# Presentation: 338839

## Electron cooling of 8 GeV antiprotons at Fermilab’s Recycler:Results and operational implications 

- June 5th, 2006
- L. Prost, Recycler Dpt. personnel

- Fermi National Accelerator Laboratory

## Outline

- Context of electron cooling at FNAL
- Electron cooling
- Electron beam properties
- Cooling of antiprotons
- Cooling force measurements
- Electron cooling in operation
- Conclusion

- Lionel PROST, et al.

## Fermilab complex

- The Fermilab Collider is a Antiproton-Proton Collider operating at 980 GeV

- Lionel PROST, et al.

## Antiprotons and Luminosity

- The strategy for increasing luminosity in the Tevatron is mostly to increase the number of antiprotons
- Provide a third stage of antiproton cooling with the Recycler

- Lionel PROST, et al.

## Antiprotons flow (Recycler only shot)

**Antiprotons ****flow ****(Recycler only shot)**

**Accumulator**

**Recycler**

**Tevatron**

- Transfer from Accumulator to Recycler

- Shot to TeV

- Keep Accumulator stack <100 mA  Increase stacking rate

**2600e9**
**400 e10**
**200 mA**

**100 mA **

- Lionel PROST, et al.

## Beam Cooling in the Recycler

- The missions for cooling systems in the Recycler are:
- The multiple Coulomb scattering (IBS and residual gas) needs to be neutralized
- The emittances of stacked antiprotons need to be reduced between transfers from the Accumulator to the Recycler
- The effects of heating because of the Main Injector ramping (stray magnetic fields) need to be neutralized

- Lionel PROST, et al.

## Performance goal for the long. equilibrium emittance: 54 eV-s

**GOAL**

**MAX**

- Stochastic cooling limit

- 20% lower

**36 bunches****at 2 eVs per bunch**

**36 bunches****at 1.5 eVs per bunch**

- Lionel PROST, et al.

## Recycler Electron Cooling

- The maximum antiproton stack size in the Recycler is limited by
- Stacking rate in the Debuncher-Accumulator at large stacks
- Longitudinal cooling in the Recycler
- Stochastic cooling only
- ~140×1010 for  1.5 eVs bunches (36)
- ~180×1010 for 2eVs bunches (36)

- Longitudinal stochastic cooling has been complemented by Electron cooling

- Lionel PROST, et al.

## How does electron cooling work?

- In the moving frame we have a mixture of gases of “hot” antiprotons and “cold” electrons.
- Transfer of energy through Coulomb interactions

- Lionel PROST, et al.

## How does electron cooling work? (cont’)

- At electron energies up to ~300 keV:
- Direct electrostatic acceleration of electrons with energy recovery.
- A strong longitudinal magnetic field accompanies electrons from the cathode to the exit of the cooling section. Magnetic field assures the transport of the electron beam while retaining low temperature of the electrons.

**Typical parameters of all existing low-energy electron coolers:**
**electron kinetic energy: 2-300 keV**
**electron beam current: up to 5 A**
**Cooler length: 1-3 m**
**Magnetic field: 0.1-0.5 T**

- Lionel PROST, et al.

## What makes the Fermilab system unique? 

- It requires a 4.36 MV DC power supply. We have _chosen_ a commercially available electrostatic accelerator.  As a consequence we had to develop several truly new **beamline, cooling, and solenoid technologies:**
- Interrupted solenoidal field: there is a magnetic field at the gun cathode and in the cooling section, but no field in between.  It is an angular-momentum-dominated transport line;
- Low magnetic field in the cooling section: 50-150 G.  Unlike low-energy coolers, this will result in non-magnetized cooling – something that had never been tested;
- A 20-m long, 100-G solenoid with high field quality

- Lionel PROST, et al.

## Fermilab cooler – main features

- Electrostatic accelerator (Pelletron) working in the energy recovery mode
- DC electron beam
- 100 G longitudinal magnetic field in the cooling section
- Lumped focusing outside the cooling section

- Lionel PROST, et al.

## Pelletron

- (MI-31 building)

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

- Lionel PROST, et al.

