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Generation of high intensity electron beams from a duoplasmatron discharge
Generation of high intensity electron duoplasmatron discharge
beams from a
received 8 May 1975
P Varga, M Bruck and R Bruckmiiller,
lnstitut fiir Allgemeine Physik, Technical University Vienna, Austria
High intensity electron beams are usually produced by thermionic cathodes. For applications where increased energy spread is tolerable the use of plasma cathodes can be advantageous. In the present paper the dependence of beam qualities on discharge parameters for a Duoplasmatron electron gun is investigated. With extraction potentials of l-10 kV a maximum current density of 7 A cm -= with a total current of 0.5 A can be produced. The measured microperveances are in the region of 0.53
1. Introduction
The low pressure arc-type Duoplasmatron discharge has been developed for use as an ion source by Ardenne’, FrGhlich* and Demirchanov3 and since has been the subject of numerous investigations. On the contrary, the application of the Duoplasmatron as an electron source has been treated rather scantily.2*4-6 The present paper is intended to show the dependence of the quality of the extracted electron beam on the principal parameters: arc current, arc voltage, axial magnetic field strength, neutral gas density, and extraction potential. According to the above mentioned investigations the Duoplasmatron discharge can be described in the following way (Figure 1): The discharge between the hot cathode and the anode is compressed by means of a Zwischenelectrode (intermediate electrode) and an axially symmetric inhomogeneous magnetic field. The magnetic field can be produced either by a solenoid or a permanent magnet. In the discharge, two regions can be distinguished, i.e. the cathode and the anode regions. Between these regions, which differ considerably in charge carrier densities, usually one or more double layers are set up. The cathode region corresponds to a common diode discharge between the hot cathode and the quasi-anode formed by the double-layer at the entrance of the Zwischenelectrode channel. For the anodic discharge region this double layer acts as a quasi-cathode which produces a dense electron beam due to its concave form and the onset of the magnetic field. Because of the increased wall-recombination in the Zwischenelectrode channel the losses of ions in the plasma must be compensated. This is achieved by the formation of one or more additional double layers which assure enhanced ionization (Wasserrab7, Langmuir’). The onset of the magnetic field in the Zwischenelectrode channel reduces the wall losses by steadily decreasing the radial diffusion of the charge carriers in the anode region. The primary electrons emerge from the quasi-cathode and interact with the anodic plasma particles by inelastic and Coulomb collisions and also collectively with the plasma. This results in a partial thermalization of primary electrons and some heating of the secondary electrons. Vauucm/volume
25lnumber
9/l
0, 1975.
Pergamon Press LtdlPrinted
Cathode
Zwischenelectrode
I
! 1 Anode I I
Extroctionelect;ode
Figure 1. Discharge region of Duoplasmatron
electron source and development of potential and magnetic field strength along axis.
Charged particles leave the plasma mainly in the axial direction because of their reduced radial mobility due to the magnetic field. Since the electrons’ mobility is higher than that of the ions, a potential hill is produced between the anode and the region of highest magnetic flux density’. The height of this hill increases with the temperature of the secondary electrons”. This shape of the potential produces an ion flux in the direction to the cathode as well as to the anode. A fraction of the ions and electrons reaching the anode leaves the source through the anode orifice and forms a plasma jet. The density of the anodic plasma is usually 1013-1014 cme3. in Great Britarn
421
P Varga, M Bruck and R Bruckmdler:
Generation of high intensity electron beams from a duoplasmatron
discharge
2. Electron extraction from a Duoplasmatron
The formation of an electron beam is determined by the shape of the emission surface, the beam expansion due to space charge, and the energy spread of the extracted electrons. 2.1 Shape of emission surface. By application of a high potential between the source and extraction electrode, electrons can be extracted from the Duoplasmatron. The extraction electrode acts in respect to the source plasma like a probe. Thus, a layer is built up which shields the electrode against the plasma. This layer contains a surplus of negative charge carriers if the electrode is positive in respect to the plasma. Under the simplifying assumption of a flat extraction surface, one finds that the current density j- of the electron beam is independent of the applied potential and is only a function of plasma density np and the mean electron velocity in the plasma:
J- =np.e
J
ExtractIOn
electrode _-
G+-
.inl
k.T_ 2.n.m_
T_
= electron temperature; m- = electron mass. On the other hand, the thickness of the extraction layer depends on the applied potential according to the space charge ru1e11*12. The shape of the emission surface which determines the beam formation is influenced by the extraction potential. To form a parallel or slightly convergent electron beam the emission surface should be flat or slightly concave. To fulfil this condition with technically feasible potential gradients the anode is shaped as an expansion electrode (see Figure 1). Thus, the plasma density in the extraction region is reduced to lo”1Ol2 cmm3. Contrarily to particle sources with a fixed emission surface, the determination of an optimal electrode shape is possible for plasma sources in the case of specified extraction parameters only. If extended parameter regions should be covered the most suitable electrode shape must be found empirically.
