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Traps for antiprotons, electrons and positrons in the 5 T and 1 T magnetic fields
G. Testera & Genoa group
AEGIS main magnetic field (on axis) : from Alexei
Traps for antiprotons, electrons and positrons in the 5 T and 1 T magnetic fields
Penning type electric field
2 ztrap: about 6 rtrap
rtrap
z
Half length
of the harmonic region:
about 1.6 rtrap
ztrap
z
Malmberg type electric field:
2 ztrap>>rtrap
The antiproton catching trap used in ATHENA:
r=1.25 cm
Length 35 cm
Catching and cooling antiprotons
a) Degrading
5 108 electrons
preloaded in the catching trap
Solenoid - B = 5 Tesla
e-
t=0
Antiprotons
antiprotons
Degrader
Cold electron cloud
[cooled by Synchtrotron Radiation, t ~ 0.15s]
99.9% lost
b) Reflecting
0.1%
E<10kV
t=200 ns
c) Trapping
t=several tens s
c) Cooling
[through Coulomb interaction]
The dimensions of the AEGIS antiproton catching trap
Length of the antiproton catching trap
Antiproton axial energy max
Velocity v
AD bunch length t
L (length of the 10KeV potential well )
Real trap Length
: 10 KeV
: 1.4 106 m/s
: 200 ns (up to about 500 ns)
: L=0.5 v t= 35 cm
: 40 cm
Trap radius :
Antiproton cyclotron radius
= 2.8 mm (10 KeV)
beam radial size
= ? mm
Beam centering
Note: rtrap>> cloud radial size
rtrap
=1.5 cm (max 2 cm)
Positron injection???
y
x
Electrons in the antiproton catching trap
rp
Cold cloud at equilibrium
a=zp/rp
zp
f (a )
a
r
40 1
 V0
3q N (1.6rtrap ) 2
3
p
Radial size of the plasma
Trap size
Potential well
Number of particles
N=108
Rtrap=2 cm
N =5 108
Rtrap=2 cm
rp= 1mm
Zp=1.77 cm
V0=200 V
rp = 2 mm
Zp = 3 cm
V0 =200 V
change rp with RW
change zp with V0
The position of the trap in the 5 T magnet
The region with uniform magnetic field
Bz  (5.0822)
 5.0822
The position of the trap in the 5 T magnet
The region with uniform magnetic field
eMovable degrader
Bz  (5.0822)
 5.0822
10KV
A movable degrader with few different foils:
in situ optimization of the catching efficiency
ATHENA results
FWHM=10mm
Additional Al thickness mm
Degrading materials in ATHENA:
67 mm silicon beam monitor
25 mm Stainless steel
130 mm Al foil (last degrader)
Summary of the parameters of the antiproton catching trap
Electrode voltages : 10 KV on HV1, HV2
+ - 200 V on the others electrode (DAC+ amplifiers)
HV2 connected also to +-200 V
HV1, HV2 L=2.5 cm
7 electrodes for Penning trap
11 additional electrodes
1 electrode radially split in 4 sectors
Total : 21 wires (200 V) coaxial cables
2 HV cables (10 KV)
 Voltages connected through filters
 Voltage rise and fall time: hundreds msec
 Fast pulses ( 10 ns rise time, duration about 100-200 ns) added to some
electrodes to dump the particles
 Plasma mode detection
 RW voltages superimposed to the trap voltages
Positron transfer
In the e+ accumulator B=0.15-0.2T
The last trap can be Malmberg or Penning (as we like)
e+ cooling : only buffer gas
No significant radiation cooling (B too low)
Radial compression through RW wall during accumulation with gas cooling
Accumulation time : 300-500 sec
Stop accumulation : gas pump out
3 108 e+
R e+ = max 5 mm
L e+ = 2-3 cm (?)