## Commissioning Milestones Highlights (2005)

- Feb, 25th – Installation Complete
- Mar, 7th – All systems ready for commissioning
- Charging system, gun, pulser work
- Mar, 17th – 4.3 MeV, 0.5 A pulsed beam to collector (U-Bend mode, low losses)
- Regulation system works properly
- Apr, 20th – First DC beam (few mA) in Recycler beam line
- Jun, 3rd – 4.3 MeV, 0.2 A DC recirculating in the full line.
- Jul, 9th – First observation of electron beam interacting with antiproton beam
- Jul, 15th – Electron Cooling of 8 GeV antiprotons has been demonstrated
- Jul, 16th – Electron cooling is used for a collider shot
- Jul, 26th – 0.5 A DC in the full line. _All commissioning milestones are met_.

- Lionel PROST, et al.

## Beam quality:  Longitudinal temperature

- The cooling process is determined by an effective energy spread consisting primarily of  two components, the electron energy spread at a fixed time and the Pelletron voltage ripple
- The energy spread is determined by IBS (the main contributor) and by density fluctuations at the cathode. According to simulations, at currents 0.1 – 0.5 A the energy spread is 70 – 150 eV.
- The Pelletron voltage ripple is 200 - 300V r.m.s. (probably, fluctuates from day to day). The main frequency is 1.8 Hz, which is much shorter than a cooling time.
- Hence, the effective energy spread is equal to these two effects added in quadratures.

- Lionel PROST, et al.

## Beam quality: Electron angles in the cooling section 

**Angles are added in quadrature*

- Lionel PROST, et al.

## Back one year ago... July 05 electron beam status

- Goal for the rms angular spread (<0.2 mrad) had not been met
- 0.5 A DC beam was not stable
- 200 mA only
- Reliability, reproducibility were still a problem
- BUT...
- 	... (first) cooling attempt was successful !

- Lionel PROST, et al.

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

- Lionel PROST, et al.

## Cooling force – Experimental measurement methods

- Two experimental techniques, both requiring small amount of pbars (1-5 × 1010), coasting (i.e. no RF) with narrow momentum distribution (< 0.2 MeV/c) and small transverse emittances (< 3  mm mrad, 95%, normalized)
- ‘Diffusion’ measurement
- For small deviation cooling force (linear part)
- Reach equilibrium with ecool
- Turn off ecool and measure diffusion rate
- Voltage jump measurement
- For momentum deviation > 2 MeV/c
- Reach equilibrium with ecool
- Instantaneously change electron beam energy
- Follow pbar 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 stashes

- Lionel PROST, et al.

## Example: 500 mA, nominal settings, +2 kV jump (i.e. 3.67 MeV/c momentum offset), on axis 

- Example: 500 mA, nominal settings, +2 kV jump (i.e. 3.67 MeV/c momentum offset), on axis

**~3.7 MeV/c**

**2.8 ****× 10****10**** pbar**
**3-6 ******** mm mrad**

- Traces (from left to right) are taken 0, 2, 5, 18, 96 and 202 minutes after the energy jump.

- Lionel PROST, et al.

## 15 MeV/c per hour

- Extracting the cooling (drag) force

- Evolution of the weighted average and RMS momentum spread of the pbar momentum distribution function

- Lionel PROST, et al.

## Cooling force measurements carried out

- Three types of measurements:
- Various electron energy jumps
- Description of the drag rate as a function of the antiproton momentum deviation
- For various electron beam positions
- Various electron beam positions (w.r.t. antiproton beam)
- Mostly at 100 mA
- Various electron beam current
- On axis (mostly) *i.e. electron beam and antiproton beam are centered*
- ‘By-product’
- Drag rate as a function of the transverse emittance
- Keeping the transverse emittance low throughout the measurement has been sometimes challenging
- Difficulties measuring the ‘real’ emittance at very low p/p and low number of particles

- Lionel PROST, et al.

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

***Error bars ******≡ statistical error from the slope determination***

- Lionel PROST, et al.

## Electron cooling drag rate - Theory

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

- Non-magnetized cooling
- force model

- Lab frame quantities

- Lionel PROST, et al.

## Comparison to a non-magnetized model

_**Constant**_**:**
**	****Coulomb log,   ********** = 10**
_**Fitting parameters**_**:**
**	****Electron beam current density,   *****J******cs***
**	****Lab frame RMS energy spread,   ***********E***
**	****Lab frame RMS angular spread, ***********e***

**100 mA, nominal cooling settings (both data sets)**

- Lionel PROST, et al.