5
0
IO
cm
Figure 2. Duoplasmatron
300
electron source.
B,,,= 0 5 T t
2.2 Energy spread of extracted electrons. The energy spread of extracted electrons usually is considerably larger for plasma sources than for thermionic sources due to their energy distribution in the plasma and mainly because of the increasing instabilities of the discharge at higher discharge current and voltage. If beam-plasma interaction in the extraction or drift region takes place the reaction on the source plasma has to be considered as an effect increasing the energy spread. 3. Results 3.1 The influence of discharge parameters on the extracted current. Figure 2 shows the Duoplasmatron source. As the working gas, argon has been used. The axial symmetric inhomogeneous magnetic field was produced either by a solenoid or by permanent magnets, the diameter of the extraction orifice was 0.8 mm. The discharge parameters were varied between the following values: 0.5 < ZE < 3A; 30 V < U, < 100 V; 2.10-’ < px < 8.10-’ torr; 0 < B,,, < 0.5 T. The extraction potential was varied from 1 to 10 kV. At 10 kV the maximum extracted electron current I_ was 0.5 A. Figure 3 shows the dependence of I- on the pressure pK in the cathode region and the discharge current Z,. These measurements were made at an extraction potential of 2 kV. The general 422
Figure 3. Dependence of extracted electron current on pressure and discharge current for argon discharge.
features can be understood in the following way ‘O: At low discharge currents the losses of charge carriers are mainly caused by the classical diffusion transversal to the magnetic field lines. The plasma density and therewith the electron current increase with the growing discharge current. Plasma instabilities occur at critical conditions depending on neutral gas pressure, magnetic field strength, and discharge current. They cause increased diffusion losses (Bohm diffusion), thus considerably lowering the plasma density and therewith the extractable current. Further increasing of the discharge current raises the plasma density again in spite of higher losses.
P Varga, M Bruck and R Bruckmidler:
Generation of high intensity electron beams from a duoplasmatron discharge
that for a beam current up to 50 mA, the compensation field can be considered as an asymmetric bell-shaped field according to Glaser14* The measured imaging properties of this lens system proved to be in good correspondence with the calculated ones if the location of the smallest beam diameter (beam waist) is assumed as the image point. At higher beam currents the image points are usually nearer to the source than expected. The reason for this behaviour might be the growing effect of the space charge with increasing beam current. Usually it is an advantage, especially at high discharge currents, to produce the source magnetic field by a solenoid to be able to optimize the electron yield by variation of the magnetic flux (camp. Figure 4).
P,=5xlci2torr
c t
,
4. Comparison electron guns
of the Duoplasmatron
source with thermionic
The microperveance PP can be stated as a measure for the intensity behaviour of a particle source : pP = 106. I. V312
100 60 o-05
0 125
025
0
‘5
(>5
Brnm ‘T Figure 4. Development of extracted electron current with magnetic field strength for various discharge currents. (Argon discharge.)
(A.V-3’2)
Electron guns with a microperveance. higher than 0.1 are qualified as ‘highly perveant’15. Figure 6 shows the microperveances calculated from the obtained beam currents depending on the beam energy. For a thermionic source acting in the I
field strength for electron extraction depending on the discharge current. The decrease of I- after passing this value can also be explained by the above mentioned mechanism: for a certain discharge current the current density varies according to the magnetic field strength up to the onset of abnormal diffusion. Figure
4 shows
the existence
of an optimal
magnetic
3.2 Influence of the fringing magnetic field on the beam formation. When a solenoid was used the fringing field in the extraction region was kept low by employing soft magnetic material for the anode plate; no substantial influence on the beam could be observed, The beam, however, is evidently influenced by the focusing effect of the compensation field in front of the anode if a permanent magnet is applied (Figure 5). It was shownI LL
-
Lrt-
-
Ia
\ \ \ 3
c
7
a
9
L IO
Figure 6. Development of microperveance varying with extraction
T
voltage for various discharge currents. (Argon discharge.)
