Production of antihydrogen atoms by mixing antiprotons with a cold, confined,
positron plasma depends critically on parameters such as the plasma density and
temperature. We discuss non-destructive measurements, based on a novel,
real-time analysis of excited, low-order plasma modes, that provide
comprehensive characterization of the positron plasma in the ATHENA
antihydrogen apparatus. The plasma length, radius, density, and total particle
number are obtained. Measurement and control of plasma temperature variations,
and the application to antihydrogen production experiments are discussed.Comment: 5 pages, 4 figures, to be published in Phys. Rev. Let

Amoretti, M.

Amsler, C.

ATHENA Collaboration

Bonomi, G.

Bouchta, A.

Bowe, P. D.

Carraro, C.

Cesar, C. L.

Charlton, M.

Doser, M.

Filippini, V.

Fontana, A.

Fujiwara, M. C.

Funakoshi, R.

Genova, P.

Hangst, J. S.

Hayano, R. S.

Jorgensen, L. V.

Lagomarsino, V.

Landua, R.

Lindelof, D.

Macri', M.

Madsen, N.

Manuzio, G.

Montagna, P.

Pruys, H.

Regenfus, C.

Rizzini, E. Lodi

Rotondi, A.

Testera, G.

van der Werf, D. P.

Variola, A.

English

arXiv

Production of antihydrogen atoms by mixing antiprotons with a cold, confined, positron plasma depends critically on parameters such as the plasma density and temperature. We discuss non-destructive measurements, based on a novel, real-time analysis of excited, low-order plasma modes, that provide comprehensive characterization of the positron plasma in the ATHENA antihydrogen apparatus. The plasma length, radius, density, and total particle number are obtained. Measurement and control of plasma temperature variations, and the application to antihydrogen production experiments are discussed

