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Investigation of plasma produced by laser and electron pulse ablation
gJnl ELSEVIER
Surfaceand Coatings Technology74- 75 (1995) 580-585
Investigation of plasma produced by laser and electron pulse ablation T h . W i t k e a, A. L e n k a, B. S c h u l t r i c h a, C. S c h u l t h e i s s b a Fraunhofer-Institutffir Werkstoffphysik und Schichttechnologie (IWS), Dresden, Germany b Kernforschungszentrum, Karlsruhe, Germany
Abstract Short laser pulses with high energy are a very promising tool for controlled ablation of materials, both for structurization and for deposition. They are especially suited for the ablation of non-conducting and of complex materials. The plasmas induced by the laser irradiation are distinguished by their high degree of ionization and excitation. A new source with comparable pulse properties is represented by the channel spark device. It delivers pulsed high current and self-focused electron beams (about 15 keV, 1 kA, 100 ns). The plasma produced by lasers and by electron pulses has been compared for an aluminium target. The state of the plasma has been characterized by the emitted optical radiation, spectral resolved. Considering the short pulse times, the high expansion velocity and the small dimensions, a special device combining high temporal, high spatial and high spectral resolution has been applied. The identity and the fraction of the dominating species have been determined in this way. Keywords: Plasma technology; Thin film deposition; Electron pulse ablation; Plasma spectroscopy; Short time investigation
1. Introduction Short laser pulses with high energy are a very promising tool for controlled ablation of materials, both for structurization and for deposition. They are especially suited for the ablation of non-conducting and of complex materials. The peculiarities of the laser induced ablation are mainly based on two effects, by introducing the pulse energy in the target over a small area and over a small penetration depth (10 to 100nm for metals) within a short time. The result is a very high energy density produced in the surface region leading to very high instantaneous ablation rates. Furthermore, the ablated material may gain additional energy by interaction with the laser beam. The resulting plasma beam with its high degree of ionization and with kinetic energies between some 10 eV and some 100 eV is very well suited for the deposition of high quality films. The same principle, the ablation by concentrated irradiation with high intensity pulses, is realized by a new source, the channel spark [-1,2]. It works with pulsed high current and self-focused electron beams. The energetic pulse parameters are comparable to the laser light pulses. However, there are two important differences: the larger penetration depth (above the order of a micrometer) and the modified interaction with the 0257-8972/95/$09.50© 1995ElsevierScienceS.A. All rights reserved SSDI 0257-8972(95)08308-1
ablated vapour. The advantages of the channel spark are its simple construction and the effective energy transfer into the target surface. For deposition technology the plasma beam is the decisive element of the PVD equipment. Hence, the characteristics of the plasma and its dependence on the primary irradiation are of principal importance. In the following, the plasmas produced by two lasers are compared with those by electron ablation.
2. Experimental arrangement The principal arrangement for the channel spark is given in Fig. 1. The electron pulse is produced by a triggered hollow cathode discharge and is extracted by an accelerating high voltage via a glass capillary. For the acceleration of the electron beam a gas filled tube is necessary. Positive gas ions, generated by impact ionisation, compensate the negative space charge of the electrons and prevent a splitting of the electron beam. Pulse times below 100 ns and power densities of about 10 8 W cm -2 are achieved. By use of a digitizing oscilloscope the current in the electron accelerator was mea-
Th. Witke et aL /Surface and Coatings Technology 74-75 (1995) 580-585
581
p h o t o by IRO-camera
(DiCAM-2) with objective(f = 180 mm)
aluminiu
target
I,mm electron beam accelerator d~c=
tube (gLas)
x ~ 100ns
Im~<~1 kA E ~ 10keY power density < 10s W/cm 2
L - digitizing
.2
p~2Pa
I
electron source gas discharge
t
oscilloscope
°
-HV R1
T
trigger discharge
Fig. 1. Experimental arrangement of the channel spark.
sured a n d the trigger signals for the high speed m e a s u r i n g devices were generated. C o n s i d e r i n g the short pulse times, the high e x p a n s i o n velocity a n d the small dimensions, a special device c o m b i n i n g high temporal, high spatial a n d high spectral resolution has been applied which allows new insights
into the energetical c o n d i t i o n s within the fast propagating p l a s m a plume. The so called high speed framing camera is s h o w n in Fig. 2. A long-distance microscope (Questar Q M 1 ) is c o m b i n e d with four electronic highspeed cameras. The object is pictured by the microscope with high magnification over a long w o r k i n g distance.
