Bekefi, G.

Shefer, R.E.

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STARK BROADENING INDUCED BYTHE INTENSE ELECTRIC FIELD OFA PULSED RELATIVISTIC MAGNETRONbyR. E. Shefer and G. BekefiPreprint PFC/JA-78-9Plasma Research ReportPRR 78/41November 1978STARK BROADENING INDUCED BY THE INTENSE ELECTRICFIELD OF A PULSED RELATIVISTIC MAGNETRONbyR. E. Shefer and G. BekefiDepartment of Physics and Research Laboratory of ElectronicsMassachusetts Institute of TechnologyCambridge, Massachusetts 02139ABSTRACTThe time-resolved Stark broadened spectrum of the H line. ofhydrogen has been observed in a pulsed microwave field of 20nsecduration, a frequency of 4.6GHz, and a field strength of -l00kV/cm,generated by a relativistic electron beam magnetron. The resultingline broadening can be used to determine the electric field strengthand its spatial mode structure.This work was supported by the U.S. Air Force Office of ScientificResearch (Gran't AFOSR77-3143B), and in part by the National ScienceFoundation (Grant ENG77-00340-AOl).-1-Novel microwave sources1-5 capable of delivering up to 4GW ofpower, generate rf electric fields of several hundred kV/cm in as-sociated microwave guiding structures. In this note we explorethe feasibility of observing these fields through the Stark broaden-ing of atomic spectral lines. Such measurements have a two-foldpurpose. First, they provide a means of checking the predictionsof Stark broadening theory in time-dependent, sinusoidal rf fields.And secondly, once these checks have proved satisfactory, the result-ing line broadening can be used in the determination of the absoluteelectric field strength and of its spatial mode structure in thewaveguide system in question. To date all experiments of this typehave been performed at electric field strengths at least an orderof magnitude lower than those reported here. The accompanyingtheory and numerical calculations of spectral line shapes6-10 arelikewise known in a regime that may not be fully applicable to thepresent experiments. For example, an important quantity in the11,12theory is the ratio of the frequency of precession of the atomicdipole moment about the direction of the electric field, to the fre-quency of the rf oscillations. This ratio written out in fullR E (3nhe /me) (E/w) (1)is then a statement concerning the degree of adiabaticity of themotion. [Here e and m are the charge and mass of the electron, re-spectively, n is the principal quantum number of the upper state ofthe transition; E is the electric field strength, and w is the angu-lar frequency of the rf perturbation.] In experiments reported- 2 -6 7 8hitherto the parameter R<5; in our measurements R~170. Furthur-more, in the earlier experiments, the electric field was either acontinuous wave or, if pulsed, there were several thousand rf periodsper pulse. In the present experiment the pulse is comprised of only-75 periods of the rf oscillation, a fact that may have to be in-corporated in a comprehensive analysis.The experimental arrangement is illustrated in Fig. 1. Thepower from the magnetron is extracted from one of the six resonators(which form its slow wave structure1 ' 2 ) and is guided by means of asection of evacuated S-band waveguide towards the transmitting horn.The 17nsec long wave pulse of frequency 4.6GHz travels down theguide in the principal TE1 0 mode. A quartz capillary Geissler tube(0.75mmID), filled with 4Torr of spectroscopically pure hydrogenpasses down the middle of the rectangular waveguide, with the tubeaxis oriented parallel to the rf electric field. The discharge tubeis provided with stainless steel electrodes situated outside thewaveguide, so that a weak plasma can be struck with a dc field typi-cally equal 200W/cm and at a dc current of -lOmA. The light from thedc discharge is used to align the optics. But, more importantly, itis employed as*a "keep-alive" during the actual experiments. Bymaintaining the discharge tube lit one is assured of having copiouselectrons to initiate the rf breakdown, with the result that fairlygood shot-to-shot reproducibility is achieved. The diameter of thedischargetube is