Temperature: 300K or eV
e+
The space charge electric field cancels the applied axial electric field
inside the plasma
V
Space charge energy: several tens of eV
z
e+ transfer from the accumulator in the main magnet
e+ in the accumulator
V
z
V1
1
1
mv 2f  mv i2  q(V1  V2 )
2
2
V2
Kinetic energy in the flight direction >> velocity spread in the cloud (temperature)
Space time gradient of the accelerating field: optimized to give the “same energy” to all the
positrons
Fligtht time << Coulomb explosion time
Energy in the flight direction >> space charge energy
Mirror effect of the magnetic field
e+ energy in the flight direction: several hundreds eV
vflight = 6 106 m/s
(100 eV)
= 1.9 107 m/s (1 KeV)
e+ catching in the main magnet
5 Tesla region
V3+D
V1= 500 V (example)
Transfer line
V2 (=0)
V3
After catching all the
voltages can be lowered
Only D is important
Transfer flight time : few hundreds ns
Positron catching trap : initial L about 30 cm
No antiprotons in trap during e+ transfer
Fast e+ cooling in 5 T magnetic field
Collection of e+ in a shorter trap in the region with high B homogeneity
Compression by RW (several tens sec)
At the same time: catch and cool pbar again
The position of the trap in the 5 T magnet
The region with uniform magnetic field
e+
Movable degrader out
Bz  (5.0822)
 5.0822
Could we have 10 cm more 5T magnetic field with 10-4 homogeneity?
The solenoid can be longer
Summary of the parameters of the positron catching trap
Electrode voltages : 1 KV max added to
+ - 200 V (DAC+amplifiers)
7 electrodes for Penning trap
10 additional electrodes
1 electrode radially split in 4 sectors
Total: 23 (1.2 KV) coaxial cables
(or more, it depends on the length of the magnetic
field)
 Voltages connected through filters
 Voltage rise and fall time: hundreds msec
 Fast pulses ( 10 ns rise time, duration about 100-200 ns) added to some
electrodes to dump the particles
 Plasma mode detection
 RW voltages superimposed to the trap voltages
Entrance electrode: fast pulse
Adiabatic transfer
Shaping time : 50 msec
Wait at each stage: ms
several rotation
5
10
15
Single to double particle injection: see last meeting
Pbars lye on axis all the time
Normal “adiabatical” transfer
Positrons have to jump off axis to be shooted on the porous target to form
positronium
Diocrotron excitation of
the positron plasma
Ps*
(J. R. Danielson and C. M. Surko Physics of Plasmas 13 (2006), 123502)
100 mK region
10 cm
22.5 cm
Trap radius 2.5 cm (big trap)
0.8 cm (small trap)
Particle transfer in the 1 Tesla magnetic field
Transfer length 1m (-40 cm, +60 cm)
About 40 transfer electrodes (l=2.5 cm each)
Big radius trap:
17 coaxial cables +- 200 V
Small radius traps pbar manipulation : 10 coaxial cables
e+ manipulation
10 coaxial cables
+- 50 Volts
+- 50 Volts
e+ acceleration toward the target : 5 (or more) cables (3KV?)
pbar cooling and Rydberg accelerator: 25 coaxial cables
+- 50 Volts
1KV for about 100 msec
Additional electrodes after the trap???
MCP+ phosphor screen: can be installed in the place of the grating system
They have to be removed when the grating will be
installed
Still preliminary summary of the needed cables:
this is the minimum number….
100
mK
4K
20
50 V
5
3 KV
e+ acceleration (only for few tens ns)
25
50 V-1 KV
1KV only for 100 microsec
17
200 V
big trap before the small ones
40
200 V
transfer region
23
1.2 KV
e+ catching
21
200 V
pbar catching
2
10 KV
pbar catching
Trap mechanics+ electronics & control system
HV pulser
DAC+amplifiers
Fast pulser
Faraday cup amplifiers
MCP+Phosphor+CCD
Plasma mode detection (FPGA based)
RW & plasma manipulation
Labview boards
Assuming to build the electronics
: 162 KEuro
Pumps, feedthroughs, vacuum gauges,flanges,movable feedthr. : 90 KEuro