## Comparison to a non-magnetized model (cont’)

- Results from the fits

|  | Electron beam vertical offset, mm |  |  |
| --- | --- | --- | --- |
|  | 0 | 1.5 | 2 |
| JCS, A cm-2 | 1.2 | 0.7 | 0.3 |
| e, mrad | 0.19 | 0.25 | 0.25 |
| E, eV | 370 | 370 | 370 |

- RMS energy ripple, RMS angular spread
- ‘Best’ estimations (250 eV, 160 rad) from measurements
- Beam current density
- Factor of ~5 higher than best estimate (assuming _uniform current density_)

- Lionel PROST, et al.

## Better model for determining the current density in the cooling section ?

- For 100 mA, the beam current density distribution is _NOT_ uniform
- Use SuperSAM gun simulations to estimate on-axis current density
- Electron beam is quite uniform and linear (in phase space) over a limited emitter surface

**This ‘model’ reduces the discrepancy between ‘measured’ and expected current density in CS by a factor of ~2**

- Lionel PROST, et al.

## Electron cooling in operation

- In the present scheme, electron cooling is typically not used for stacks < 200e10
- Allows for periods of electron beam/cooling force studies
- Over 200e10 stored
- Electron cooling used to ‘help’ stochastic cooling 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)

**This procedure is intended to maximize lifetime**

- Lionel PROST, et al.

## Electron cooling in operation (cont’)

- Electron cooling
- prior to extraction

- Stochastic cooling only

- 	Electron cooling between transfers

**Transverse emittance****3 ******** mm mrad/div**
**Momentum spread****1.25 MeV/c /div**
**Longitudinal emittance****50 eVs/div**
**Pbar intensity****75e10/div**

- Lionel PROST, et al.

## 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**

- Lionel PROST, et al.

## 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 pbar bunch in an area of the beam where the angles are low

**5 mm offset**

**2 mm offset**

**Area of good cooling**

- Lionel PROST, et al.

## Issues related to electron cooling and large stacks

- Since started to use the electron beam for cooling, we have dealt with two main problems
- Transverse emittance growth
- During mining
- Lifetime degradation
- When the beam is turned on and/or moved towards the axis (i.e. strong cooling)

**MINING**

- Lionel PROST, et al.

## Emittance growth during mining

- Emittance growth likely due to a quadrupole instability (Burov *et al.* )
- Growth rate  ***xy** I**e** I**p* , (***xy* coupling parameter)
- Increase tune split to reduce ***xy*

**0.414/0.418 (H/V) **
**0.451/0.468 (H/V)**

**(A)**

**(B)**

_**Initial growth rate**_**:**** **
**(A)****  ****36 ******** mm mrad per hour**
**(B)****  ****3 ******** mm mrad per hour**

**/ ~10**

- Lionel PROST, et al.

## Lifetime degradation throughout a store

**Pbar  intensity**

**Lifetime****(1 hour running average)**

**500 hours**

**60 ****× 10****10**
**400 hours**

- Lionel PROST, et al.

## Present Recycler performance with electron cooling

- Lionel PROST, et al.

## Evolution of the number of antiprotons available from the Recycler (~1 year period)

**Mixed mode operation**

**Ecool implementation**

**Recycler only shots**

- Lionel PROST, et al.

## Conclusion (I)

- Fermilab has a unique operational electron cooling system for cooling of 8.9 GeV/c antiprotons
- Since the end of August 2005, electron cooling is being used on (almost) every Tevatron shot
- Increases of stash sizes are a direct consequence of the ability to cool the beam efficiently
- Electron cooling allowed for the latest advances in the TeV peak luminosity
- Emittance growth during the mining process has been almost completely eliminated by changing our operating point (tune space)
- Theoretical model prediction
- More tune space investigations in the near future
- Lifetime degradation is mitigated by a progressive cooling procedure
- Focus of upcoming studies

- Lionel PROST, et al.

## Conclusion (II)

- Cooling force has been measured and compared to a non-magnetized model
- Reasonable agreement with expectations
- Uncertainties in the electron beam properties make this agreement no better than within a factor of 2-3

- Lionel PROST, et al.

## People of Ecool

**Recycler department head:**
- Paul Derwent
**Recycler deputy department head:**
- Cons Gattuso*
**Ecool Safety officer:**
- Mike Gerardi
**Recycler department personnel:**
- Valeri Balbekov
- Dan Broemmelsiek*
- Alexey Burov†
- Kermit Carlson‡
- Jim Crisp
- Martin Hu*
- Dave Neuffer
- Bill Ng
- Lionel Prost*
- Stan Pruss*

**Recycler department personnel (cont’):**
- Sasha Shemyakin (GL)*
- Mary Sutherland*
- Arden Warner*
- Meiqin Xio
**Other AD departments:**
- Brian Chase
- Paul Joireman
- Ron Kellett
- Brian Kramper
- Valeri Lebedev
- Mike McGee
- Sergei Nagaitsev
- Jerry Nelson
- Greg Saewert
- Chuck Schmidt
- Alexei Semenov
- Sergey Seletskiy
- Jeff Simmons
- Karl Williams

- * Main experimentalists (experimental studies, data analyses,...); † Primary ecool  theorist (theoretical analyses); ‡ Primary technical support (tech support coordination,...)

