1. Low Energy Positron Toroidal Accumulator
Laboratory of Nuclear Problems
Electron Cooling Section
Low Energy Positron Toroidal Accumulator
Positron Source and Trap
Primrose Njingana
Gerrit Olivier
Sfundo Khanyile

2. Contents 1. Brief background on LEPTA project
2. Cryogenic source of positrons
3. Positron trap

3. LEPTA Facility kicker septum cooling section positron trap
collector
septum
cooling section
106100sec=108 e+
e-gun
positron trap
Helical quadrupole
O-Ps
22Na106 e+ per sec
104 Ps per sec
positronium
detector

4. 2. Positron Source
For the positron source we are using a 22Na source which was prepared at the proudly South African Ithemba labs.
The 22Na undergoes β+ decay, thereby emitting positrons. Because the positrons emitted by the 22Na source has a very broad energy spectrum, we need to moderate the positrons to generate a monochromatic beam of slow positrons.

5. To do this we use a cryogenic moderator, consisting of frozen neon.
When the fast positrons pass through the moderator they are slowed down to thermal speeds creating a monochromatic beam of slow positrons.

6. The Cryogenic Moderator of PositronsNe
22Na
T ~ 5 K e+ Ne
The moderator has an efficiency of 1%, in other words 1% of the positrons emitted by the source falls in the required energy band.

7. Positron Energy Spectrum

8. After the positrons are moderated by the frozen neon, they get filtered further with correcting coils that are placed so that they create a magnetic force acting on the positrons..
Corrections coils
Direction of force on positrons
Slow positron
Fast positron
Positron source
The fast positrons are not affected enough to pass through the connecting pipe but the path of the slow positrons are bent so that they can pass through the pipe, thereby filtering out the fast positrons

9. 3. Positron Trap
Why do we need a positron trap?

10. Design of the trap
The trap consists of 8 electrodes with different potentials. The positrons pass through area 1 and 2 but cannot pass the potential wall created by the last 5 electrodes.
Area 1
Area 2
Area 3 eU z
10-3
Pressure, Torr N2 е+
10-4
10-6
Here the positrons accumulate until there are a sufficient amount and can be extracted.

11. Particle storage and “the space charge limit”
Area 1
Area 2
Area 3 eU z
10-3
Pressure, Torr N2 е+
10-4
10-6
When a lot of positrons accumulate they decrease the depth of the potential well in area 3 which causes the accumulation of less positrons.

12. Particle storage and “the space charge limit”
Area 1
Area 2
Area 3 eU z
10-3
Pressure, Torr N2 е+
10-4
10-6
By using a dynamical control potential in area 3 we get rid of this problem and double the possible amount of positrons that can accumulate in the trap.

13. Positron trap
Positron source

14. The positron source does not have the desired intensity yet so measurements and experiments are currently being done with electrons using an electron gun and a collector behind the solenoid.
Because there is no magnetic field in the space between the trap and the collector the beam gets spread out. Initially we detected a strong current on the collector, this was because the collector was too far away from the trap and the beam was spread out.
Electron Beam
Collector

15. Thank you very much to Roman Pivin and Sergey Yakovenko for showing us more about Lepta and helping us on the project!!