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Pottinger, Sabrina

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Southampton (e-Prints Soton)

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When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g.AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, paginationhttp://eprints.soton.ac.ukUNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF ENGINEERING AND APPLIED SCIENCES AERONAUTICS AND ASTRONAUTICS Doctor of Philosophy INVESTIGATION OF STEADY STATE CHARACTERISTICS OF HOLLOW CATHODE INTERNAL PLASMAS  USING OPTICAL EMISSION SPECTROSCOPY By Sabrina Pottinger 11 Electron bombardment ion thrusters are at the forefront of advan<:es in alternative forms of spa<:e-<:raft  propulsion.  They offer  advantages over <:hemi<:al  ro<:kets  in terms of mass savings and pro-pellant utilisation and are suited to missions requiring high spedfk impulse and high delta V. The te<:hnology has been su<:<:essfully  tested on the Deep Spa<:e  1 mission and has been proposed for use on more demanding missions to the outer planets. Electron bombardment ion thrusters may require lifetimes in ex<:ess  of 50,000 hrs for future mis-sions.  In order to extend lifetime and enhan<:e performan<:e the operation of <:ritkal <:omponents must be thoroughly investigated.  One su<:h  <:omponent is the hollow <:athode  (HC) acting as the primary electron sour<:e for the thruster. The use of alternative propellants su<:h  as argon and krypton must be investigated as they offer <:ost  savings over xenon propellant.  Any adverse effects that arise due to the use of these propel-lants must be identified.  The underlying physks of HC operation must be in<:orporated into the development of a<:<:urate  theoreti<:al models.  To this end experimental data sets may be used for <:ode  validation, hen<:e  avoiding further <:ostly experimental <:haraderisation and testing. The hollow <:athode  internal plasma has been investigated using optkal emission spedros<:opy. Localised measurements were re<:orded at three positions along the hollow <:athode axis.  The insert temperature and plasma parameters have been <:alculated for various discharge currents, mass flow rates and electrode separations. The ratio of <:ontinuum radiation was used to calculate the insert surface temperature and the ratio of dis  <:rete  spedral line  radiation was  used to  calculate the eledron temperature and ionisation fradion. The determined electron temperature values enabled the electron density to be mlculated. The dependen<:e of plasma parameters and insert temperature on hollow <:athode operating parameters has been assessed. CHAPTER 2.  BACKGROUND AND LITERATURE REVIEW  13 discharge produced and hence the thruster performance. The lifetime of the HC is critical in dictating the life of the thruster as a  whole, for this reason efforts have been made to study the behaviour of the component and improve its performance as mentioned in the previous section. The electron source in the early ion thrusters was a tungsten or tantalum filament wire, which emitted electrons thermionically [24].  Oxide cathodes consisting of a spiral ribbon of nickel tungsten mesh coated with alkaline earth carbonates that formed the oxide emitter were also used [6].  These electron sources were soon superseded by HCs for the following reasons, •  They are rugged and mechanically simple, •  Possess long lives well in excess of 20,000hrs, •  Restart reliably and repeatedly even after long shutdown periods, •  Can be easily tested on the ground and may even be exposed to the atmosphere. The HCs in use today consist of a  porous tungsten insert impregnated with a mixture of earth metal oxides that lower the workfunction of the material allowing electron emission. The HC insert is surrounded by a tantalum tube closed off at one end with what is referred to as an orificed tip.  The orifice diameter is  typically an order of magnitude smaller than the insert diameter(see Fig.  2.3).  This tube is then surrounded by bifilar heater coils and enclosed in a layer of insulation. The HC outer casing is stainless steel.  The HC operates by passing propellant through the dispenser tube.  The HC is  then heated to a  temperature of T  rv  10000e to facilitate the thermionic emission of electrons.  Electrical breakdown then occurs at the keeper electrode, which is at a positive potential.  The plasma generated in the region between the keeper and the HC tip then expands into the orifice and via a  mechanism which is  not understood,  an internal plasma is  generated.  The process of electrical breakdown occurs over a time period of less than one millisecond.  The increased electron emission from this plasma results in an electric field  being established across the plasma sheath adjacent to the insert surface encouraging further emission.  This electron emission mechanism is known as field enhanced thermionic emission.  The electrons released from the insert (primary electrons) interact with the propellant gas causing excitation and CHAPTER 2.  BACKGROUND AND LITERATURE REVIEW  21 The plume mode is  characterised by  a  high voltage noisy discharge;  a  bright luminous plume extends downstream of the HC tip.  The plume mode occurs at low mass flow  rates, low  discharge currents and large anode to HC separations.  The spot mode is associated with a  low  voltage, low noise discharge.  