purposely chosen to be small (0.75mmID) so as tocause a minimal perturbation to the wave. We checked this by mea-suring the power flowing from the magnetron to the horn with andwithout the discharge tube, but found no noticeable change.The light from the discharge is focused onto the 10OPm wide'.3-entrance slit of a half meter Jarrell-Ash monochromator. The lightfrom the exist slit enters the photomultiplier tube shielded bothelectrically, and against the x-ray flux emitted by the magnetronwhen the -0.4MeV electrons strike the anode block. The photomulti-plier dynodes are gated off except for a -lpsec wide time intervalspanning the microwave pulse. In this way we prevent saturation ofthe photomultiplier by the light from the steady keep-alive dis-charge. The output from the photomultiplier is displayed on a fastoscilloscope triggered by the discharging of the Marx generator,which is the source of energy for the magnetron., 1 3Figure 2 shows a set of oscilloscope traces of a single shot:that is, the light intensity from the hydrogen-filled tube, themicrowave power emanating from the horn antenna, the magnetroncurrent, and the magnetron voltage. We see that the microwaveburst is accompanied by a pronounced signal from the photomultiplier(of approximately 2V peak) indicating the occurrence of rf ioniza-tion. We note that the keep-alive discharge produces a photomulti-plier signal of only 20mV. Thus, the microwave field increases thelight intensity by two orders in magnitude, which is in part due toproduction of additional electrons and in part due to electronheating by the microwaves. We point out that there is no measur-able delay between the onset of the microwave pulse and the onsetof rf breakdown. Measurements made in other gases indicate thatat the level of rf field strengths with which we are dealing here(~lOOkV/cm), the "breakdown lag" is less than 2nsec.Figure 2a gives the relative light intensity for a given set-ting of the monochromator wavelength. To obtain the line shape ofthe H line, the monochromator is advanced to a new wavelength set-ting and all measurements like those shown in Fig. 2 are repeated.Some thirty successive shots lead to the line profile illustratedin Fig. 34. Despite not able shot-to-shot variations, a clearlydefined broadened line profile emerges. We cannot establish thelevel of the continuum with precision and therefore we can onlyestimate the value of the line width. We find that the full widthat half power (FWHP) is approximately 6A. This is to be comparedwith the instrumental profile shown in Fig. 3c whose full width0is 1.OA.We see from Fig. 2a that the intense burst of light duringthe microwave pulse is followed by long lasting "afterglow" lightradiated by the plasma as it slowly decays by diffusion and recom-bination processes. Figure 3b shows the line shape at the peak ofthis afterglow, that is, some 150nsec after the microwave burst hasceased. It is seen that the line width had decreased dramaticallyand equals, within the accuracy of this determination, to the in-strumental line width. This is most gratifying: it demonstratesthat the line broadening shown in Fig. 3a is indeed predominantlydue to the rf &lectric field, and that Stark broadening by the ,plasma ions (and electrons) is not an important contribution. [Anindependent estimate suggests that the upper limit on the charged14 -3 15particle density is -.O cm which results in a line-broadeningcontribution of no more than 0.4A.]We do not have at this time a detailed line shape calculationcorresponding to the parameters of our experiment. In lieu ofthis we take tie quasi-static theory as a first approximation inour attempt to relate the half power width AX12 to the peak elec--5-tric field amplitude E0 of the microwave signal. The appropriateexpression is122 2 2 2AX1 /2  [3XE h(n2-n 2 )/2r2 mec]E (2)where X is the wavelength of the transition whose upper and lowerstates are n 1and n2 For the H line one finds thatAX 1/2 = 0.0579 E (3)0where now AX 1/2 is given in A and E is in kv/cm. Using the mea-0sured value, AX 1/26A, yields E =104kV/cm. This is to be comparedwith the value of 102kV/cm