David B. Cassidy. Department of Physics and Astronomy, University of California, Riverside, USA. Varenna, July 09

Transcription

1 Experimental production of many- positron systems David B. Cassidy Department of Physics and Astronomy, University of California, Riverside, USA Varenna, July 09

2 Allen P. Mills, Jr. (the boss) Blame me for these lectures

3 Collaborators Tomu Hisagawa Vincent Meligne Harry Tom UCR Rod Greaves First Point Scientific Inc.

4 Overview Lecture 1: beams, traps and the generation of intense positron pulses Lecture 2: techniques for characterising and utilising intense positron pulses Lecture 3: experiments As much as possible I will only talk about things that we have already done, or that we are in the process of doing.

5 Many-positron systems? A positron plasma is technically a many positron system, but it behaves in the same way as an electron plasma. Positrons are like wives (or husbands): having more than one at a time seems like a lot (i.e., even two = many ). I will consider many-ps systems, wherein the Ps atoms are created with a temporal overlap and may or may not interact with each other.

6 First Point Scientific modular beam/trap Source Trap Accumulator 50 mci 22 Na source Neon moderator Efficiency ~ 1% > 10 6 e + /sec Nitrogen buffer gas e + lifetime ~ 2 seconds Single particle RW compression efficiency ~ 10%, 4 Hz duty cycle > 50,000 e + /pulse SF 6 cooling gas (~ 1 x 10-7 Torr) e + lifetime > 500 seconds efficiency ~ 80% Maximum 10 8 e + Typical pulse, 2 x 10 7 e + Pulse out ~ 15 ns FWHM RSI 77, (2006)

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9 Moderator: W or Ne? Neon moderators are efficient but lead to a large energy spread in the beam, which affects trap efficiency. They also require costly equipment (> 50K$). Tungsten is less efficient but is cheap and simple to use and has a lower energy spread. Moderator conversion trap total efficiency efficiency Neon 1% 20% 1 0.5% 5% 1/8 Tungsten 0.1% 40% 1/5 0.05% 20% 1/20

10 Standard Neon moderated source OFHC mount Na-22 source capsule Solid Neon layer We find best efficiencies (~ 1%) when moderator is grown quickly at high pressure (~10-4 Torr, 5 minutes). Radioactive material Ti window moderated positrons

11 Energy spread of neon moderated beam, measured in the trap normalised counts E (FWHM) (volts) Field at source ~ 75 Gauss V S1 (volts) Magnetic field (Gauss)

12 1.0 B = 200 G normalised counts moderator 10 days old moderator 3 hours old VS1 (volts) Energy spread is fairly constant over time, but only if you keep the buffer gas out of the source chamber

13 G G G Y Axis (cm) X Axis (cm) G G G Beam has a hollow profile at maximum extraction efficiency due to cone shaped moderator

14 22 Na is not enough for some experiments Accelerator based source being installed at UCR (1) ion source (2) RFQ deuteron accelerator (1.5 MeV) (3) radiation shielding tank (4) target chamber with carbon target

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16 12 C(d,n) 13 N thick target yield: RF power limits the duty cycle of the RFQ, so that the average current will be ~ 0.5 ma. Activity produced at 1.5 MeV is 0.8 Ci Factor of 30 increase over present source. (25 mci) In theory we could have a lot more but this is what we will try first (could be some problems, see Weber lecture)

17 Some problems could be solved using Mills concept for 13 N gas extraction and bright source production

18 Sources: 13 N vs 22 Na Present accumulator configuration can only hold ~ 10 8 positrons. Sodium source takes ~ 5 minutes to fill accumulator. Stronger source won t change achievable positron densities, it will only increase the data acquisition rate This would be nice, but it is not critical. For Ps BEC work it will be more important

19 Potential structure of the (2 stage) positron trap: See Surko Lectures for details

20 Single particle compression: Why does this work in such a broadband manner at higher amplitudes? Positron lifetime is increased from 0.5 to 2 seconds, trapping efficiency increased and spot size reduced. Mysterious but very useful

21 Plasmas formed in accumulator by stacking many trap pulses number of posit trons (10 6 ) (11V) (10V) (9V) Fill time (sec)

22 The positron beam density can be controlled via the rotating wall electric field in the accumulator y (mm) High density beam n 2D ~ cm -2 x (mm) Low density beam n 2D ~ cm -2

23 Beam profile changes as it becomes more compressed and becomes less flat topped central den nsity (arb.) MHz 7 MHz 3 MHz X Axis (cm)

24 Strong drive regime compression interrupted by zero frequency modes (see Surko lectures) 10 correction coils on correction coils off central density (arb.) compression frequency (MHz)

25 Correction coils reduce effects of zero frequency modes central density (arb.) Y coil (amps) X coil (Amps)

26 Limitations of compression not understood central density (arb.) approximate resonant frequency (MHz) Resonance frequency 12.0 Linear Fit Accumulator magnetic field (Gauss) G 600 G 700 G 800 G 900 G compression frequency (MHz)

27 Electrode structure in accumulator 2π π/2 3π/2 π n 2D ~ cm -2 Inlet stage 1& gate Stage 3: multi-ring trap electrodes Accumulation voltage (V) 30 Inlet n 2D ~ cm -2 Stage 1 & 2 Stage 3 gate distance along accumulator axis (inches) dump voltage (V)

28 2 kv buncher produces sub-ns pulses HV pulse input magnet coil buncher rings positrons 5 cm PMT output (mv) Buncher on Buncher off Short pulse critical for Ps 2 experiments time (ns) PMT output (mv)

29 pulsed magnet coils window Pulsed magnet provides spatial compression Bunched positrons target position Phosphor screen High Voltage Input 5 cm Magnet current (A) A ~ 1.5 Tesla time (ms) But we need more density

30 New system recently installed for beam remoderation Accumulator beam imaging chamber movable phosphor screen buncher rotatable K cold head electrostatic lenses pumping restriction magnetic field termination retractable LEED/Auger spectrometer cryo pump

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32 Buncher, field termination and electrostatic lenses all on one flange

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34 Extraction of beam from magnetic field Final spot size on target predicted to be ~ microns. Remoderator efficiency ~ 10%. Beam areal density > cm -2 (factor of 10 improvement, in worst case scenario)

35 (a) (b) Light intensitty (arb.) light intensity (arbitrary) X Axis (cm) Y Axis (cm) X Axis (cm) Y Axis (cm) Positron beam profiles measured in the imaging chamber (a) just after the accumulator (in an axial magnetic field of ~ 700 Gauss) and on a phosphor screen placed in the position of the remoderator foil (b) where the field is essentially zero.

36 Steering can be accomplished using a restricted gamma ray counter even when imaging is impossible Gamma rays from foil position Direct imaging in remoderator foil position Y coil current (A) Y coil current (A) X coil current (A) X coil current (A)

37 Still to do Study Ni remoderator foil treatment Optimise electrostatic beam transport and focusing Study space charge effects to produce the highest density pulses we can without affecting the timing Configure accumulator for larger pulses in preparation for Ps BEC experiments

38 Conclusion We have seen how to produce positron pulses with areal densities over cm -2 and a sub ns width The question then becomes what can we do with them? That will be the subject of the next lectures Thank you for your attention.