# Presentation: 419805

## Electron cooling at the RecyclerStatus report

- Lionel Prost
- DOE Tevatron Operations Review
- March 21, 2006

## Electron cooling status - Summary

- The electron cooling system installation was finished on February 25th, 2005, at which date commissioning began.
- Cooling of 8 GeV antiprotons with the electron beam was first demonstrated on July 15th, 2005 and was reported at the COOL’05 workshop held in Galena, IL.
- Electron cooling was used for almost all collider shots since
- On October 5th, 2005, Recycler-only shots were implemented.
- Availability and reliability of the electron cooling system is important for this mode of operation to be profitable
- It effectively passed the burden of cooling from the Accumulator onto the Recycler
- Fermilab now has a world-record _operational_ electron cooling system
- 4.3 MeV, up to 1.5 A DC (*U-Bend mode*)
- Beam power: 6.5 MW (New world record)

## Electron cooling system setup at MI-30/31

**Added section **

- (includes additional pumping)

**Beam line accommodates both U-bend and vertical bend magnets**

**Fast acting valves **

**Magnetically shielded to protect e beam from fields imposed by the MI bus**

## Pelletron

- (MI-31 building)

- Cooling section solenoids
- (MI-30 straight section)

## All design parameters have been met 

- Electron kinetic energy 			4.34 MeV
- Uncertainty in electron beam energy 	 0.3 %
- Energy ripple				500 V rms
- Beam current		 		0.5 A DC
- Duty factor (averaged over 8 h)		95 %
- Electron angles in the cooling section
- (averaged over time, beam cross section, and cooling section length),  rms 		0.2 mrad

- _All design parameters have been met _

## Running at high current (> 0.2 A) induces full discharges (~1-2 per week) until the Pelletron needs to be reconditioned.

- 24 hours

- No full discharges
- 5 recirculation interruptions

**2 nTorr**
**2 nTorr**
**0.4 MV**
**0.2 A**

**Beam current**

**Decel. side pressure**

**Accel. side pressure**

**Pelletron voltage**

## Electron angles in the cooling section 

**Angles are added in quadrature*

- Part of shutdown activities

## Electron beam status

- The electron beam stability and duty factor are adequate for providing cooling when needed
- Days of operation without trips for beam current <200 mA
- ‘Natural’ interruptions are taken care of automatically (Java application)
- Duty factor >95% for beam current up to 500 mA
- Conditioning of the Pelletron takes a shift (8 h) and can be done when the Recycler has a low stack
- Less than once per month at low current (i.e. < ~200 mA)
- Once per 2-4 weeks at high current (i.e. > ~ 200 mA)
- Actually depends on number of full discharges
- Beam angles are low enough
- Confirmed by the fact that we successfully cool antiprotons !

## First e-cooling demonstration – 07/15/05

## Run II project milestones

- 						Plan		Actual
- Commissioning begins		02/01/05	03/01/05
- U-bend commissioned		03/14/05	04/15/05
- Full beam line commissioned	04/04/05	05/04/05
- A 0.5-A DC beam		           	07/08/05	07/26/05
- Cooling of antiprotons 		09/08/05	07/15/05
- Electron cooling operational	01/02/06	_10/03/05_
*	**(contribute to HEP)*

## Electron cooling in operation

- In the present scheme, electron cooling is typically not used for stacks < 200e10
- Over 200e10 stored
- Electron cooling along with stochastic cooling is used to maintain a certain longitudinal emittance (i.e. low cooling from electron beam) between transfers or shot to the TeV
- ~1 hour before setup for incoming transfer or shot to the TeV, electron beam adjusted to provide strong cooling (progressively)
- The electron beam current is set to 100 mA
- Adequate for present number of stored antiprotons
- Longitudinal emittance of the bunch before mining and extraction is typically < 70 eVs
- Some indirect interaction may exist between electron cooling and stochastic cooling