- Lionel PROST, et al.

## EXTRAS

- Lionel PROST, et al.

## Setup of Fermilab’s Electron Cooler

- Lionel PROST, et al.

## Electron beam parameters (for cooling)

- Electron kinetic energy 			4.34 MeV
- Uncertainty in electron beam energy 	 0.3 %
- Energy ripple				250 V rms
- Beam current		 		0.1 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

- Lionel PROST, et al.

## Simplified electrical schematic of the electron beam recirculation system

- Beam power	                      2.15  MW
- Current loss power	         21.5 W
- Power dissipated in collector 1.6  kW

- For I= 0.5 A, I= 5 A:

- The beam power of 2 MW requires the energy recovery (recirculation) scheme

- Lionel PROST, et al.

## Electrostatic generator: the Pelletron (developed by NEC)

- ‘Improved’ Van de Graaff generator
- Charge carried by a chain (metal cylinders joined by nylon links) instead of a rubber belt
- Induction system to charge the chain (instead of rubbing contacts or corona discharges)

- Lionel PROST, et al.

## Preview of what’s inside the pressure vessel

**High-voltage column with grading hoops partially removed to show the accelerating tube (right) and the charging chains (far center).**

- Lionel PROST, et al.

## Diagnostics

- YAG crystal, OTR monitors throughout the beam line
- Beam size (shape), distribution
- Used to compare to optics models
- 1 multi-wire scanner
- Beam size and shape after 180° bend
- Removable apertures in the cooling section
- Between each of the ten cooling section solenoid
- Beam size and angle
- BPMs
- Between each of the ten cooling section solenoid + 16 in other beam lines (accel, supply, return, transfer, decel)
- Can measure both pulsed and DC beam
- Capable of monitoring both electrons AND pbars

- Lionel PROST, et al.

## OTR Detectors for the Medium Energy Electron Cooler 

- Detector characteristics
- 5 μm foil
- Lower current limit 20mA
- Resolution 50 μm
- Applications
- Real-time charge density distribution and beam size measurements
- Measurement of beam initial conditions in the acceleration section
- Beam ellipticity measurements
- Beam temperature measurements with pepper-pot

- Beam Image from OTR at full current (acceleration tube exit)

- Beam profile versus Lens current on acceleration side

- Lionel PROST, et al.

## Neighborhood with the Main Injector

- Magnetic fields of busses and MI magnets in the time of ramping causes an extensive motion of the electron beam (up to 0.2 mm in the cooling section and up to 2 mm in the return line)
- MI radiation losses sometimes result in false trips of the ECool protection system

- Electron beam motion and MI losses at R04 location in the time of MI ramping. 0.55 Hz oscillation is due to 250 V (rms) energy ripple.

- 2 sec

**MI bus current**

**MI loss**

**X**

**Y**

**1mm**

- Lionel PROST, et al.

## Cooling is not magnetized

The role of the magnetic field in the cooling section is to preserve low electron angles, 

- Cooling is not magnetized
- The role of the magnetic field in the cooling section is to preserve low electron angles,

- Low magnetic field in the cooling section

- Transverse magnetic field map after compensation (*B**z* = 105 G).

- Simulated angle of an 4.34 MeV electron in this field. RMS angle is < 40 rad.

- Lionel PROST, et al.

## Beam size measurements in the Cooling Section

- 11 movable orifices (not in phase) in the cooling section

- The scrapers are diaphragms of 15 mm diameter, located every 2 m.
- While only one of them is in place, the beam is shifted in some direction until it touches the scraper. The bpm data for the beam center             is taken at this point.
- The beam is shifted in other direction, and the center coordinates at touch are detected again; usually 8 directions are used. Then, the entire procedure is repeated for other scrapers.
- From these data, the beam ellipse and the scraper offsets are found for every scraper involved.
- Initial conditions for the beam envelope are fitted for these ellipses.
- A cylindrical boundary might not guarantee low angles in the middle of the beam because of aberrations

- radial angle

- tangential angle

- density

- Lionel PROST, et al.