Collaboration, ATHENA

Amoretti, M

Amsler, C

Bonomi, G

Bouchta, A

Bowe, PD

Carraro, C

Cesar, CL

Charlton, M

Doser, M

Filippini, V

Fontana, A

Fujiwara, MC

Funakoshi, R

Genova, P

Hangst, JS

Hayano, RS

Jorgensen, LV

Lagomarsino, V

Landua, R

Lindelof, D

Rizzini, EL

Macri', M

Madsen, N

Manuzio, G

Montagna, P

Pruys, H

Regenfus, C

Rotondi, A

Testera, G

Variola, A

Werf, DPVD

UCL Discovery

P H Y S I C A L R E V I E W L E T T E R S week ending1 AUGUST 2003VOLUME 91, NUMBER 5Positron Plasma Diagnostics and Temperature Control for Antihydrogen ProductionM. Amoretti,1 C. Amsler,2 G. Bonomi,3 A. Bouchta,3 P. D. Bowe,4 C. Carraro,1,5 C. L. Cesar,6 M. Charlton,7 M. Doser,3V. Filippini,8 A. Fontana,8,9 M. C. Fujiwara,10 R. Funakoshi,10 P. Genova,8,9 J. S. Hangst,4 R. S. Hayano,10L.V. Jørgensen,7 V. Lagomarsino,1,5 R. Landua,3 D. Lindelo¨f,2 E. Lodi Rizzini,11 M. Macrı´,1 N. Madsen,2 G. Manuzio,1,5P. Montagna,8,9 H. Pruys,2 C. Regenfus,2 A. Rotondi,8,9 G. Testera,1,* A. Variola,1 and D. P. van der Werf7(ATHENA Collaboration)1Istituto Nazionale di Fisica Nucleare, Sezione di Genova, 16146 Genova, Italy2Physik-Institut, Zu¨rich University, CH-8057 Zu¨rich, Switzerland3EP Division, CERN, CH-1211 Geneva 23, Switzerland4Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark5Dipartimento di Fisica, Universita` di Genova, 16146 Genova, Italy6Instituto de Fisica, Universidade Federal do Rio de Janeiro, Rio de Janeiro 21945-970, Brazil,and Centro Federal de Educac¸a˜o Tecnologica do Ceara, Fortaleza 60040-531, Brazil7Department of Physics, University of Wales–Swansea, Swansea SA2 8PP, United Kingdom8Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, 27100 Pavia, Italy9Dipartimento di Fisica Nucleare e Teorica, Universita` di Pavia, 27100 Pavia, Italy10Department of Physics, University of Tokyo, Tokyo 113-0033, Japan11Dipartimento di Chimica e Fisica per l’Ingegneria e per i Materiali, Universita` di Brescia, 25123 Brescia, Italy,and Istituto Nazionale di Fisica Nucleare, Gruppo collegato di Brescia, 25123 Brescia, Italy(Received 14 February 2003; published 31 July 2003)055001-1Production of antihydrogen atoms by mixing antiprotons with a cold, confined, positron plasmadepends critically on parameters such as the plasma density and temperature. We discuss nondestructivemeasurements, based on a novel, real-time analysis of excited, low-order plasma modes, that providecomprehensive characterization of the positron plasma in the ATHENA antihydrogen apparatus. Theplasma length, radius, density, and total particle number are obtained. Measurement and control ofplasma temperature variations, and the application to antihydrogen production experiments arediscussed.DOI: 10.1103/PhysRevLett.91.055001 PACS numbers: 52.27.Jt, 36.10.–k, 52.35.Fp, 52.70.–m[1]. Furthermore, the space charge potential of a suffi-ciently dense (108 cm3) and extensive (length 3 cm)positron plasma considerably alters the effective electro-provides a nondestructive diagnostic based on measure-ments of the first two axial modes of a finite temperaturepositron plasma. The diagnostic has no discernible effectThe ATHENA Collaboration recently produced anddetected cold antihydrogen atoms [1] at the CERNAntiproton Decelerator. A similar result has been subse-quently reported by the ATRAP Collaboration [3].The antihydrogen was made by mixing low energy anti-protons with a cold, dense positron plasma in a nestedPenning trap [4]. A knowledge of the characteristics ofthe positron plasma is important for several reasons. Themost likely antihydrogen formation mechanisms, sponta-neous recombination and three-body recombination,have different dependences on both the density and thetemperature of the positron plasma [5]. Knowing theseparameters is crucial in helping to elucidate the antihy-drogen formation mechanism. Control and simultaneousmonitoring of the positron plasma temperature allow theantihydrogen formation reaction to be effectively turnedoff while maintaining overlap between antiprotons andpositrons. This provides a good measurement of the totalbackground signal for our unique antihydrogen detector0031-9007=03=91(5)=055001(5)$20.00 static potential in the positron trap and thus the dynamicsof the antiprotons in the nested trap.Harmonically confined one component plasmas at tem-peratures close to absolute zero are known to form sphe-roids of constant charge density [6]. In our case the shapeis a prolate ellipsoid characterized by the aspect ratio  zp=rp, where zp and rp are the semimajor axis andsemiminor axis, respectively [Fig. 1(a)]. A cold fluidtheory [7] relating the frequencies of the low-orderplasma modes to the density and the aspect ratio ofspheroidal plasmas was confirmed experimentally forlaser cooled ion plasmas [8,9] and successfully appliedto cold electron plasmas [10]. Work on finite temperatureelectron plasmas demonstrated that mode detection couldbe used as a diagnostic of density and aspect ratio and thatfor a plasma of known density and aspect ratio the fre-quency of the quadrupole mode is dependent on theplasma temperature [11,12].Here we describe an extension to the above work which2003 The American Physical Society 055001-1FIG. 1 (color online). (a) Trap electrodes with the heatingand mode detection electronics. The shape of the positronplasma prolate ellipsoid is shown schematically. (b) The axialpotential of the ATHENA nested trap is shown and the rangesof axial motion of the positrons and the antiprotons indicatedschematically.P H Y S I C A L R E V I E W L E T T E R S week ending1 AUGUST 2003VOLUME 91, NUMBER 5on the normal evolution of the plasma, so it can be usedduring antihydrogen production. We have developed amodel in which the plasma length can be extractedfrom the shape of the resonance when the dipole modeis excited. Thus the aspect ratio, density, and length canbe measured and the radius and positron number ob-tained. We show that we can monitor induced changesin the temperature while ensuring that the normal evolu-tion of other plasma parameters is not changed. Using thismonitor and suitable radio-frequency excitation of thedipole mode we can set the plasma temperature duringthe interaction between positrons and antiprotons.In ATHENA, the positrons and antiprotons are con-fined by cylindrical electrodes of radius rw  1:25 cm.These are inserted in a cryostat and immersed in a 3 Tmagnetic field. The positrons are held in the central partof the trap by a harmonic axial potential [see Fig. 1(b)]. Inthe high magnetic field the positrons cool by synchrotronradiation [13]. We have not measured the temperature ofthe positron plasma but note that the lower limit is set bythe temperature of the trap electrodes (15 K).The first two axial modes (dipole and quadrupolemodes) are excited by applying a sinusoidal perturbationto one electrode with an electromotive force Vt  vtej!t.The oscillation of the plasma induces a current in thepickup electrode [14,15] and a voltage Vr  vr!ej!t isdetected across the resistance Rr [Fig. 1(b)]. Ex-perimentally the ratio TL!  vr!=vt is measured asa function of the drive frequency ! by means of a net-work analyzer. A narrow stepwise frequency sweep is055001-2made of the voltage source across the resonance frequencyof each mode. The excitation amplitude vt is of the orderof 100 V and the dwell time for each 5 kHz step is3.96 ms. The number of scan steps is usually 50. Thechoice of scan width is a balance between followingscan-to-scan changes in mode frequency and avoidingperturbations to the normal evolution of the plasma. Tofollow changes in the plasma over a longer time scale anautomatic frequency tracking code has been implementedto allow the excitation to be locked to the mode frequen-cies. For each frequency step, the amplitude and phase(relative to that of the driving signal) of the voltageinduced by the plasma motion are acquired. The crosstalk signal between the transmitting and receiving elec-trodes is acquired without positrons and subtracted fromthe signal measured with the plasma present.The plasma number density n and aspect ratio  can beextracted from the zero-temperature analytical model [7]using the measured frequencies (!1; !2) of the dipole andquadrupole modes. We have developed an equivalent cir-cuit model which explicitly includes the plasma dimen-sions. This method yields directly the plasma length when and n are known. In contrast, other equivalent circuitapproaches utilizing tuned circuits and assuming smallcloud dimensions measured the total number of particlesin Penning traps [15]. Our model describes the signalinduced on an electrode by the coherent oscillations ofthe dipole mode when an external driving force is applied.In particular, we write [16]TL gt; zpgr; zpRrRs  j!L1!21=!2: (1)The dimensionless functions gr and gt depend on theshape of the plasma and on the geometry of the trap.They describe the effects of the finite plasma extensionboth on the mode excitation and detection. gt; zpVt=2rw is the electric field acting on the center of mass of theplasma when the dipole mode is excited by applying avoltage Vt to the transmitting electrode with the otherelectrodes grounded. The function gr; zp is related tothe current Ir induced by the dipole mode oscillation onthe receiving electrode, Ir  Negr; zpvcm=2rw, wherevcm is the velocity of the particles due to the dipole modeand e is the positron (electron) charge. TL is obtained bymeasuring T0L  ATL, where A is the net gain of theelectronics chain and it is independent of the