Table 1 Parameters of the lasers, the channel spark and the produced plasmas
Pulse energy Pulse time Spot diameter Wavelength Power density Electron temperature Degree of ionization AI in% A1+ in% A1++ in% Electron density
Nd-YAG-laser Rofin-Sinar RSY 25Q
Excimer-laser Siemens XP 2020
Channel spark KFK TPA-CSM 20/1.5/6
6 mJ 120 ns 120 gm 1.06 gm 4.5 x 10s W cm -2 1.2 eV 1.13 2 83 15 24 x 1016cm 3
2J 50 ns 2.5 mm 308 nm 8 x 10s Wcm -2 1.3 eV 1.08 0 92 8 -~1 x 1017cm-3
1J 100 ns 22 mm g5 x 108 Wcm z 1.3 eV 1.4 2 56 42 ~ 2 x 1016cm 3
582
Th. Witke et aL /Surface and Coatings Technology 74 75 (1995) 580-585
image splitter 22 % intensity per channel 12 % intensity loss
/ Queatar QM1 Long-Dlatance Microscope
"
e
/
/
PCO Camera DICAM-2
working range: 0,5 ... 1,5 m magnification: max. 12 format: 18 mm diffraction limited
with: MCP, Taper, Videomodule
I
t
[
channel t channel 2
J
channel 3 channel 4
For all channels independent adjustable: delay: 0 ns... 1 ms (1-ns-steps) width: 5 ns ... 1 ms (1-ns-steps) gain
PC with frame-grabber
Fig. 2. Schematic representation of the high speed framing camera.
By a beamsplitter this picture is identically projected on the inputs of the four cameras. Over micro-channelplates and tapers of optical fibres, the images are transmitted to CCD-videomodules storing the pictures (780 × 580 pixel, 8 bit). The delay after the trigger signal, the width and the gain are separately adjustable for every channel. The performance figures of the camera system are as follows: (a) widths adjustable in 1 ns steps from 5 ns to 1 ms; (b) delays adjustable in 1 ns steps from 0 ns to 1 ms; (c) usable lateral resolution maximum 10 ~tm (with 2 additional lenses); (d) observation distance between 50cm and 150cm; and (e) filter insertion possible in every channel.
3. Results
For restricting the broad range of complex processes, the discussion is concentrated on experiments with a Nd-YAG-laser and an excimer-laser in the intensity range around 108W cm 2, especially interesting for many technological applications. The standard conditions are summarized in Table 1. The processes are
intensity 4 I
Si
3
IAI ++
A, + +
[AI++
ltl
1
si+
A,+tl A
II
2
s,+ ll.;/'//t
1 0
-
. 400
I
,
I
500
u
,
UUUL I
600
,
I
700
wavelength / nm Fig. 3. Emission spectrum of the channel spark plasma.
discussed for aluminium because of its clear emission spectrum of the plasma. The irradiation was done in vacuum or in a controlled atmosphere. By estimating average values for the electron temperature, the degree of ionisation and the electron density, it is possible to get a first impression of the ablation process. The plasma temperature and the degree of ionisation of the channel spark are in the same range as the values for the lasers. These parameters were estimated by Boltzmann-plot from the emission spectrum of the plasma (Fig. 3) I-3,4].
Th. Witke et al./Surface and Coatings Technology 74-75 (1995) 580-585
timing:
de~,:
1 ch~nl~ ~) no
I
2
3
4
c;heunnel 2 ~ no
¢~tJurtn~ 3 ~0 ne
oh~ned 5 0 rim
583
4
Fig. 4. Dynamics of a channel spark plasma.