## Scraper Measurements Dec 1 (nominal settings, 500 mA)

**SCC00**

**SCC70**

**SCC60**

**SCC50**

**SCC40**

**SCC30**

**SCC20**

**SCC10**

**SCQ01**

**SCC90**

**SCC80**

**Beam radius ~ 4.5 mm**
**Averaged rms angle <0.2 mrad**

- Lionel PROST, et al.

## Comparison of two focusing settings 

- Envelope (fit) along the scrapers 0-5

**One lens changed by 2 A**
**Average rms envelope angle  is 0.5 mrad**

**Nominal**
**Average rms envelope angle  is 0.2 mrad**

- Lionel PROST, et al.

## Why sudden new interest in high energy cooling?

- The existing stochastic cooling technology is band-width limited (10 GHz or so).
- The lack of progress in bunched-beam stochastic cooling
- The advance in electron gun and collector technology (experience of low energy e-cooling), and in recirculation of DC beams.
- The advance in recirculating linac technologies.
- The advance in linear optics on beams with a large angular momentum.

- Lionel PROST, et al.

## Simulation of cooling demonstration

**Without cooling -- the momentum distribution remains flat over 0.3% span for 30 minutes**
**Coasting beam, IBS+ECOOL simulation, εn = 2 mm mrad, Ie=0.1 A, rms angular spread = 0.5 mrad**

- Lionel PROST, et al.

## Recycler measured momentum distribution using Schottky

**1.5e11 pbars, ε****n**** = 2 mm mrad**
**Momentum acceptance (flat central part): about 0.5% (+/- 22 MeV/c)**

- Lionel PROST, et al.

## Drag rate as a function of the electron beam current3.67 MeV/c momentum deviation, on axis, nominal cooling settings

- Not a real fit

**Drag force on axis appears to be independent of the electron beam current**
********* ******Quite consistent with equilibrium longitudinal emittance measurements***

- The drag force is nearly constant at 0.1 – 0.5 A, while in simulations the current density at the axis is twice higher at 0.5 A than at 0.1 A.

- Lionel PROST, et al.

## Drag rate as a function of the transverse emittance1.84 and 3.67 MeV/c momentum offsets, 100 - 500 mA e-beam, on axis

**Scattered in the data likely dominated by the difficulties in getting similar machine conditions**

- Lionel PROST, et al.

## Emittance growth during mining

**e-beam: 500 mA, +3.5 mm offset**
**pbars: 180e10**

**e-beam: 500 mA, +3 mm offset**
**pbars: 180e10 **

**Stochastic cooling system was turned off when mining, e-beam (when used) remained on**

- Dampers are on for all measurements

**pbars: 114e10 **

**Initial rate:****17 ******** mm mrad/hour**

***Instrumentation problem***

- Lionel PROST, et al.

## Emittance growth during mining reduced by ~10

- Changed working point for the tunes (in order to split them more), from 0.414/0.418 (H/V) to 0.453/0.473 (H/V)

**Electron beam current**
**Horizontal emittance**
**Electron beam position**
**Longitudinal emittance (circle)**
**Vertical emittance (circle)**
**Pbar intensity****(circle)**

**Stochastic cooling system is turned off before mining**

**~2 ******** mm mrad/hour**

**Phase density when mining = 0.9**

**Mining**

**227e10**

**100 mA**

- Lionel PROST, et al.

## Correlation with presence of electron beam & cooling ?

**40 minutes of electron cooling on axis at 300 mA**

**Lifetime drops and recovers ~1 hour later**

**But phase density has increased by ~2 !**

**Pbar intensity****(1 × 10****10****/div)**
**Longitudinal emittance****(20 eV s/div)**
**Vertical emittance****(2 ******** mm mrad/div)**
**Electron beam current****(0.1 A/div)**
**Lifetime****(1 hour running average) [circle]**
**(500 h/div)**

**3.8 ******** mm mrad**

**68 eV s**

**158 × 10****10**

**0 h**

**2000 h**

- Lionel PROST, et al.

## Comparison with cooling force measured at low-energy coolers

- Comparison with data for normalized longitudinal cooling force measured at low energy coolers adapted from
- I.N. Meshkov, Phys. Part. Nucl., 25 (6), p. 631 (1994).

- Red triangles represent Fermilab’s data measured at 0.1 A. The current density is estimated in the model with secondary electrons.

- Lionel PROST, et al.