Figure
5. Application of a permanent magnetic field strength along the axis.
magnet
Development
of
space-charge limited regime, however, the microperveance by definition-independent of acceleration voltage-remains constant. Figure 7 shows a comparison of the investigated Duoplasmatron source with other types of electron guns in respect of microperveance and extracted electron current density. The most striking feature of the Duoplasmatron compared with thermionic electron guns is the high current yield being practically independent of the extraction potential. Within this investigation the highest current achieved was 0.5 A. Beam currents of 1 A and more are feasible at higher cost (as higher discharge currents, forced cooling of the extraction electrode, etc6). Figure 8 shows the measured fwhm AEllz and the fwtm AElllo of the energy distribution function against the discharge voltage at constant discharge current ZE.Also, the energy spread increases with growing discharge current (Figure 9). Both facts can be explained in terms of the increasing instability of the discharge with increasing discharge current or discharge voltage. 423
P Varga, M Bruck and R Bruckmiiller:
Generation
of high intensity electron beams from a Duoplasmatron
Cathode Duoplas
discharge
llmlt
matron
~,CIGW(IYB tubes
Figure 7. Maximum current density vs microperveance types of electron source (Brewer15).
for various
A A
Figure 9. Measured dependence of fwhm and fwtm of energy distribution function vs discharge current. both applicable. With regard to the qualification of the Duoplasmatron for the production of electron and ion beams as well as photons (visible to vacuum uv) this configuration is suitable for many applications in atomic, surface, and plasma physics. With rising beam intensity the energy spread of the extracted electrons increases. In less favourable working conditions instabilities might occur which are excited either in the source plasma itself and are transferred to the electron beam or which are generated in the drift region because of beam-plasma interactions. 60
Acknowledgements We gratefully acknowledge the support of this work by the Austrian ‘Fonds zur Forderung der wissenschaftlichen Forschung’ (Forschungsprojekt 15 I5), the ‘ijsterreichische Studienand the interest of Prof Dr F gesellschaft fur Atomenergie’, Viehbock. Acknowledgements are especially due to Dr H Winter for many valuable suggestions. References
L!L V
’ M Ardenne, Tabellen der Elektronenphysik, Zonenphysik and Ubermikroskopie. Deutscher Verlag der Wissenschaften, Berlin (1956). 2 H Frohlich, Nukleonik, 1, 1959, 183.
Figure 8. Measured dependence of fwhm and fwtm of energy distribution function vs discharge voltage.
3 R A Demirchanov, Report BNL 767, 1962, 224. 4 M Ardenne, Explle Tech Phys, 9, 1961, 227. 5 G Gautherin and A Septier, J Phys Radium Phys appl, 12, 1962,
Usually an energy spread of 30-70 eV is to be expected with the stated source parameters. On the other hand, the energy spread for electrons extracted from thermionic sources is only a few eV for higher beam currents.
212A. ’ M A Zavyalov ci al, Znstr Exp Tech (USA), 14, 1971, No. 2, Pt 2. ’ T Wasserrab, Z Physik, 128, 1950, 575. ’ I Langmuir, Phys Rev, 33, 1929, 954. 9 C Lejeune, Thesis, Paris (1971). lo H Winter and B H Wolf, Plasma Phys, 16, 1974, 791. ’ ’ C Child, Phys Rev, 32, I9 1I, 492. ” I Langmuir, Collected Works of Z Langmuir, Vol 3 (edited by
5. Summary The special features of this configuration compared with thermionic guns are: high beam intensities are possible at low extraction potentials; steady and pulsed operations are also 424
G Suits). McMillan (Pergamon Press), New York (1961). I3 P Varga, Diplomarbeit, TH Wien (1975). ” A B El-Kareh and J C J El-Kareh, Elecrron Beams, Lenses and Optics, Vol 1. Academic Press, New York (1970). I5 G Brewer Focusing of chargedparticles, Vol 2 (edited by A Septier), Chap 3.2. Academic Press. New York (1967).