plasmaproperties. We used electrons to fine-tune this parameterby comparing the number obtained by this diagnosticwith the number measured on a Faraday cup. Electronswere chosen as they can be loaded faster, with a widerrange of total number N. The inductance L of the equiva-lent circuit is related to the plasma length,L  32r2wmne2z3p; (2)055001-2P H Y S I C A L R E V I E W L E T T E R S week ending1 AUGUST 2003VOLUME 91, NUMBER 5where m is the positron (or electron) mass [16]. Theresistance Rs characterizes the damping rate of the mode. and n are determined independently by the frequencyanalysis. The power transmitted through the plasma jTLj2is related to Rs and zp by Eqs. (2) and (1). Thus a fit to themeasured transmitted power yields zp. The radius rp andthe total number N can now be found. An example of anapplication of this diagnostic to positrons is shown inFig. 2. Here the total number obtained with the model isplotted against the number found by extraction to aFaraday cup. In this regime the linearity and good corre-spondence in the absolute number show that both themodel and its implementation constitute a complete, real-time, nondestructive plasma diagnostic. Typical proper-ties of the ATHENA positron plasma for antihydrogenproduction were 7 107 positrons at a density of about1:7 108 cm3 in a plasma 	 3:2 cm long (2zp) with aradius of about 0.25 cm and a storage time of severalhundreds of seconds. The maximum change in the plasmaparameters during the mixing cycle of 190 s was lessthan 10%.Because of the importance of temperature in antihy-drogen production, we have also investigated whether thisdiagnostic system can be used to detect changes in theplasma temperature. Previous authors have demonstrated,with work on similar electron plasmas [11,12], that tem-perature changes manifest themselves in frequency shiftsof the quadrupole mode. In particular, they findkT  mz2p5!h22  !223!2p22!22d2fd21; (3)]7 [10dumpN0 2 4 6 8 10]7 [10modeN0246810FIG. 2. The total number of positrons obtained using themodes diagnostic is plotted against the number obtained byextracting the positrons to a Faraday cup.055001-3where f  2Q1=2  1p=2  1 and Q1 is aLegendre function of the second kind, !h2 is the quadru-pole frequency with heating applied, and!p is the plasmafrequency. The values of , !p, and zp are evaluated inthe cold fluid limit. Provided that these parameters do notvary significantly when the plasma temperature changes,a measured shift in the quadrupole frequency can be usedto calculate the magnitude of the temperature change.We have used the mode diagnostic system to investigatethe response of a cold and dense positron plasma withoutantiprotons to heating, implemented by applying an ex-citation near the dipole frequency (21 MHz) to one of thetrap electrodes [Fig. 1(a)]. Off-resonance heating pulseswere not effective. The excitation is a variable amplitudesignal that is swept from 20 to 22 MHz at a repetition rateof about 1 kHz. This is done to ensure that the dipolemode frequency is covered and that the plasma reachesthermal equilibrium. Previous authors report an equili-bration rate of some tens of kHz in similar conditions[17]. Figure 3 shows the behavior of the quadrupolefrequency when the initially cold plasma is subjected toheat off-on cycles. Application of the excitation results ina rapid, voltage-dependent rise in the quadrupole fre-quency. When the excitation is removed, the quadrupolefrequency returns to a value in step with the evolution ofthe unperturbed plasma which is also shown in Fig. 3. Theunperturbed plasma evolution is characterized by a slowdecrease in the frequency of the quadrupole mode andcorresponding decrease in aspect ratio and density. Thisis consistent with a slow expansion of the plasma.The heating and the cooling times are faster than ourTime [s]0 10 20 30 40 50 60 [MHz]π/2 2ω32.93333.133.233.333.4FIG. 3. The quadrupole mode frequency versus time for nor-mal evolution () and for two heat off-on cycles (). Thefrequency shift corresponds to an increase of the plasmatemperature of about 150 meV.055001-3Time [s]0 100 200 300 400 500 600 700 800 [MHz]π/2 2ω3333.233.433.633.834(a) [mV]dV0 1 2 3 4 5 6 7 8T [meV]∆k0100200300400500(b)FIG. 4. (a) Time evolution of the quadrupole mode frequency during a heating off-on cycle. The different frequency shiftscorrespond to different heating amplitudes Vd. The drift of the frequency in the unperturbed intervals is due to the plasmaexpansion, and it is consistent with the normal evolution of the unperturbed plasma (see also Fig. 3). (b) Dependence of thetemperature variation on radio-frequency signal amplitude for the same cycle.P H Y S I C A L R E V I E W L E T T E R S week ending1 AUGUST 2003VOLUME 91, NUMBER 5sampling interval resulting