Coaxial electron beams and multiply charged ion beams are used to study dielectronic recombination. The study includes recombination of boron, carbon and sulfur. 3 refs., 7 figs. (WRF

Datz, S.

Dittner, P. F.

Miller, P. D.

Pepmiller, P. L.

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(Chapter in Bock entitled "Atomic Excitation and Recombination in ExternalFields," M. H. Nayfeh and C. Clark, eds., Harwood Academic Publishers,London & New York (1985))By acceptance of this article, thepublisher or reciptent acknowledgesthe U.S. Government's right toretain a nonexclusive, royalty-freelicense in and to anv copyrightcovering the ar*.,cle.OX 39 -DIELECTRONIC RECOMBINATION MEASUREMENTS ONMULTICHARGED IONSCONF-8410237—4DE86 002255P. F. DITTNER, S. DATZ, P. D. MILLER, ANDP. L. PEPMILLER, Oak Ridge National Laboratory,Oak Ridge, TN 37831Our efforts are directed principally at the measurementof dielectronic recombination in multiply charged ions.To do this, we chose a merged beam approach to take ad-vantage of our ability to produce high charge state,MeV/amu ions and a high current, high energy (1-3 keV)electron beam. The merged beam apparatus (outlined inFig. 1) is constructed such that in the interactionregion the ion beam is coaxial with and embedded withinthe electron beam for a distance of 84 cm.The ion beam from the ORNL EN-tandem acceleratorenters the interaction region through an axial, 0.64 mmFARADAY CUP -APERTUREELECTRON ^ ^-ELECTROSTATICQUN CYLINDER DEFLECTORFIGURE 1. Schematic Diagram of Merged Beam ApparatusOF m TOP.F. DITTNER, S. DATZ, P.D. MILLER AND P.L. PEPMILLERdiameter hole in the cathode of the electron gun. Afterexiting the interaction region, the ion beam is subjectedto charge state analysis. In our earlier work, we usedmagnetic deflection; in our more recent efforts, we useelectrostatic deflection. The initial charge state ofthe ion beam, q+ is deflected into a Faraday cup. Thecup is connected to a current integrator and the outputpulses are counted by a sealer. Ions that have picked upan electron (charge = (q-l)+) are deflected onto a solidstate position sensitive detector (PSD). The ions havingcharge (q-l)+ arise from electron pickup of the q+ ionsfrom the residual gas, slit edge scattering, and thesought after effect, DR.The source of the electron beam is a double griddedPierce-type high-intensity electron gun which is designedto produce a convergent, laminar electron beam. The gunis operated in the space charge limited mode where thespace charge limited current, Ic, is given in terms ofthe cathode to anode voltage, Vc, by Ic = PVC3/2. Theconstant P (the "perveance") is determined by theelectrode geometry and here equals 10~6. The electrongun is magnetically shielded from the solenoidal field ofthe interaction region. The emerging electron beam comesto a focus ~ 7 mm from the anode where it has a diameter(containing 95% of the beam) of 3.15 mm. It enters acoaxial solenoidal magnetic field which is adjusted toestablish Brillouin flow (e.g., ~ 0.18 T for 1 keVelectrons) in which the beam radius stays constant andthe beam rotates as a solid of revolution about its axiswith the Larmor frequency, wL. Under Brillouin flow,the longitudinal velocity of an electron is independentof radius, the radial velocity is zero, and the azimuthalDR MEASUREMENTS ON MULTICHARGED IONSvelocity is equal to O>L tl[Ties tne radial position of aparticular electron. Surrounding the electron beam is acoaxial cylinder 84 cm long having an inside diameter of7.9 mm, which is normally electrically grounded. Follow-ing the interaction region., defined by the length of thecoaxial cylinder and the solenoidal field, the electronsexpand (due to space charge repulsion) and strike thechamber walls.The experimental procedure consisted of optimizingthe electron beam, at a particular Vc, and counting theA(9~l)+ and AQ+ beams while stepping through the relativeenergies of interest by changing the energy of the ionbeam. This optimization was carried out by adjusting theorientation of the solenoid and the cylinder and makingsmall adjustments to the solenoidal field such that thecurrent to the cylinder was a minimum (<0.01 I c). It wasthen verified by observing the position spectrum whileswitching the electron beam on and off that the electro-static field, due to the electron space charge, producedno steering of the ion beam. We believe that minimumion beam steering indicates minimum