Intense ion beam K α measurements on PBFAIIJ. Maenchen, T. A. Mehlhorn, D. F. Wenger, R. J. Leeper, D. J. Johnson, and T. R. Lockner Citation: Review of Scientific Instruments 59, 1706 (1988); doi: 10.1063/1.1140140 View online: http://dx.doi.org/10.1063/1.1140140 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/59/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Development of the laser evaporation ion source for lithium beam generation on the Particle Beam FusionAccelerator (PBFA-II) Phys. Plasmas 6, 3697 (1999); 10.1063/1.873629 Proton beam targets shot on PBFAII Rev. Sci. Instrum. 63, 4779 (1992); 10.1063/1.1143560 Simulation and interpretation of ion beam diagnostics on PBFAII Rev. Sci. Instrum. 59, 1709 (1988); 10.1063/1.1140141 Rutherford magnetic spectrograph for intense ion beam measurements on PBFAII Rev. Sci. Instrum. 59, 1700 (1988); 10.1063/1.1140138 Lithium plasma generation for PBFAII ion diodes AIP Conf. Proc. 146, 654 (1986); 10.1063/1.35818

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Intense ion beam Ku measurements on PBFA .. II J. Maenchen, T. A. Mehlhorn, D. F. Wenger. R. J. Leeper, D. J. Johnson, and T. R. Lockner

Sandia National Laboratories, P. O. Box 5800, Albuquerque, New Mexico 87185

(Presented on 15 March 1988)

Inertial confinement fusion research efforts at Sandia National Laboratories center on generating and focusing high~intensity light ion beams on the Particle Beam Fusion Accelerator PBF A-II. A time-integrating three-frame ion beam spatial monitor was developed for these experiments to determine proton and lithium ion beam uniformity and orbit characteristics by imaging the ion­induced Ka line radiation. The three views of the ion beam are made off axis in three diode quadrants at a 3.8-cm radius. This spatial monitor, in conjunction with other beam diagnostics, allows experimenters to determine the high-voltage focusing characteristics of the ion diode.

I. EXPERIMENTAL ARRANGEMENTS

PBFA-II is a 36~module accelerator configured to drive a single ion diode load symmetrically through two vacuum feeds (see Fig. 1), presently coupling 5 MV and 3 MA in a 44-ns, 12-TW pulse at the half-energy level. The ion beam is generated on the inner cylindrical wall of the anode (15.5-cm radius, 10 em tall) and is radially accelerated toward the axis, forming a cylindrically symmetric solid ion beam. The ion beam is divided into four azimuthal quadrants of which this diagnostic package records three: One quadrant of the beam converges to a 3.8-cm-radius conical target. The sec­ond focuses fully onto a thin (5000-A. gold) Rutherford scat­tering target on axis monitored above by a magnetic spec­trometer l and below by a multiframe filtered CR39 focal spot camera,z and then reexpands onto a - 3.8-em-radius inverted cone Ka target. The third converges similarly to the first but is apertured at large radius to diagnose beam orbit details (as discussed in Ref. 3).

II. APPARATUS

The purpose of the x-ray pinhole camera system is to identify the spatial distribution and orbit variation of ions originating from different anode positions. This knowledge may be used to improve the focusing characteristics of the ion beam. The system contains three cameras using a com­mon S-cm-thick, lead bremsstrahlung shield and film plane, each with adjustable magnification and pinhole diameter. The three lines of sight are separated by 90° in azimuth and are centered on a 3.8-cm-radius, imaging Ka radiation emit­ted by proton and lithium ion bombardment of aluminum and titanium targets, The targets are thin foils (25-,um AI, 50-,um Ti) supported on 250-,um stainless steel conforming to conic sections 37° from beam normal such that the target cone intersects the midplane of the ion beam at the 3.8-cm pinhole camera line-of-sight radius. These targets are each irradiated by a full-height ( 18° vertical) 80· azimuthal quad­rant of the ion beam, and the pinhole cameras each view a beam-target interaction region of ± 2.5-cm elevation with­in a ± 40° azimuthal window. For positional verification an array of 35 2-rnm-diam holes is punched through the targets at O~, 5-, and lO-mm elevations off the midplane at angular increments of7S.

The beam-induced Ka radiation from the target is imaged vertically through pinholes located 15 em above the midplane onto a common film plane 2S em above the mid­plane. The film cassette is a black anodized aluminum case holding an optical filter followed by a thin-emulsion film (Kodak SB5 or RAR 2492). For aluminum targets the opti­cal filter is 4.9-,um Al (0.58 transparency, 0.71 for 4.5-keV lines), while for titanium targets a 50~.um Be optical filter is used (0.95 transparency, 0.17 for 1.5-keV lines).

