simulate the LHC parameters, if possible.

## Some more interesting data

- Compare zero level with gate on and off

- Photocatode recovery is not an issue

## Tests at the Tevatron

- Bunch spacing 396 ns (21 buckets)

- Bunch length ~1600 ps

**Circumference: 21 ********s**

- 1113 RF buckets
- 12x3 filled

**~2.8 ********s gap**

- RF bucket spacing ~19 ns

- 139 buckets

## Tests on the Tevatron

- - PMT is gated on for a period where we want to count photons
- - LeCroy scope tallies times of arrival of individual photons
- - Tevatron Electron Lens (TEL) produces gap in longitudinal distribution
- - Microbunches are visible in end of abort gap
- Level of microbunches is ~107 particles / rf bucket
- - No structure visible in front half of abort gap
- Pbars are injected in front half, so kicker cleans that part

## PMT Photon Counting Setup

- Gate Timing

- Beam RF clock and revolution marker

- HP Pulser

- PMT

- LeCroy Scope

- PMT Amplifier (x100)

- Tunnel

## Single photon counting at the Tevatron 

- Single photon counting at the Tevatron

- Bunch 36 is 2 s to the left of this plot

- bins (2 ns)

- Arbitrary units

- • 1 s gate
- • average of 3 gaps
- • counts  	       II accumulated over I  I  1000 turns

- Microbunches are clearly visible

**+ centre of RF buckets**

## Timing of the AGM

- One 100 ns long sample every turn (1% duty cycle)
- 33 samples to map entire abort gap.
- Abort gap mapped 33 times in the 100 ms integration time.
- “Interleaving” samples necessary for detecting charges falling across two samples.
- Alternatively, use 825 ns long samples (max window allowing 1 sample per turn).
- Entire gap is now mapped in 4 turns.
- 275 averages in 100 ms
- This maximizes S/N, but needs more complex data acquisition (multiple integrators)

## AGM Summary

- -    Can be easily switched at the required speed
- -   S/N ratio seems more than adequate
- Fast photocatode recovery (~100’s ps)
- Tested at the ALS and the Tevatron
- Can monitor the accumulation of charges in the gap, rather than just detecting when threshold level is reached
- Can also be used to monitor the unbunched beam around the ring
- Can monitor bunch tails and ghost bunches at a reduced resolution
- -   Specifications for data acquisition hardware

- MCP-PMT is a viable technique

## Fundamental limit to resolution

- Impulse Response Function for gateable PMT’s is on the order of 100 ps. The MCP-PMT could be conceivably used for AP measurements.
- The 50 ps time resolution required, could be perhaps achieved by inverse filtering

**Why not use a MCP-PMT for accelerator physics studies too ?**

**~100**

**afterpulses**

## Diagnostics for measuring longitudinal parameters of the LHC beams

**- Abort Gap Monitor**

**- Debunched Beam**
**- Bunch Tails**
**- Ghost Bunches**

** ****Bunch Core**
**   ****(“real time”)**

## Radiation Flashes

- Caused by untrapped particles lost on the aperture during ramp.
- The momentum cleaning system (collimators in IP7) takes care of most of it. Additionally, beam loss is not so concentrated as in a beam dump.
- Maximum allowable charge density between bunches: 2•105 p/ps. Much higher than in the abort gap, but needs monitoring all around the ring.

- An MCP-PMT based instrument can map entire ring in 100 turns