Lo, D.H.

Petrasso, R.D.

Wenzel, K.W.

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PFC/JA-91-34 Rev.Comment on "Cluster-Impact Fusion"Daniel H. Lo,t Richard D. Petrasso, Kevin W. WenzeltMarch 1992Plasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139This work was supported by LLNL Subcontract B116798, U.S. DOE Grant No. DE-FG02-91ER54109, DOE Magnetic Fusion Energy Technology Fellowship Program (t), andDOE Fusion Energy Postdoctoral Research Program (t).To be published in Physical Review Letters, March 30 1992.Comment on "Cluster-Impact Fusion"Beuhler, Friedlander, and Friedman (BFF) reported anomalously hugeD-D fusion rates while bombarding deuterated targets with (D20)+ clusters(N - 25-1000) accelerated to -325 keV1'2 [i.e. ;0.3 keV lab energy for D in(D20)+0 ]. However, from our analysis of BFF's fusion product spectra, weconclude that their D lab energy was ~50 keV. Therefore, no gross anomaliesexist. Also, from our analysis of the BFF beam-ranging experiments through500 ptg/cm 2 of Au,2 we conclude that light-ion-beam contaminants (e.g. D+of order 100 keV) have not been ruled out.BFF showed D-D proton peaks (~3 MeV) with widths (FWHM) of about330 keV obtained with a surface barrier detector (SBD)." 2 BFF1 attributethis width to differential energy loss of 3-MeV protons passing through the50-pg/cm 2 Al front layer of the SBD at various angles. However, we cal-culate this effect to be only ~1 keV.' Broadening due to energy stragglingis ~5 keV. 4 About 20 keV is the electronic detector noise specified by themanufacturer; however, in our experience, -50 keV is easily obtained. Thus,a broadening of still ~330 keV must be attributed to some other process,which we discuss henceforth.In the BFF experiments, the cluster beam hits a deuterated target -1.5cm from a 300-mm 2 SBD (Fig. 1). Thus the SBD subtends an angle (at itsextreme) of about 600. The D-D proton energy in the lab frame depends onthe D-D center-of-mass velocity (a function of the initial D energy) and theangle (0) between the proton and the initial deuteron directions (see Table1 and Fig. 1). From Table 1 and the extent of the BFF proton peak, weconclude that the fusing deuterons have a lab energy of ~50 keV. (A smallereffective angle, which should apply to BFF, would imply a higher energy,thus the "> " in > 50 keV.)As a test for beam contaminants, BFF argue that ranging their clus-ter beam through 500 pg/cm2 of Au will eliminate "cluster fusion" but notlight-ion contaminant fusion (from D+, D+, D+). This argument is not gen-erally valid. A contaminant of, e.g., ~100 keV D+ will indeed penetrate500 pg/cm 2 of Au; however, it loses 70 keV in ranging through,3 and thefusion yield is reduced by -20 times. [The same reduction in fusion yield1holds for D+ (D+7) at ~200 keV (300 keV).] Indeed BFF did observe in suchan experiment very roughly an order of magnitude decrease in the fusionrate (the experiment statistics are poor), but they interpret this as proofthat light contaminants are not causing their observed rate.2,5 We disagree.Furthermore, while BFF believe they have eliminated oxygenated light con-taminants from the ion source up to the first stages of their accelerator, wefeel that contaminants formed by, for example, ionization in the acceleratortube subsequent to splash-back from the apertures have not been convinc-ingly precluded."'7 Finally, we note that with a ~100 keV D+ (~300 keV D+)contaminant, for instance, only of order 1 D+ (D+) per 3000 (10000) clusters[(D 20)+ at 1 nA] is needed to produce the BFF fusion rate.Therefore, from our analyses of the BFF data, from the negative theoreti-cal results,6 ,8 and from the negative cluster experiment with post-accelerationmass and energy analyses, 7 we conclude that a light-ion contaminant has notbeen ruled out. To do such will probably require post-acceleration mass andenergy analyses. 7We appreciate helpful comments from Prof. T. Tombrello of Caltech anda discussion with Dr. L. Friedman of BNL.D. H. Lo, R. D. Petrasso, K. W. WenzelMIT Plasma Fusion Center, Cambridge, MA 02139' R. Beuhler, G. Friedlander, and L. Friedman, Phys. Rev. Lett. 63, 1292(1989).2 R. Beuhler, Y. Chu, G. Friedlander et al., J. Phys. Chem. 94, 7665 (1990).3 H. Andersen and J. Ziegler, Hydrogen: Stopping Powers and Ranges in AllElements (Pergamon, New York, 1977).4 R. D. Evans, The Atomic Nucleus (Krieger, Melbourne, FL, 1982).s R. Beuhler, Y. Chu, G. Friedlander et al., Phys. Rev. Lett. 67, 473 (1991).' M. H. Shapiro and T. A. Tombrello, Phys. Rev. Lett. 65, 92 (1990).7 M. Fallavier, J. Kemmler, R. Kirsch et al., Phys. Rev. Lett. 65, 621(1990).8 C. Carraro, B. Chen, S. Schramm, and S. Koonin, Phys. Rev. A42, 1379(1990).2Table 1: D-D proton energy (MeV) depends on the D-D c.m. velocity (afunction of the D lab energy, ED) and the lab angle, B (Fig. 1). AE is theenergy (MeV) extent of protons collected by an SBD at 90* when it subtendsan angle of ;::60*, a value appropriate for the BFF experiments.ED(keV) 0* 60* 900 1200 1800 AE1 3.062 3.042 3.023 3.003 2.984 .03910 3.151 3.087 3.025 2.964 2.904 .12350 3.323 3.176 3.035 2.900 2.772 .276100 3.464 3.249 3.048 2.858 2.681 .391(Are. 300m//PRoJECTILEFigure 1: Geometry of BFF experiment.3-v-TAR GET 60

Comment on "cluster-impact fusion"

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Comment on "cluster-impact fusion"

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