beams from a
received 8 May 1975
P Varga, M Bruck and R Bruckmiiller,
lnstitut fiir Allgemeine Physik, Technical University Vienna, Austria
High intensity electron beams are usually produced by thermionic cathodes. For applications where increased energy spread is tolerable the use of plasma cathodes can be advantageous. In the present paper the dependence of beam qualities on discharge parameters for a Duoplasmatron electron gun is investigated. With extraction potentials of l-10 kV a maximum current density of 7 A cm -= with a total current of 0.5 A can be produced. The measured microperveances are in the region of 0.53
1. Introduction
The low pressure arc-type Duoplasmatron discharge has been developed for use as an ion source by Ardenne’, FrGhlich* and Demirchanov3 and since has been the subject of numerous investigations. On the contrary, the application of the Duoplasmatron as an electron source has been treated rather scantily.2*4-6 The present paper is intended to show the dependence of the quality of the extracted electron beam on the principal parameters: arc current, arc voltage, axial magnetic field strength, neutral gas density, and extraction potential. According to the above mentioned investigations the Duoplasmatron discharge can be described in the following way (Figure 1): The discharge between the hot cathode and the anode is compressed by means of a Zwischenelectrode (intermediate electrode) and an axially symmetric inhomogeneous magnetic field. The magnetic field can be produced either by a solenoid or a permanent magnet. In the discharge, two regions can be distinguished, i.e. the cathode and the anode regions. Between these regions, which differ considerably in charge carrier densities, usually one or more double layers are set up. The cathode region corresponds to a common diode discharge between the hot cathode and the quasi-anode formed by the double-layer at the entrance of the Zwischenelectrode channel. For the anodic discharge region this double layer acts as a quasi-cathode which produces a dense electron beam due to its concave form and the onset of the magnetic field. Because of the increased wall-recombination in the Zwischenelectrode channel the losses of ions in the plasma must be compensated. This is achieved by the formation of one or more additional double layers which assure enhanced ionization (Wasserrab7, Langmuir’). The onset of the magnetic field in the Zwischenelectrode channel reduces the wall losses by steadily decreasing the radial diffusion of the charge carriers in the anode region. The primary electrons emerge from the quasi-cathode and interact with the anodic plasma particles by inelastic and Coulomb collisions and also collectively with the plasma. This results in a partial thermalization of primary electrons and some heating of the secondary electrons. Vauucm/volume
25lnumber
9/l
0, 1975.
Pergamon Press LtdlPrinted
Cathode
Zwischenelectrode
I
! 1 Anode I I
Extroctionelect;ode
Figure 1. Discharge region of Duoplasmatron
electron source and development of potential and magnetic field strength along axis.
Charged particles leave the plasma mainly in the axial direction because of their reduced radial mobility due to the magnetic field. Since the electrons’ mobility is higher than that of the ions, a potential hill is produced between the anode and the region of highest magnetic flux density’. The height of this hill increases with the temperature of the secondary electrons”. This shape of the potential produces an ion flux in the direction to the cathode as well as to the anode. A fraction of the ions and electrons reaching the anode leaves the source through the anode orifice and forms a plasma jet. The density of the anodic plasma is usually 1013-1014 cme3. in Great Britarn
421
P Varga, M Bruck and R Bruckmdler:
Generation of high intensity electron beams from a duoplasmatron
discharge
2. Electron extraction from a Duoplasmatron
The formation of an electron beam is determined by the shape of the emission surface, the beam expansion due to space charge, and the energy spread of the extracted electrons. 2.1 Shape of emission surface. By application of a high potential between the source and extraction electrode, electrons can be extracted from the Duoplasmatron. The extraction electrode acts in respect to the source plasma like a probe. Thus, a layer is built up which shields the electrode against the plasma. This layer contains a surplus of negative charge carriers if the electrode is positive in respect to the plasma. Under the simplifying assumption of a flat extraction surface, one finds that the current density j- of the electron beam is independent of the applied potential and is only a function of plasma density np and the mean electron velocity in the plasma:
J- =np.e
J
ExtractIOn
electrode _-
G+-
.inl
k.T_ 2.n.m_
T_
= electron temperature; m- = electron mass. On the other hand, the thickness of the extraction layer depends on the applied potential according to the space charge ru1e11*12. The shape of the emission surface which determines the beam formation is influenced by the extraction potential. To form a parallel or slightly convergent electron beam the emission surface should be flat or slightly concave. To fulfil this condition with technically feasible potential gradients the anode is shaped as an expansion electrode (see Figure 1). Thus, the plasma density in the extraction region is reduced to lo”1Ol2 cmm3. Contrarily to particle sources with a fixed emission surface, the determination of an optimal electrode shape is possible for plasma sources in the case of specified extraction parameters only. If extended parameter regions should be covered the most suitable electrode shape must be found empirically.
