1. Parameters of ejected beam
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

2. Which parameters of ELENA beam are important for experiments?
Beam parameters important for users:
Beam energy
Repetition rate
Number of antiprotons in a bunch
Beam size and (to significantly less extent) divergence
Bunch length
Stability of beam position at the end of line
Other parameters can be added on request of experiments...
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

3. Beam energy choice
ELENA extraction energy is chosen 100 keV (13.7 MeV/c) and limited by:
space charge limit for intensity of antiproton beam, N ~ β2γ3
transverse and longitudinal beam emittances blow up due to intra beam scattering (IBS) with growth rate 1/τ ~ exp(const·β-3)
beam emittance blow up due to residual gas scattering Δε ~ 1/γ-2β-3 which impose strong requirements to high vacuum in machine 3·10-12 Torr
high quality of electron beam (ultra low transverse and longitudinal temperature) used for cooling of antiprotons
Possibility to use electrostatic elements for beam transfer from ELENA to experiments -> bunch delivery to different experiments during one turn in machine is possible!
Foil thickness for separation of transfer line and trap vacuum
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

4. Repetition rate AD cycle is about 100 sec now
ELENA cycle is expected about 25 sec, well fitting in AD cycle
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

5. Bunch length
After discussions with experiments the bunch length of 300 nsec (1.3m at 100 keV) is accepted to achieve effective capture of antiprotons in a trap with typical size
One can make bunch shorter (with bigger energy spread in it, which we less concern) by implementing proper programming of RF voltage during bunch compression
By implementing proper programming of RF voltage during bunch compression one can make bunch shorter with bigger energy spread in it
The bigger momentum spread will cause the problems in beam transfer line due to big dispersion resulting a big gap in electrostatic deflector, making this device difficult and expensive
The problems caused by (relatively) big momentum spread can be solved by bunched beam cooling (to be discussed later) yet other problem exists
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

6. Space charge limitation
The shorter bunch is the bigger is incoherent (Laslett) tune shift
Here rp=1.55 ·10-18 m, β and γ are relativistic factors, Nb is a number of particles in a bunch, εx,y is a beam emittances and C is the ring circumference . The coefficients F1 and F2 depend on bunch profiles and equal to 1 for uniform transverse and longitudinal beam distributions, and bigger for other (more realistic) distributions. The beam emittance for the first case corresponds to 100% of beam, and for the second case to 95% of beam.
The bigger tune shift is, the bigger is the area in a betatron diagram populated by particles, the more chance is to loose part of them (see slide 8)
For ELENA ring we put into design conservative estimate of ΔQ=0.1 with possibly achievable ΔQ=0.2
If one wants shorter bunch, the emittances will be bigger => optimal compromise must be chosen
Example 1: ΔQ=0.1, F=2 (Gaussian beam in transverse plane), bunch length lb=1.3 m (300 ns, beam has a uniform distribution over its length) , beam emittances εx= εy =4π mm mrad, C=30.4m, the bunch intensity limit is N=0.75·107.
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

7. Space charge limit in ELENA for realistic beam distribution over the bunch length
Example 2: ΔQ=0.1, F=2 (Gaussian beam in transverse plane), bunch length for 95% of beam is lb=1.3 m (300 ns, beam is bunch with use of one RF harmonic) , beam emittances εx= εy =4π mm mrad, C=30.4m, the bunch intensity limit is N=0.40·107.
Example 3: ΔQ=0.1, F=2 (Gaussian beam in transverse plane), bunch length for 95% of beam lb=1.3 m (300 ns, beam is bunch with use of two RF harmonics, main and doubled) , beam emittances εx= εy =4π mm mrad, C=30.4m, the bunch intensity limit is N=0.61·107.
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

8. Beam distribution over the length at the end of bunch compression
Bunching on harmonic h=4, F2=nmax/nav=1.87
Courtesy by Steve Hancock
Bunching on harmonic h=4+8, F2=nmax/nav=1.22
Courtesy by Steve Hancock
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