An intense luminous spot is  observed near the orifice.  The spot mode occurs for high mass flow  rates, high discharge currents and small anode to HC separations. The transition between the two modes occurs rapidly if operating Voltage Increasing mass flow rate Current Figure 2.5:  Current voltage characteristics for the spot to plume mode transition parameters such as mass flow  rate and discharge current are varied and is accompanied by hysteresis as shown by Fig.  2.5. A  variety of theories have been put forward in an attempt to explain mode change char-acteristics.  The physical phenomena that may be responsible for  plume  to spot mode transitions are listed below. •  Movement of the HC sheath boundary •  Expansion of current emission from the orifice •  Keeper current collection mechanism •  Anode current collection mechanism CHAPTER 2.  BACKGROUND AND LITERATURE REVIEW  52 cal pyrometry cannot be applied to current impregnated tungsten inserts because of the differences in the thermal characteristics of the materials.  Accurate temperature profiles are of great importance if He life limiting factors are to be fully understood.  Studies into the production of high energy ions have yet to explain the presence of ions with energies greater than the discharge potential, which may cause He failure from tip erosion.  Spec-troscopic investigations have used externally placed diagnostic equipment,  therefore the data collected is integrated along the line of sight of the optics used i.e.  along the length of the He. As a consequence the plasma may not be resolved in the axial direction and in-formation regarding gradients in plasma parameters and temperature distributions cannot be collected.  The lack of data pertaining to the internal plasma hinders the development of reliable He simulations, which require experimental results for code validation. The previous experiments performed on Hes have highlighted the areas of He operation which require further investigation.  The mechanisms responsible for observed phenomena such as increased electron emission due to the use of enclosed keepers and the production of high energy ions have yet to be identified.  A complete theory of mode change characteristics needs to be developed.  This theory must incorporate the influence of orifice geometry on mode transitions.  Also, the applicability of research findings obtained for Hes operated in diode configuration to Hes operated in full thruster configuration must be assessed, as only one mode (the spot mode) is  observed in thrusters.  This implies that the characteristics of He operation must be decoupled from external factors including anode geometry and test facility background pressure in order to develop a universal theory for He operation. The plasma processes occurring within the orifice require extensive investigation. The small scale of the plasma (typically diameters less than Imm) and the presence of high energy ions  has prohibited experimental investigations.  The contribution made to the electron current due to orifice processes is  not known and the ion fractions within the orifice need to be determined.  Within the insert region experimental investigation has been prohibited for the same reasons.  Temperature gradients for the insert surface and electrons within the He need to be determined and their dependence on operating parameters assessed. Radial distributions of the internal plasma have yet to be investigated. CHAPTER 2.  BACKGROUND AND LITERATURE REVIEW  .53 The chemistry of the tungsten impregnate material may provide an insight into the end of life mechanisms for the component.  The mobility of barium atoms and ions  requires investigation in order to explain observed deposits at the rear of HCs. A precise mechanism for plasma breakdown has yet to be established.  In order to under-stand the HC start up mechanism, the region where breakdown initially occurs and the transfer of this plasma to the insert region of the HC needs to be investigated. The research objectives of this investigation outlined in Chapter 1 will aim to address the dependence of plasma parameters and insert temperature on axial position.  The influence of external factors such as electrode geometry and the inert gas propellant used will  be investigated.  These results will  provide a  basis for  comparison with results obtained [or the HC discharge plasma, which includes the plasma within the orifice and inter electrode space as well as the insert region.  This comparison will provide an insight into the processes which occur within the orifice and between the electrodes and will indicate the difference between the HC internal plasma and the orifice plasma. The following chapter will describe the equipment developed and the techniques used in order to record spectra at various positions along the axis of a modified T6 HC with the aim of calculating temperature distributions and plasma parameters. CHAPTER 3.  EXPERIMENTAL APPARATUS  65 •  The gas sampling cylinder was evacuated by opening valve D and gradually opening and then  closing  valve  E  until the PC readout displayed  a  pressure of less  than 10mbar.  Valves D and E were then closed. •  The propellant gas bottled was then opened and the regulator outlet pressure set to the desired value.  The regulator outlet valve and valve A were opened to charge the feed  line with propellant.  