obtained from measuring the microwavepower output from.the horn antenna (Fig. 2b), and then integratingPoynting's flux over the waveguide cross section. -In conclusion then, we have reported preliminary observationstheof Stark broadening in sinusodal rf fields where/intensity greatlyexceeds that employed by earlier workers. Detailed line shape mea-surements and detailed line shape calculations are yet to be done.Nonetheless, the technique may prove useful as a diagnostic of thestrength of such microwave fields. By focusing different sectionsof the discharge tube onto the spectrometer slits, and by properlyorienting the tube relative to the direction of the electric field,one may be able to map out the entire spatial distribution of thefield. This may be advantageous in more complex situations wherethe mode structure may not be well-known.- 6-r -REFERENCES1. G. Bekefi and T. J. Orzechowski, 11hys. Rev. Letters 37, 379,(1976).2. A. Palevski and G. Bekefi, Plasma Research Report No. PRR 78/32July 1978, Plasma Fusion Center, Massachusetts Institute ofTechnology, Preprint PFC/JA-78-5 (to be published in Phys.Fluids).3. N. F. Kovalev, B. D. Kol'chugin, V. E. Nechaev, M. M. Ofitserov,E. I. Soluyanov and M. I. Fuks, Sov. Tech. Phys. Lett. 3(10),430 (1977) [Pis'ma Zh. Tekh. Fiz. 3, 1048 (1977)]; also V. E.Nechaev, M. I. Petelin and M. I. Fuks, Sov. Tech. Phys. Lett.3(8), 310 (1977) [Pis'ma Zh. Tekh. Fiz. 3, 763 (1977)].4. A. N. Didenko, A. S. Sulakshin, G. P. Fomenko, Yu. G. Shteinand Yu. G. Yushkov, Pis'ma Zh. Tekh. Fiz. 4, 10 (1978).5. A.. N. Didenko, A. S. Sulakshin, G. P. Fomenko, V. I. Tsvetkov,Yu. G. Shtein and Yu. G. Yushkov, Pis'ma. .Zh. Tekh. Fiz. 4,823 (1978).6. V. E. Mitsuk, Sov. Phys. Tech. Phys. 3, 1223 (1959).7. W. W. Hicks, Lawrence Berkeley Laboratory Report LBL-2470,December 1973, University of California (unpublished)8. W. W. Hicks, R. A. Hess and W. S. Cooper III, Phys. Rev. 5,490 (1972).9. A. Cohn, P. Bakshi and G. Kalman, Phys. Rev. Letters 29, 324(1972); 31, 620 (1973).10. P. Bakshi, G. Kalman and A. Cohn, Phys. Rev. Letters, 31, 1576(1973).11. G. V. Sholin, Sov. Phys.-Doklady 15, 1040 (1971)-. 7 -12. E. K. Zavoiskii, Yu. G. Kalinin, V. A. Skoryupin, V. V. Shapkinand G. V. Sholin, Sov. Phys.-Doklady 15, 823 (1971).13. T. J. Orzechowski and G. Bekefi, Phys. Fluids 19, 43 (1976);also Plasma Research Report No. PRR 78/28, July 1978, PlasmaFusion Center, Massachusetts Institute of Technology PreprintPFC/JA-78-4 (to be published in Phys. of Fluids).14. P. Felsenthal and J. M. Proud, Technical Report RADC-TR-142(June 1965), Techniques Branch, Rome Air Development Center,Air Force Systems Command, Griffiss Air Force Base, New York(unpublished) .15. G. Bekefi, editor "Principles of Laser Plasmas," Wiley 1976page 558.- 8-FIGURE CAPTIONSFig. 1.Fig. 2.Fig. 3.- .9 -Schematic of the experiment.Oscilloscope traces of (a) the relative light intensityfrom the hydrogen discharge; (b) the peak microwavepower emitted by the horn; (c) the current drawn betweenthe magnetron cathode and anode; and (d) the voltage ap-plied across the magnetron electrodes. In (a) note theshort light pulse followed by a long afterglow.(a) The Stark-broadened profile of the H line of hydro-gen, observed during the microwave pulse; (b) the lineprofile of H in the afterglow, -150nsec after the micro-wave pulse; (c) the instrumental line profile. In (b)note that the H line sits atop a pronounced continuum.zZz< w0Z0 00WO0..>-(1)I0LuHC)Lu0D0:T &ci:0Ldzu0w 0J 0*J WUCH-J0JCo0a-00U)LA 0a. Wr o<[ wZ C/) fli'- O 'UI-J-J0H0-rq44.r4Enu-bi0-LtiQ'00U-H0C)(I)0-J:::)00 0-PHOTOMULTIPLIEROUTPUT (IV/div.)MICROWAVE POWER(200 MW/div.)MAGNETRON-CURRENT (8.8KA/div)MAGNETRONVOLTAGE (251 kV/div.)---,--- -40 nsecTIME -0Fig. 2Shefer & Bekefi(a)(c)(d)I-- 77-----I 0(Ce0('a)-1200(b0-8 -4 0WAVELENGTH (A)Fig. 3Phefeg & BekefiHc-nwHH0-J.4 12

Stark broadening induced by the intense electric field of a pulsed relativistic magnetron

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Stark broadening induced by the intense electric field of a pulsed relativistic magnetron

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