RUTHERFORD MAGNETIC SPECTROGRAPH

NEUTRAL ION SPOT

SIX FRAME dEldx ION

PINHOLE CAMERA

PIN DETECTOR ARRAY

FIG. 1. PBFA·II ion diode assembly. Symmetric top and bottom vacuum transmission lines feed a common ion diode load. Ion beam diagnostic ac· cess is within the top and bottom 27° cathode cones, surrounded by leakage electron bremsstrahlung emission.

1706 Rev. Sci.lnstrum. 59 (8), August 1988 0034·6748/88/081706-03$01.30 © 1988 American Institute of Physics 1106

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oL-~ __ L-~ __ J-____ ~ __ ~-L __ ~~ ____ ~

o 2 3 4 5 6

x lem)

FIG. 2. Raw density pinhole camera image.

m. DATA ANALYSIS

The film is developed in the manner described by Henke et al. 4 and digitized on a scanning microdensitometerS using a 300~,um step and N oA. = 0009 optics. The array is trans~ ferred to a VAX for analysis by the VIDA (Visual Ion Diag­nostic Analysis) program. Ii Each pinhole camera image (Fig. 2) is searched for the annular film edges (showing opaque in the scan). The common accelerator and specific camera axis are then identified by a rros search. The spatial fiducial array of 2-mm holes in the target provides azimuthal orientation and fine tuning of camera spatial positioning er­rors. The film density is the sum of chemical color, brems­strahlung (multi-MeV), and line (few keY) photon­induced signals. We are using line-induced net film density calibrations from Henke et al.4

,7 and Koppel and Boyle,1{ assembled by Palmer,9 to convert film density to photon fiuence (photonsl,um2

) on the film plane. The SB5 film sen­sitivity to bremsstrahlung creates a high background (N.D.-2.6), causing the Ka photons to drive the film into saturation, The RAR 2492, while less sensitive to line radi­ation, has reduced bremsstrahlung color (N,D,-O.4), yielding a better image,

As the pinhole camera film plane is horizontai, the line of sight axial, and the source conical, the camera magnifica­tion and solid angle vary with both radius and azimuth, The VIDA program unfolds this geometry to convert the film illumination (photonsl,um2

) to radiated fiuence as a func­tion of target position.

The thick target yiel.ds of proton and lithium ions im­pinging on these tilted targets are shown in Fig. 3 for radial ion paths and our 90· observation geometry, 10 The yields are derived from cross-section datalH4 (13% variation) with a shape prescribed by Cohen and Harrigan. ls The lithium yields are scaled proton data evaluated as u(Li, E)

= z2(J'(p, E I A). Figure 4 shows the envelope of possible proton-aluminum yields at 5 Me V for aU possible vertical angles by which a nonoverfocused ion beam (focal length greater than the anode radius) may intersect the target. This is the dominant uncertainty in unfolding the ion beam fluence on target. The discrepancy between the yields shown is due to the coupled ion energy deposition and target self-

1707 Rev. Sci. Instrum., Vol. 59, No.8, August 1988

'---r- ---+----i---l---L.-.. -.. c=!--=-""". -4----1 . i

FIG. 3. Thick target yield calculations for ions incident on targets inclined at 37' from beam normal, observed at 90". From the left these are P on AI, P on Ti, Li on AI, and Li on Ti.

absorption varying together with ion angle, Clearly this ion angle uncertainty (typically ± 47% variation) must be re­duced to use the diagnostic with confidence.

The masked quadrane provides the information re­quired to minimize the ion angle uncertainty. The ions from one quadrant of the diode are apertured at r = 12 cm by a 1.3-cm-wide, f3 = 26° slot cut in a 250-,um Mylar beamstop, Fiducials identify the azimuthal orientation where the heli­cal slot crosses the beam midplan.e. Angular deflection on the target (fJ) identifies the ion source elevation by z = r(J tan /3. The pinhole camera image in the masked quad­rant is converted to target yield in the manner discussed above. A covariance search identifies the highest intensity distribution centroids of the target Ka fiuence, which are then used as an average ion intercept location on the target. Connecting this axial position Ztgt with Zmask ballistically generates an average vertical ion angle functiont/>(z} used to unfold the incident ion fiuence from the target radiated fluence. It is important to note that this function changes from shot to shot due to variations in diode voltage, current, magnetic topology, beam neutralization, and anode curva­ture, so t/>(z) is used to unfold the 3.8-em-radius data for one

(:I

~ 5

C 2 4

~ 'II C

3 0 .... 0 .c ~ '0 2 :i >-

il -3 -2 ·1 0 2

Target Vertical Position (em!