The Negretti needle valve was  set to the required value and valves Band C were opened. •  To begin data acquisition a command was sent via the computer keyboard, immedi-ately followed  by the closing of valve C and opening of D. •  The gas flow,  set  by the needle valve,  was directed into the sampling cylinder.  The rate of rise of pressure within this cylinder was then used to calculate the mass flow rate. •  When data acquisition was completed the mass flow rate was automatically displayed on the PC monitor.  Further measurements were taken by evacuating the sampling cylinder, setting the needle valve to the desired value and res  ending the command to begin data acquisition. 6.6..----------------, y  = .{).osalx' +  O .S842x  +4.9557  6.4  .. .. ..  Ri  ~ O:9999 .. ... ............ . . =  6.2  c :0  6 CD &  5.B 1  5.6 ~ 5.4 5.2  .. ...... .. .... ....... .. ......... ... . +Calibrallon 1 • Calibration 2 o Calibration 3 5~----~------,-----,~----~ 023 Mass Row Rate (mgls) 4 Figure 3.12:  Mass flow  rate calibrations for krypton propellant CHAPTER 5.  EXPERIMENTAL RESULTS I ~ ~---------------------------------------. 350  ..................... .... . ........... ..... ................ . ......... ..... . 300  ........ ................ ................................... . 250 ~200 "  11. 150 100 +mdot= 1.0  Ill/I's • mdot=3.0  Ill/I's 50 0+--+--~~~--~-+--+--r~r-~~==T==r=9 2  4  6  8  10  12  14 Anode Current (Al 98 Figure .5.10: Discharge power  as a function of current for argon propellant in geometry 2 ~Tr I I= ·-+- ==V =. (A ~ )~ m= OO =t== l ~ . l= Ill/I' ~ S ___  ~~ V ~ (~ ~ moo 7=t=~ l ~. l = lll/I' = s,- 1 ------------~ 30  fl- O - V (A) moot=3.0 Ill/I's -&- V(~  mdot=3.0  Ill/I'~ .... ... ... ... .. ..... .... .... . ~  : ~ i~~i----F¥H:4t:~f 1l!  15  ...... ... .... .... ... ........... ...... .... ..................... ...... ....... . ~1 0 .~  .... ............................... . 5  ...... .................. ....... ..... ~~  .. 1  ...  1  .. 2  4  6  8  10  12  14 Anode Current (A) Figure 5.11: Voltage current characteristics for argon propellant in geometry 2 CHAPTER 5.  EXPERIMENTAL RESULTS I  109 The krypton spectra display intense lines for  wavelengths longer than 740nm and higher intensity spectral lines are observed at shorter wavelengths than in the case of an argon discharge (see Fig.  5.20), with peak intensities over 500 counts.  The xenon spectra show increased activity in the visual region of the spectrum relative to the argon and krypton spectra, as shown in Fig.  5.22.  There are a high number of lines in the wavelength range A = 240 - 500nm, although the intensities are lower than those observed for the krypton discharge the line width is  reduced in most cases but still not to the level  necessary for identification purposes. CHAPTER 8.  CONCLUSIONS AND FUTURE WORK  194 resolution as well  as  time resolved measurements that allow the evolution of the plasma to be investigated and quantitative analysis of the spectra to be performed.  This will provide information on the state of the plasma at various axial positions prior to, during and directly after breakdown. APPENDIX C.  PROGRAM FOR INSERT TEMPERATURE CALCULATION  21.5 CHARACTER(len=lOO)::filel, file2,  file3 !Input Data, polynomial curve fit filel='C: \Results\Input Files\Arl.dat' !Writes data:  wavelength, intensity and temperature file2='C:\Results\Temp.dat' !Writes temperature values to file file3='C:\Results\  Temperature.dat' nodata=392 jmax=392 kmax=O mmax=76636 ALLOCATE(Temp(mmax,l)  ) ! Body of WallTemp ! Read in data over wavelength range 650 to 900nm ! Opening of the data file OPEN(30,FILE=filel,IOSTAT=er,STATUS='old',ACTION='read') ! Opening of the new data file OPEN(31,FILE=file2,IOSTAT=er,STATUS='unknown',ACTION='readwrite') ! Opening of the new data file APPENDIX C.  PROGRAM FOR INSERT TEMPERATURE CALCULATION  216 OPEN(32,FILE=file3,IOSTAT=er,STATUS='unknown',ACTION='readwrite') ! Initialise variables i=l !Reading wavelength and intensity values DO i=l,nodata,l READ(30,*) RawData(i,l), RawData(i,2) END DO CLOSE(30,IOSTAT=er) !Temperature calculation k=l j=l n=O kmax=jmax WRITE(*,*)tlJ= tI,jmax WRITE(*,*)tlk= tI,kmax DO k=l,kmax,l DO j=l,jmax,l IF(j .GE.(k+  1) )THEN !WRITE(*,*) RawData(k,l),RawData(j,l) T=O.014413043*( (RawData(k,l  )-RawData(j,l))  / APPENDIX C.  PROGRAM FOR INSERT TEMPERATURE CALCULATION  217 (RawData(k,l )*RawData(j ,1))) /Log( (RawData(k,2)/ Raw Data(j ,2))* ((RawData(k,  1) ** 5) / (RawData(j,l  )**5))) IF(T.LT.-1)THEN T=T*(-l) END IF IF(T.LT. 10000 .AND. T.GT.O)THEN WRITE(32,*) RawData(k,1),RawData(k,2),RawData(j,1), RawData(j,2), T WRITE(31,*) T n=n+1 END IF END IF END DO END DO !Mean temperature and standard deviation calculated !Read temperature values from beginning of file Rewind(31) m=l DO  m=l,mmax,l READ(31,*) Temp(m,l) END DO APPENDIX C.  PROGRAM FOR INSERT TEMPERATURE CALCULATION  219 IMPLICIT NONE INTEG  ER(KIND=4) ::  j,N ! Compute mean mean =  0 DO j =  l,N,+l mean =  mean+Temp(j,l) END DO mean =  mean/real(N) ! Compute variance variance =  0 DO j =  l,N,+l variance =  variance+(Temp(j,  1  )-mean)* (Temp(j, 1  )-mean) END DO variance =  variance/real(N-1) ! Compute standard deviation sd =  DSQRT(  variance) END SUBROUTINE statistics 

Investigation of steady state characteristics of hollow cathode internal plasmas using optical emission spectroscopy

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