misalignment betweenthe ion and electron beams. The deviation of the ratio,R, of (q-l)+ ions to q+ ions from a monotonic trend withion beam energy gives a measure of the DR cross section.In Fig. 2, we plot this ratio for two ion energy "sweeps"of a B 2 + beam (14-18 MeV) merged with the electron beam.In the first sweep, Vc = 1022 V (solid squares); in thesecond sweep Vc = 1079 V (open circles). The linesthrough the data in Fig. 2 are to guide the eye. It canbe seen that the DR signal shifts to higher ion energiesas the electron energy is increased, i.e., the signalP.F. DITTNER, S. DATZ, P.D. MILLER AND P.L. PEPHILLERdepends on the relative energy. To emphasize this pointfurther, we plot this ratio, in Fig. 3, for the two B 2 +sweeps versus the relative energy, E're^ . In Fig. 4,this ratio is plotted for a C 3 + beam (17.5 - 23 MeV)merged with an electron beam at Vc = 1079 V. The back-background ratio, Rg, can be fitted by a monotonic trend(H0"9l0BKL-0WGSJ-9877FIGURE 2. Ratio, B 1 +/B 2 +, as a function of B 2 +ion energy for Vc = 1022 V (solid squares andVc = 1079 V (open circles).0RNL-0WG8V9 -~33 8 ~. • i8 •• 3fo o0 2 6 8 <0 12 !4 16 18E' , I'JNCORRMeV)FIGURE 3. Ratio, B 1 +/B 2 +, vs. relative energy.For potential drop correction to the relativeenergy scale.DP. MEASUREMENTS OH MULTICHARGED IONSl ine as in Fig. 4o Subtracting RB from R yields thesignal rat io Rs. Rs is related to the DR cross sectiono» bywhere pe is the electron density, pj is the ion density,v r is the relative veloci ty, v-j is the veloci ty of theions, and V and A are the volume and cross sectional ar-aof the interaction region, a cylinder whose radius isthat of the ion beam r-j, and having the length of theelectron beam, L. Since P-J and v̂  are constant wi thin V,O«"»L-OV»G 83-98 75E'(1(UNCORH! (eV>20 16 12 10 8 6 4 2 118 19 20 21C 1 * IOM ENERGY IMe2JFIGURE 4. Ratio C2+/C3+ as a function of C3+ion energy for Vc = 1079 V. For potential dropcorrection to the relat ive energy scale, seeRef. 1.P.F. DITTNER, S. DATZ, P.D. MILLER AND P.L. PEPMILLERthe ion current I-,- = Jp-jV^dA = P-JV-JA. Approximating p eby an average electron density "pe> times a distributionin relative velocities f(v r), both being independent ofposition within V, we can write, Rs = fpeL<vra>)/vi,where the triangular brackets denote the average overf(v r). Thus, from the measured quantities we can calcu-late <vrcj> at every ion energy or relative energy, i.e.,<vra> = Rsv-j/peL.The best possible resolution in relative energy forthis cathode configuration would be >2 eV; about 0.5 eVarising from the non-zero radius of the ion beam in thespace charge limited electron beam and ~2 eV from thecathode temperature (~ H O C K ) coupled with a compressionfactor of ~16.In our first reported measurements,1 we arrived at .our signal level by simply subtracting a smooth extrapo-lated background drawn under the resonance region. Thistreatment indicated a resolution of ~3 eV, but subsequentimprovements in measuring technique have shown this tobe overly optimistic.Our gun contains a gridded cathode; the initial ideawas to modulate the electron beam to determine the back-ground. This technique did not prove feasible becauseturning the electron beam on and off modulated the back-ground gas density and gave rise to a large spurioussignal even at energies where signals were not physicallypossible.An alternative scheme has proved more successful.Here we modulate the voltage applied to the guide tubesurrounding the merged beam region. For half the timethe tube is grounded and for the other half the tube isDR MEASUREMENTS ON MULTICHARGED IONSraised in voltage such that the electron beam energy isshifted to be off resonance. The result of such anexperiment is shown for S 5 + in Fig. 5. Several jumpsappear in the data which are much beyond the range ofstatistical error. These jumps probably occur becauseof slit edge scattering which may vary somewhat with ionbeam steering at different energies. (Mote that in ourconfiguration the ion beam may be scattered at thenarrow entrance aperture in the cathode.) These "jumps"ORNL-OWG 84-17888— E(S5-)(MeV>36 33 31 30 29 28 27 26 25 24 232.01.52 i.Oa.0.5'»i ' i r i r i iS5*(2p6 3s)— S4'(2p6 3s n/) A *Vco.hod. = 6 4 2 V* - T U B E A T G R O U N D• - T U 8 E A T • 2 8 0 VI I I I I I I I I I I I I I I I I I I I I I ! I5 10 15 2 0 2 5FIGURE 5. Ratio (Sk+/S5+) as a function ofS 5 + ion energy (top) and relative