in the observed discontinu-ities of the frequency shifts on the time scale of Fig. 3.Furthermore, for these heating amplitudes, withoutantiprotons we observe no discernible positron loss withour positron annihilation detector which covers a solidangle of 80% of 4 [18]. Figure 4(a) shows the responseof the plasma quadrupole frequency during heat off-oncycles with different amplitudes Vd of generated heatingvoltage [directly proportional to Vh of Fig. 1(a)]. Thus thedependence of temperature increase T on Vd was found,and the result is shown in Fig. 4(b). For the data reportedhere the minimum measurable temperature was about15 meV due to the frequency step size used (20 kHz).The minimum step size of 5 kHz would give a sensitivityof a few meV. The observed linear dependence of thetemperature rise on Vd is in contrast with the lineardependence on the power (and thus on V2d) that one wouldexpect. This could be most likely due to nonlinear effectsof the on-resonance heating. A nonlinear regime couldalso explain the fact that the temperature rise in Fig. 4(b)appears to extrapolate to zero at finite Vd.The utility of the mode diagnostic system as applied toantihydrogen production is immediately apparent if weconsider that the radiative reaction rate is proportional ton and to T1=2, while the three-body reaction rate variesas n2 and T9=2 [5]. In order to gain insight into theexperimental production mechanism and to ensure repro-ducible results, careful monitoring of any changes inthese parameters is essential. The ability to reproduciblyadd heat to the positron plasma, and to thereby control thereaction rate, is also very desirable. We have alreadyutilized the heating and temperature diagnostic to ‘‘turnoff ’’ antihydrogen production in ATHENA [1], providinga useful null measurement.055001-4The implementation of automated mode analysis mea-surement has allowed us to log the plasma parameters atall times when positrons are in the trap. The system iswideband and does not require that any resonant circuitrybe mounted internally on trap electrodes. We are thus freeto choose a wide range of depths for our harmonic posi-tron trap and are therefore able to tune the shape of ourpositron plasma at will. Knowing the size of the positroncloud is crucial to assuring a good spatial overlap betweenantiprotons and positrons in the reaction region. Thespace charge potential of our positron plasma flattensthe potential inside its volume along the z directionreducing the potential barrier seen by the antiprotons inthe mixing trap by several volts. Information about theeffective potential is important in helping to understandthe antiproton cooling and interaction dynamics.We have extended the plasma mode diagnostic methodto provide comprehensive characterization of a cold,dense positron plasma employed in the ATHENA anti-hydrogen experiment. The method has already been uti-lized to great advantage in ATHENA and promises to bean essential element of future experiments. The tech-nique, while particularly useful for nondestructive mea-surements on difficult-to-produce species such aspositrons, has immediate applicability to other Penningtrap plasmas.This work was supported by INFN (Italy), CNPq andFAPERJ (Brazil), MEXT (Japan), SNF (Switzerland),SNF (Denmark), and EPSRC (United Kingdom).*Corresponding author.Electronic address: testera@infnge.ge.infn.it055001-4P H Y S I C A L R E V I E W L E T T E R S week ending1 AUGUST 2003VOLUME 91, NUMBER 5[1] M. Amoretti et al., Nature (London) 419, 456 (2002).[2] S. Maury, Hyperfine Interact. 109, 43 (1997).[3] G. Gabrielse et al., Phys. Rev. Lett. 89, 213401(2002).[4] G. Gabrielse, S. L. Rolston, L. Haarsma, and W. Kells,Phys. Lett. A 129, 38 (1988).[5] M. H. Holzscheiter and M. Charlton, Rep. Prog. Phys. 62,1 (1999), and references therein.[6] L. Turner, Phys. Fluids 30, 3196 (1987).[7] D. H. E. Dubin, Phys. Rev. Lett. 66, 2076 (1991).[8] D. J. Heinzen et al., Phys. Rev. Lett. 66, 2080 (1991).[9] J. J. Bollinger et al., Phys. Rev. A 48, 525 (1993).[10] C. S. Weimer, J. J. Bollinger, F. L. Moore, and D. J.Wineland, Phys. Rev. A 49, 3842 (1994).055001-5[11] M. D. Tinkle et al., Phys. Rev. Lett. 72, 352 (1994); M. D.Tinkle, R. G. Greaves, and C. M. Surko, Phys. Plasmas 2,2880 (1995).[12] H. Higaki et al., Phys. Rev. E 65, 046410 (2002).[13] T. M. O’Neil, Phys. Fluids 23, 725 (1980).[14] C. A. Kapetanakos and A.W. Trivelpiece, J. Appl. Phys.42, 4841 (1971).[15] D. J. Wineland and H. G. Dehemelt, J. Appl. Phys. 46, 919(1975); X. Feng et al., J. Appl. Phys. 79, 8 (1996).[16] M. Amoretti et al., Phys. Plasmas 10, 3056 (2003).[17] B. R. Beck, J. Fajans, and J. H. Malmberg, Phys. Plasmas3, 1250 (1996).[18] C. Regenfus, Nucl. Instrum. Methods Phys. Res., Sect. A501, 65 (2003).055001-5