- _This procedure is intended to maximize lifetime_

## Adjusting the cooling rate

- Two ‘knobs”
- Electron beam current
- Beam stays on axis
- Dynamics of the gun varies between low and high currents
- Hence, changing the beam current also changes the beam size and envelope in the cooling section
- Electron beam position
- ‘Adjustments’ are obtained by bringing the antiprotons bunch in an area of the beam where the angles are low

**5 mm offset**

**2 mm offset**

**Area of good cooling**

*Schematic transverse profiles:*
*Pink**: pbars*
*Blue**: e-beam*

## Electron cooling between transfers/extraction

- Electron beam is moved ‘in’

- Stochastic cooling
- after injection

- 	Electron beam ‘out’ (5 mm offset)

**Electron beam current****0.1 A/div**
**Transverse emittance****1.5 ******** mm mrad/div**
**Electron beam position****1 mm/div**
**Longitudinal emittance (circle)****25 eVs/div**
**Pbar intensity****(circle)****16e10/div**

**~1 hour**

**100 mA**

**195e10**

**~60 eVs**

## Electron cooling drag rate - Theory

- For an antiproton beam with zero transverse velocity, electron beam: 500 mA, 3.5-mm radius, 300 eV rms energy spread and 200 μrad rms angular spread

- Non-magnetized cooling
- force model

**Linear approx.**

## Cooling force – Experimental measurements

- Two experimental techniques, both requiring small amount of antiprotons, coasting (i.e. no RF) with narrow momentum distribution and small transverse emittances
- ‘Diffusion’ measurements
- For small deviation cooling force (linear part)
- Reach equilibrium with electron cooling
- Turn off electron cooling and measure diffusion rate
- Adding external source of noise (constant) can lead to the determination of the entire curve
- Equilibrium distribution gives the ‘shape’ of the curve
- Diffusion rate (with the added noise) gives the amplitude
- Electron energy jump method
- Reach equilibrium with electron cooling
- Instantaneously change electron beam energy
- Follow antiprotons momentum distribution evolution
- Both methods characterize the effectiveness of electron cooling (hence, the electron beam quality) quite locally and not necessarily the cooling efficiency/rate for large stacks

## Drag Force as a function of the antiprotons momentum deviation100 mA, on axis, nominal cooling settings

## Improvements/upgrades to be implemented during the shutdown

- The control system of the bending magnets power supplies has posed various reliability problems
- It has been entirely replaced by an in-house design using an Internet Rack Monitor (IRM)
- New system was tested on the bench and showed excellent stability
- New system was also tested on two (out of 10) magnets before the start of the shutdown
- Installation of a ground bypass around the cooling section
- Uncompensated bus currents from MI ramping induce currents to the beam pipe, which in turn induce an erroneous reading of the bend fields by the NMR probes
- Noisy field readings
- Connect low resistance cable to the Recycler beam pipe
- Should reduce induced currents by a factor of ~10 (from ~100 mA to ~10 mA)
- Improve bending fields regulation

## Improvements/upgrades to be implemented during the shutdown (cont’)

- Shielding for cameras in the MI tunnel
- CCD cameras to be used with Optical Transition Radiation (OTR) detectors could not survive more than a few hours the level of radiation coming from MI losses
- Affects several diagnostics stations at the end of the cooling section and the return line
- Cameras will now be installed away from the MI beam line and shielded from direct radiation
- Upgrade of the SF6 gas recirculation system
- Present system is undersized for operational conditions
- Partly because of the larger volume
- New specifically designed all-in-one skid
- Operation at a lower gas temperature
- Reduction of down-time for maintenance and service
- Improved diagnostics to monitor the system

## Conclusion

- Although commissioning of the electron cooler started 1 month behind schedule, cooling demonstration of 8 GeV antiprotons was achieved ~2 months ahead of schedule
- Fermilab now has a world-record operational electron cooling system
- Since the end of August 2005, electron cooling is being used on (almost) every Tevatron shot
- Recycler not only stores antiprotons but cools them to the required emittances
- Increases of stack sizes are a direct consequence of the ability to cool the beam efficiently (current record 437e10)
- Electron cooling allowed for the latest advances in the TeV peak luminosity (current record 172e30 cm-2 s-1)