Volume72A, number4,5 PHYSICSLETTERS 23 July 1979

EFFECTS OF A TRANSVERSE MAGNETIC FIELD ON He—Cd~LASER OUTPUT

T. HARA, M. HAMAGAKI, K. MATSUNAGA1 and T. DOTEThe Instituite of Physicaland ChemicalResearch,Hirosawa Wako, Saitama,Japan

Received27 March 1979

The effectof a transversemagneticfield B1 on thecataphoresistypeHe—Cdtlaser wasinvestigatedexperimentally.For

a 3 mmi.d. tube,the laserpowerwasfoundto reacha maximumatB1 = 300 G.

Whena transversemagneticfield is appliedto a constitutesthearraysis madeof ferrite with a residualconstantuniform positivecolumnof a low pressure magneticflux densityof 4.2 kG. It has a cylindricaldischarge,it may be expectedthat the loss of both form with a diameterof 20 mmand a length of 14.5positiveionsandelectronsto the wall is enhanced, mm. The transversemagneticfield B1 appliedto thecausinga correspondingincreaseof theaxial electric lasertube is variedfrom 0 to 600 G, whichwas meas-field and theelectrontemperature.Beckman[1] has uredby theHall effect.studiedthis phenomenontheoretically,andKaneda Typical characteristicsof the laseroutputasa[2] hasreportedexperimentalresultsof enhancement function of thedc dischargecurrent areshowninof the axial electric field and theelectron tempera- fig. 2. In fig. 2, it is seenthat theoutput powerclear-ture in a positivecolumnby a transversemagnetic ly increasesat someB1 values. Moreover,the currentsfield. The electrontemperaturein the lasermedium at the peaksin thesecharacteristicsmove towardsexcitedby the dc dischargehasa greatinfluenceup- smallervaluesasB1 increases.WhenB1 = 300 G, theon thelaserpower. In this paper,experimentalre- peakvalueof the characteristiccurve reachesa maxi-suitsof effectsof a transversemagneticfield on the mum.Thesepropertiesare similar for all heliumpres-cataphoresis-typeHe_Cd+laserare presented. sures~He andoventemperaturesTCd. Tcd for maxi-

The experimentalapparatusis shownin fig. 1. The mumlaseroutputpoweris alwaysaround260°C,lasertube usedis of 3 mm in-borediameterand about while the optimumvalueof PHeshifts to a higherval-80 cm in length.This lasertube has two Cd ovens, ue with increasingB1.eachof which is heatedby a separateoventempera- In fig. 3,the maximumpowerof the laseroutputture controllerfor maintenanceof a uniform Cd at- obtainedfor variousvalues°~~He’TCd and‘d’ nor-om densityalong thetube.The cavity of the4416 A malizedby that withoutB1, is shownasa functionHe_Cd+laserunit is composedof mirrorswith a re-flectivity of 99.9 percent.The laseroutputpower Permanent

Magnetdetectedby a solarcell wasfedto anX— Y recorder Cathode Probe A

togetherwith thedc dischargecurrent.The transverse Anodemagneticfield is controlledby changingthedistance 7betweentwo arrayscomposedof severalpermanent ~irrormagnets,which arearrangedsymmetricallyon both c Cd

sidesof the lasertube.The permanentmagnetwhich Temperature

Permanentaddress:Instituteof PlasmaPhysics,NagoyaUniversity,Nagoya. Fig. 1. Schematicdiagramof the experimentalapparatus.

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Volume72A, number4,5 PHYSICSLETTERS 23 July 1979

Ri. =4.5 Torr

500 lcd =260°C 1.5 ,/~~°N\ • 24J~\B(Gaus a) ,,,/o ~

~400 /~00 .2 / /\ 22~

~3O0 (~>~4~o~o ~1.0~Y’ \~~20O / / ‘\ 20

475 05 I

cn 100 // / ~ 0 200 ‘.00 600_~>~ N

600 B~(Gauss)C I

0 50 100 150 200 250 Fig. 3. Theratio of themaximumpowerof thelaseroutput

Di scharge Current (mA) andthat without B1 (a) andthe axialelectric field (.) asafunctionof the transversemagneticfield. Theexperimental

Fig. 2. Laseroutput versusdc dischargecurrentfor various conditionsfor thedata of theaxialelectricfield areasfol-transversemagneticfields. lows: ~He = 4.5 Torr, TCd = 2

40°C,Id= 105 mA.

of B1.The ratio of the laseroutputpowerreaches

theoptimumvalueatB1 = 300 G. The axial electric Theseexperimentalresultsshowthat the effectsfield is also plottedin fig. 3. Theaxial electricfield. of a transversemagneticfield on thecataphoresis-

in thepositivecolumnwithout themagneticfield typeHe—Cd~laserarequalitativelyexplainedby thewasobtainedfrom thefloating potentialdifference useof the Beckmanmodel [1]: whena transversebetweentwo probeswhich were locatedin thedis- magneticfield is appliedto the lasermedium,the losschargecenterregionwith a spacingof 20 mmin the of the plasmato thewall is enhanced.This causesaaxial direction.The floating potentialdifferencebe- correspondingincreasein the axial electric field.tweentheseprobescorrespondsto thespacepoten- Therefore,anincreasein theelectrontemperaturetial differencein the plasmadue to the axial uniformi- canalso be expectedasshownby Kaneda[2]. It isty of thepositivecolumn.Whenthe transversemag- consideredthat this increaseof theelectrontemper-neticfield is applied,a changeof thefloating poten. atureresultsin a moreeffectiveexcitationto the up-tial in the plasmais observed,sincethe voltagedrop perlaserlevel andleadsto an increasein the laserbetweenthe probeand the anodewhich is grounded outputpower.WhenB1 is largerthan300 G, theis increasedby B1.From divison of this changeof shift of the plasmafrom thelaserbeamaxis becomesthe floating potentialby the tube length in thepart larger,andthen thelaseroutputpowermay decrease.with the transversemagneticfield, thevariationof Detailswith quantitativeevidencewill be reportedE~withB1 canbe estimated.Thisvalueagreeswell in the future.with that evaluatedfrom the changeof thedischarge Fromthe aboveresults,it may be worthwhiletovoltagewith B1. Fig. 3 clearlyshowsthe increaseof note that the useof B1 providesa new simplemeansE~withB1.The electrontemperaturecould notbe for laserpowermodulation.measuredwith reliability due to the movingstriationin the plasma. The authorswish to thank Mr. H. Hasegawaand

WhenB1 appliesto thepartin whichthe two Mr. H. Yamatani,studentsof theMusashiInstituteprobesare arrangedsymmetricallyatboth sidesof of Technology,for their helpwith the experiments.the tubeaxis, thechangesof the ion saturationcur-rentsare detected.Namely, the ion saturationcur- Referencesrent of the probeon the side towardswhichE~X B1points,increasesand thatof the otherdecreases.From [11L. Beckman,Proc.Phys.Soc.61(1948)515.

thesechanges,it is concludedthat the plasmadensi- [2] T. Kaneda,Phys.Lett. 63A (1977) 288.

ty shifts in thedirectionof E5 X B1.

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