Positron plasma diagnostics and temperature control for antihydrogen production

M. Amoretti

C. Amsler

G. Bonomi

A. Bouchta

P. D. Bowe

C. Carraro

C. L. Cesar

M. Charlton

M. Doser

V. Filippini

A. Fontana

M. C. Fujiwara

R. Funakoshi

P. Genova

J. S. Hangst

R. S. Hayano

L. V. Jørgensen

V. Lagomarsino

R. Landua

D. Lindelöf

E. Lodi Rizzini

M. Macrí

N. Madsen

G. Manuzio

P. Montagna

H. Pruys

C. Regenfus

A. Rotondi

G. Testera

A. Variola

D. P. van der Werf

C. A. Kapetanakos

Crossref

Positron Plasma Diagnostics and Temperature Control for Antihydrogen Production

P.D. Bowe

C.L. Cesar

M.C. Fujiwara

J.S. Hangst

R.S. Hayano

L.V. Jorgensen

D. Lindelof

M. Macri

D.P. van der Werf

BONOMI, Germano

LODI RIZZINI, Evandro

Archivio istituzionale della ricerca - Università di Brescia

Production of antihydrogen atoms by mixing antiprotons with a cold, confined, positron plasma depends critically on parameters such as the plasma density and temperature. We discuss nondestructive measurements, based on a novel, real-time analysis of excited, low-order plasma modes, that provide comprehensive characterization of the positron plasma in the ATHENA antihydrogen apparatus. The plasma length, radius, density, and total particle number are obtained. Measurement and control of plasma temperature variations, and the application to antihydrogen production experiments are discussed

et al

ZORA

Niels Madsen

Michael Charlton

Dirk van der Werf

Cronfa at Swansea University

Amsler, Claude

Bowe, P

Cesar, C L

Doser, Michael

Fujiwara, M C

Hangst, J S

Hayano, R S

Jørgensen, L V

Landua, Rolf

Lindelöf, D

Lodi Rizzini, E

Macri, M

Pruys, H S

Van der Werf, D P

CERN Document Server

http://discovery.ucl.ac.uk/1375192/1/1375192.pdf

Positron plasma diagnostics and temperature control for antihydrogen
  production

Positron plasma diagnostics and temperature control for antihydrogen production

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