5
0
IO
cm
Figure 2. Duoplasmatron
300
electron source.
B,,,= 0 5 T t
2.2 Energy spread of extracted electrons. The energy spread of extracted electrons usually is considerably larger for plasma sources than for thermionic sources due to their energy distribution in the plasma and mainly because of the increasing instabilities of the discharge at higher discharge current and voltage. If beam-plasma interaction in the extraction or drift region takes place the reaction on the source plasma has to be considered as an effect increasing the energy spread. 3. Results 3.1 The influence of discharge parameters on the extracted current. Figure 2 shows the Duoplasmatron source. As the working gas, argon has been used. The axial symmetric inhomogeneous magnetic field was produced either by a solenoid or by permanent magnets, the diameter of the extraction orifice was 0.8 mm. The discharge parameters were varied between the following values: 0.5 < ZE < 3A; 30 V < U, < 100 V; 2.10-’ < px < 8.10-’ torr; 0 < B,,, < 0.5 T. The extraction potential was varied from 1 to 10 kV. At 10 kV the maximum extracted electron current I_ was 0.5 A. Figure 3 shows the dependence of I- on the pressure pK in the cathode region and the discharge current Z,. These measurements were made at an extraction potential of 2 kV. The general 422
Figure 3. Dependence of extracted electron current on pressure and discharge current for argon discharge.
features can be understood in the following way ‘O: At low discharge currents the losses of charge carriers are mainly caused by the classical diffusion transversal to the magnetic field lines. The plasma density and therewith the electron current increase with the growing discharge current. Plasma instabilities occur at critical conditions depending on neutral gas pressure, magnetic field strength, and discharge current. They cause increased diffusion losses (Bohm diffusion), thus considerably lowering the plasma density and therewith the extractable current. Further increasing of the discharge current raises the plasma density again in spite of higher losses.
P Varga, M Bruck and R Bruckmidler:
Generation of high intensity electron beams from a duoplasmatron discharge
that for a beam current up to 50 mA, the compensation field can be considered as an asymmetric bell-shaped field according to Glaser14* The measured imaging properties of this lens system proved to be in good correspondence with the calculated ones if the location of the smallest beam diameter (beam waist) is assumed as the image point. At higher beam currents the image points are usually nearer to the source than expected. The reason for this behaviour might be the growing effect of the space charge with increasing beam current. Usually it is an advantage, especially at high discharge currents, to produce the source magnetic field by a solenoid to be able to optimize the electron yield by variation of the magnetic flux (camp. Figure 4).
P,=5xlci2torr
c t
,
4. Comparison electron guns
of the Duoplasmatron
source with thermionic
The microperveance PP can be stated as a measure for the intensity behaviour of a particle source : pP = 106. I. V312
100 60 o-05
0 125
025
0
‘5
(>5
Brnm ‘T Figure 4. Development of extracted electron current with magnetic field strength for various discharge currents. (Argon discharge.)
(A.V-3’2)
Electron guns with a microperveance. higher than 0.1 are qualified as ‘highly perveant’15. Figure 6 shows the microperveances calculated from the obtained beam currents depending on the beam energy. For a thermionic source acting in the I
field strength for electron extraction depending on the discharge current. The decrease of I- after passing this value can also be explained by the above mentioned mechanism: for a certain discharge current the current density varies according to the magnetic field strength up to the onset of abnormal diffusion. Figure
4 shows
the existence
of an optimal
magnetic
3.2 Influence of the fringing magnetic field on the beam formation. When a solenoid was used the fringing field in the extraction region was kept low by employing soft magnetic material for the anode plate; no substantial influence on the beam could be observed, The beam, however, is evidently influenced by the focusing effect of the compensation field in front of the anode if a permanent magnet is applied (Figure 5). It was shownI LL
-
Lrt-
-
Ia
\ \ \ 3
c
7
a
9
L IO
Figure 6. Development of microperveance varying with extraction
T
voltage for various discharge currents. (Argon discharge.)