FIG. 4, Thick target yield variations for underfocused 5-MeV protons on aluminum vs target elevation.

Particle-based diagnostics 1707

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E .£ 0 ...

-1

_2~ __ ~ __ ~ ____ L-__ -L __ -4 ____ ~ __ ~ __ ~

-60 -30 0 30 60 Theta (degrees)

FIG. S. Unfolded K" pinhole image (Fig. 2) projected onto a 3.8-em-radius cylindrical surface shown with kJ/cm1 contours.

shot only. The target at - 3.8-cm-radius is apertured by the axial scattering target holder, so a horizontal (¢ = 0°) ion orbit assumption is used.

Finally, the ion fluence at voltage calculated across the target conical surface is projected radially to display the inci­dent ion beam in (z, 0) as though the target were cylindrical rather than conical. Intensity variations due to I-D beam convergence on target are thereby removed and the data are expressed in the cylindrical coordinates appropriate to these ion diode experiments. Figure 5 displays the completed un­folding of the unapertured beam photograph shown earlier in various stages of analysis in units ofkJ/cm2 for the mod­eled voltage of 4.5 MeV.

IV. DATA INTERPRETATION

The masked quadrant ion orbit data allow experi­menters to vary the diode parameters to better focus all ions to the common axis. Such iterative improvements are cor­roborated by this Ka pinhole camera and by axial beam diag­nostics discussed in related papers. I ,2 As the Ka radiative yield varies with ion kinetic energy and the photon fluence scales linearly with ion charge, the modeling of ion beam propagation and coupling to target can affect the absolute ion energy analysis to differing extents for protons and lith­ium ions on aluminum and titanium targets. This modeling is performed with the PICRA Y code. 16 Ideally the thick tar­get yield should be calculated time dependently for the in-

1708 Rev. Sci.lnstrum., Vol. 59, No.8, August 1988

stantaneous voltage and multiplied by the modeled ion cur­rent-density weight at each target position and time step. Then the unfolded, time-integrated target yield should be used to scale the local current weighting. This level of com­plexity has not been performed at this time, but rather a mean ion voltage on target is estimated from the propagation model at peak target power and a single voltage unfold is performed. With these caveats the mean ion energy density on target may be derived with measurement error bars of ± 40%. Peak proton intensities recorded to date are 12 kJ/cm2 with 400-kJ total beam energy at a mean voltage of 4.5 Me V in these quarter- and half-power PBF A-II ion diode experiments on 37° off-axis conical targets. It is important to note that these unfolded Ka data suggest higher total ion beam energies than are inferred by ion current monitors; this discrepancy is under investigation. Qualitative values from shot to shot and the ion orbit infonnation are regularly used to iterate diode configurations even while the absolute values remain in question.

ACKNOWLEDGMENT

This work was supported by the U.S. DOE under con­tract No. DE-AC04-76DP00789.

'R. J. Leeper, W. A. Stygar, J. R. Lee, R. P. Kensek, J. Maenchen, D. E. Hebron, and D. F. Wenger (these proceedings).

2W. A. Stygar, R. J. Leeper, L. F. Mix, E. R. Brock, J. E. Bailey, J. Maen­chen, and T. Mehlhorn (these proceedings).

3D. J. Johnson, R. J. Leeper, W. A. Stygar, R. S. Coats, T. A. Mehlhorn, J. P. Quintenz, S. A. Slutz, and M. A. Sweeney,J. Appl. Phys. 58,12 (1988).

'B. L. Henke, F. G. Fujiwara, M. A. Tester, C. H. Dittmore, and M. A. Palmer, J. Opt. Soc. Am. B 1,828 (1984).

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'B. L. Henke, 1. Y. Uejio, G. F. Stone, C. H. Dittmore, and F. G. Fujiwara, J. Opt. Soc. Am. B 3,1540 (1986).

8L. Koppel and M. J. Boyle, Advanced Research and Applications Corp. Document No. FR-81-112-Sec. IV, 1981.

9M. A. Palmer, Sandia National Laboratories (private communication). lOT. A. Mehlhorn, Sandia National Laboratories Report No. SAND86·

0016, p. 26, 1986. "C. H. Rutlege and R. L. Watson, At. Data Nue!. Data Tables 12, 195

(1973). 12G. Basbas, W. Brandt, and R. Laubert, Phys. Rev. A 7,983 (1973). DH. Tawara, Y. Hachiya, K. Ishii, and S. Morita, Phys. Rev. A 13, S72

(1976). 14R. K. Gardner and T. I. Grey, At. Data Nucl. Data Tables 21, 515

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Miller, Bull. Am. Phys. Soc. 30,1603 (1985).

Particle-based diagnostics 1708

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