energy (bottom)for a 617 eV merged electron and ion beam. Thelower curve shows the effect of the offset inelectron beam energy at the same ion energy.P.F. DITTNER, S. DATZ, P.D. MILLER AND P.L. PEPMILLERdisappear when we plot the difference signal as in Fig.6. The effect of residual background modulation is seenin the non-zero level in the 16 to 24 eV region and maybe subtracted to give th f^nal signal values. The con-sistency of this method may be seen in Fig. 7 where wehave plotted the rates <va> derived from such measure-ments for C 3 + ions. The round points were determinedwith an electron beam moving slower than the ion beam(lab system), the square points from measurements wherethe electron beam was moving faster than the ion beam(lab system). Here, the observed peak is asymmetric with0 2 4 6 8 'lO 12 14 16 18 20 22 24Er.l ( e V )FIGURE 6. Difference signal ofratio from Fig. 5.DR MEASUREMENTS ON MULTICHARGED IONSORNL-DWG 84-17886Eugob2.22.01.81.61.41.21.00.80.60.40.20— ••• • ••i r iiC3+(is2— c2• vc =•••«P •11+ ( is 2 2s8 4 0 ; Vj >1078; Ve-Vi'111n/)y v*i•I1522p n/)]'———12E r e l(eV)16 20 24 28FIGURE 7. Measured rates (<vo>) for C 3 + DR asa function of relative energy for: • ionvelocity greater than electron velocity, and •ion velocity lower than electron velocity.a full width of ~6 eV (+2, -4). For C 3 +, we expect avery sharp peak (almost a 6 function on our scale) at7 eV.2 The observed spread around 7 eV is due toelectron beam energy resolution. Using a two-parameterfit, one describing the longitudinal and the second thetransverse distribution, we find we are able to fit therates. The same two parameters fit for all the ions used(C 3 +, 0 5 +, P^+, S 5 +, and Cl6+) thus giving us some con-fidence that we have now characterized our electron beam.P.F. DITTNER, S. DATZ, P.D. MILLER AND P.L. PEPMILLERThe theoretically predicted probability of recom-bination observed in these experiments depends on theelectric fields present in two regions: f i r s t , theelectron-ion interaction region and second, the chargeanalysis region.The electric fields in the charge analysis regioncan Stark strip Rydberg states formed in the electron-ioninteraction. The maximum n state that can survive thesefields is given by"max " <6 '31 x 1Q8 <where q is the core charge and E is the field in voltsper cm. In our first experiments1 we used a magneticfield of 0.1 T to separate the charge states of ionsmoving at ~2 MeV/u. This corresponds to a field of ~40kV/cm so that for C 3 + the highest surviving state is"max = ^6* Our more recent experiments use electrostaticdeflection fields of ~4 kV/cm corresponding to naax - 44and indeed we observe a factor of 1.8 increase in C 2 +signal at the peak under these conditions.Less well understood is the effect of small fieldsin the interaction region which mix I states for a givenn and increase the DR cross section. This effect isextensively discussed in the paper by Griffin, Pindzola,and Bottcher,3 contained in these proceedings. Theycalculate the increase in the effective DR cross sectionwhich would be obtained with complete Stark mixing ofstates. In our work we find that, for the Na-like ionsP**+, S 5 +, and Cl 6 +, the measured rates are intermediatebetween the no mixing and total mixing predictions whilefor the Li-like C 3 + and 0 5 + complete mixing seems aDR MEASUREMENTS ON MULTICHARGED IONSbetter f i t . Exact calculat ions for speci f ic f i e l d con-d i t ions remain to be done.This research was sponsored by the I). S. Departmentof Energy, Division of Basic Energy Sciences underContract No. DE-AC05-840R21400 with Martin MariettaEnergy Systems, Inc.REFERENCES1 . P. F. D i t t n e r , S. Datz, P. D. M i l l e r , C. D. Moak,P. H. Ste lson, C. Bot tcher, W. B. Dress, G. D. A l t o n ,and N. Neskovic, Phys. Rev. L e t t . 5 1 , 31 (1983);E lec t ron ic and Atomic C o l l i s i o n s , "ETchler, H e r t e l ,and S t o l t e r f o h t , eds . , North-Holland Press, 1984.2 . D. McLaughlin and Y. Hahn, Phys. Rev. A27, 1389(1983); Phys. L e t t . 88A, 394 (1981); PTiy?. Rev. A28(1983), in press.3. D. C. G r i f f i n , M. S. Pindzola, and C. Bot tcher ,t h i s volume.DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.

Dielectronic recombination measurements on multicharged ions

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Dielectronic recombination measurements on multicharged ions

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