9. What may happens if one operates close to space charge limit?
The chosen betatron tunes for ELENA are Qx=2.3 and Qy=1.3 (single particle tunes) . With space charge effect they becomes lower for particles around the beam center, while stay the same for particles at the edge
of beam. One may expect possible tune spread for
ELENA in the range ΔQ=0.1÷0.2, for bigger tune
spread some particles will be lost
How long time beam is in these dangerous conditions:
About 20 msec at the end of bunching
process plus extra 20 msec at extraction plateau
waiting synchronization between RF and extraction
kicker(s), totally about 40 msec
Crossing of 4th order resonances should not be a problem
In any case, the real intensity limitation in ELENA ring due to space charge will be known during commissioning only
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

10. Beam size at focal point
The request from experiments is to keep beam size σx,y≈1mm (95% of beam within in 2σ)
With Dx ≈0 at focal point the transverse beam size depends on beam emittance (defined in the ring) and by beta function value (defined at the end of transfer line)
The minimal value of ejected beam emittance is limited by space charge and IBS value
Bunched beam cooling helps here as well, minimizing ejected beam emittances. Preliminary estimates shows that for basic scenario with 4 bunches extracted and 4.5·106 particles in bunch emittances εx ≈ 6 π mm mrad and εy ≈ 4 π mm mrad are feasible
The beta function values have to fulfil conditions at the focal point βx≤0.16m and βy ≤0.25m to provide 1mm beam size
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

11. Bunched beam cooling in ELENA
Two main reason to implement it in ELENA:
to reduce Δp/p of extracted beam to make it transport to users easier
to reduce emittances blown up by IBS during bunch compression
The speed of bunch compression must be adjusted during commissioning, making it slow enough to use electron cooling in its full potential.
On the other side it should not be too slow, to avoid beam losses due to scattering on residual gas and due to nonlinearities of optics
Initial simulations shows that we need 2÷5 sec for bunched beam cooling to use the full potential of electron cooling. This imposes some lim itation of extraction rate from ELENA in special cases
MD’s in AD: bunch compression time was increased from 0.5 sec to 5 sec, momentum spread reduced from Δp/p =4.1·10-4 down to Δp/p =1.3·10-4 , transverse emittances are the equilibrium values.
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

12. Which beam intensity do we expect in ELENA?
About 4·107 antiprotons routinely captured at injection plateau in AD, about 3·107 are extracted
Assuming 100% efficiency of beam transfer from AD to ELENA and 60% deceleration efficiency in ELENA ring about 1.8·107 can be send to experiments
With space charge limit 0.6·107 antiprotons at least 3 or 4 bunches will be prepared for extraction to avoid space charge problems
If better deceleration efficiency will be achieved, i.e. 80% with intensity 2.4·107 particles, then one can safely extract 4 bunches. In case of possibility to operate ELENA with higher tune shift extraction of 3 bunches might be an option again.
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

13. ELENA main parameters (to be confirmed by TDR)
Momentum range, MeV/c
Energy range, MeV
Circumference, m
30.4
Intensity of injected beam
3 × 107
Intensity of ejected beam
1.8 × 107
Number of extracted bunches
3 or 4
Emittances (h/v) at 100 keV, π·mm·mrad, [95%]
6 / 4
∆p/p before extraction (bunched beam cooling), [95%]
2·10−3
Bunch length at 100 keV, m / ns
1.3 / 300
Required (dynamic) vacuum, Torr
3 × 10−12
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

14. Comments to ELENA main parameters
Some parameters fixed firmly (extracted energy)
Some can be varied during operation (extracted beam intensity, number of bunches 3 or 4)
Some will be defined during commissioning (maximal intensity in one bunch, beam emittances hence beam size at focal point). The reason for that is not well known IBS rates and electron cooling rates
Bunch length might be improved in case we can operate with Laslett tune shift higher than 0.1
Beam intensity might be improved with years due to better deceleration efficiency (AD case)
Extraction on harmonics h=1 or h=2 looks difficult yet possible with some beam manipulations and can be developed later
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii

15. Thanks for your attention!
20 November 2012, ADUC - ELENA Meeting
Pavel Belochitskii