Figure
5. Application of a permanent magnetic field strength along the axis.
magnet
Development
of
space-charge limited regime, however, the microperveance by definition-independent of acceleration voltage-remains constant. Figure 7 shows a comparison of the investigated Duoplasmatron source with other types of electron guns in respect of microperveance and extracted electron current density. The most striking feature of the Duoplasmatron compared with thermionic electron guns is the high current yield being practically independent of the extraction potential. Within this investigation the highest current achieved was 0.5 A. Beam currents of 1 A and more are feasible at higher cost (as higher discharge currents, forced cooling of the extraction electrode, etc6). Figure 8 shows the measured fwhm AEllz and the fwtm AElllo of the energy distribution function against the discharge voltage at constant discharge current ZE.Also, the energy spread increases with growing discharge current (Figure 9). Both facts can be explained in terms of the increasing instability of the discharge with increasing discharge current or discharge voltage. 423
P Varga, M Bruck and R Bruckmiiller:
Generation
of high intensity electron beams from a Duoplasmatron
Cathode Duoplas
discharge
llmlt
matron
~,CIGW(IYB tubes
Figure 7. Maximum current density vs microperveance types of electron source (Brewer15).
for various
A A
Figure 9. Measured dependence of fwhm and fwtm of energy distribution function vs discharge current. both applicable. With regard to the qualification of the Duoplasmatron for the production of electron and ion beams as well as photons (visible to vacuum uv) this configuration is suitable for many applications in atomic, surface, and plasma physics. With rising beam intensity the energy spread of the extracted electrons increases. In less favourable working conditions instabilities might occur which are excited either in the source plasma itself and are transferred to the electron beam or which are generated in the drift region because of beam-plasma interactions. 60
Acknowledgements We gratefully acknowledge the support of this work by the Austrian ‘Fonds zur Forderung der wissenschaftlichen Forschung’ (Forschungsprojekt 15 I5), the ‘ijsterreichische Studienand the interest of Prof Dr F gesellschaft fur Atomenergie’, Viehbock. Acknowledgements are especially due to Dr H Winter for many valuable suggestions. References
L!L V
’ M Ardenne, Tabellen der Elektronenphysik, Zonenphysik and Ubermikroskopie. Deutscher Verlag der Wissenschaften, Berlin (1956). 2 H Frohlich, Nukleonik, 1, 1959, 183.
Figure 8. Measured dependence of fwhm and fwtm of energy distribution function vs discharge voltage.
3 R A Demirchanov, Report BNL 767, 1962, 224. 4 M Ardenne, Explle Tech Phys, 9, 1961, 227. 5 G Gautherin and A Septier, J Phys Radium Phys appl, 12, 1962,
Usually an energy spread of 30-70 eV is to be expected with the stated source parameters. On the other hand, the energy spread for electrons extracted from thermionic sources is only a few eV for higher beam currents.
212A. ’ M A Zavyalov ci al, Znstr Exp Tech (USA), 14, 1971, No. 2, Pt 2. ’ T Wasserrab, Z Physik, 128, 1950, 575. ’ I Langmuir, Phys Rev, 33, 1929, 954. 9 C Lejeune, Thesis, Paris (1971). lo H Winter and B H Wolf, Plasma Phys, 16, 1974, 791. ’ ’ C Child, Phys Rev, 32, I9 1I, 492. ” I Langmuir, Collected Works of Z Langmuir, Vol 3 (edited by
5. Summary The special features of this configuration compared with thermionic guns are: high beam intensities are possible at low extraction potentials; steady and pulsed operations are also 424
G Suits). McMillan (Pergamon Press), New York (1961). I3 P Varga, Diplomarbeit, TH Wien (1975). ” A B El-Kareh and J C J El-Kareh, Elecrron Beams, Lenses and Optics, Vol 1. Academic Press, New York (1970). I5 G Brewer Focusing of chargedparticles, Vol 2 (edited by A Septier), Chap 3.2. Academic Press. New York (1967).