LTE within the plasma is the assumption for this procedure. According to Ref. [3], a transient and inhomogeneous plasma has to fulfil the validity criteria of a stationary and homogeneous plasma. For the channel spark plasma an electron density of approximately 2 x 1016 cm -3 can be estimated from the line width of the Al+-line at 559nm. This electron density is high enough for formation of LTE [3]. Moreover, the channel spark plasma, as an inhomogeneous and transient plasma, has to fulfil the LTE conditions of an inhomogeneous, stationary plasma and of a homogeneous, transient plasma. Using the channel spark plasma parameters, an interaction constant of about 0.3 mm and an equilibration time near 50 ns can be estimated. In its mean lifetime (50 ns-250 ns after eruption) the plasma parameters do not change rapidly in this area and time. Moreover, the lines of the Boltzmann plot of the different ionisation stages are approximately parallel. Therefore, the assumption of LTE is possible. Since the plasma production by channel spark shows some fluctuations, the plasma parameters are average values of more than 20 discharge processes. Especially, the acceleration and the production of the electron beam is not exactly the same process for every run. The target position and the target material also influence the plasma parameters because the target is a part of the anode of the accelerator. The emission spectrum shown in Fig. 3 contains lines of nitrogen, carbon and silicon besides the aluminium spectrum. Silicon stems from the glass tube of the
accelerator system, nitrogen is the working gas, and carbon (as hydrocarbons) is a main part of the rest gas. Fig. 4 shows the dynamics of a channel spark plasma. The camera channels were activated in a successive mode. The timing of the channels is given in the legend. It seems that the plasma plume moves rather slowly with velocities below 1000 m s-1 compared with typically 10000 m s -1 for laser ablation under these conditions. At the end of the ablation process arc discharges between the plasma and the target can be observed. They occur about 300ns after the electron pulse. In this time, a positive space charge has developed in front of the target surface leading to unipolar arcs. The HSFC was completed with Barlow lenses for higher magnification. A characteristic series is shown in Fig. 5 giving an impression of the formation and life of the arc discharges near the target surface. The lifetime of the discharges is in the range 200-600 ns and the size varies between 50 and 300 lam. In this way target material is emitted from the erosion areas of the arcs on the target surface for much longer than the electron pulse duration. Insight into the plasma composition can be gained by running the four cameras at the same time with suitable filters (Fig. 6). The filters were selected according to the emission spectra of the plasma. Filter choice and camera timing are given in the legend of the figure. A clear distinction is revealed between the propagation of the excited and luminating species ablated from the target (picture 2 and 3) and those from other sources (picture
584
Th. Witke et aL/Surjitce and Coat&gs Technology 74 75 (1995) 580-585
timing:
2eons
1
2
3
4
ch~ne~ I lOOns
chennet 2 lOOns
chanrte~3 lOOns
channe~4 lOOns
Fig. 5. Observation of plasma target discharges with higher lateral resolution of the high speed framing camera.
4). The latter are overwhelmingly ejected from the capillary as is proved by the considerable fraction of silicon ablated from the walls of the glass capillary. This plasma stream with a pressure much higher than the basic pressure of only 2 Pa is sufficient to deflect the flux of the evaporated material.
4. Conclusions
Short and intense laser or electron pulses are effective means in thin film technology. They are distinguished by the high excitation of the impinging coating particles, the possibility for transition of stoichiometry (even of complex compositions) and the missing restriction on conducting materials. Laser pulsed deposition based methods are now well established and widely investigated. New fields are opened by using equivalent electron pulses realised in the channel spark device. Its most important advantage is the much lower instrumental
expense. However, besides much similarity some differences in the formation and propagation of the depositing plasma stream must be considered. For laser or electron pulses of about 100 ns and 108 W cm 2 directed on a target, the material ablates mainly by thermal evaporation. The corresponding surface temperatures are of the order of the boiling temperature and amount to some thousand kelvin. By interaction with the incident laser or electron beam the vapour is highly excited and nearly completely ionized. Plasma temperatures beyond 1 eV (corresponding to temperatures above 10000 K) are achieved and an essential fraction of the particles is in higher ionization states. In vacuum or low pressure atmosphere the laser induced plasma plume propagates with a high velocity of about 10000 m s -1. Under comparable conditions for the channel spark device, the plasma propagation is suppressed by some space charge effects. These are manifested by the appearance of unipolar arcs long after the end of the electron pulse. Furthermore, the influences of the plasma stream
Th. Witke et al./Surface and Coatings Technology 74-75 (1995) 580-585
1: channel 1 without fllte~
timing:
delay: 150ns
50ns
.................. , ~ .................* 1
,, ......... ~ 2 "~ ............
585
2: channel 2 fllte~ 570 nm; A ~ 5 nm light from AI ++ -ions
3: chann~ 3
filter 620 nm; A ; 5 nm light from hi + -ions
-~ 3
• ......... ~ 4
4: channel 4 filter 500 nrn; A ~ 20 nm light from N 2 (working gas), s i ( a c c ~ ' a t o r tube),
C 2 (rest gas)
Fig. 6. Composition of a channel spark plasma.
expelled and the electron pulse from the accelerator capillary on the ablation plasma must be considered. These hydrodynamic effects together with some plasma irregularities inside the capillary tube are responsible for the observed fluctuations of the channel spark discharge and are the object of further investigation and optimization.
References [ 1] M. H6bel, J. Geerk, G. Linker and C. Schultheiss, Appl. Phys. Lett., 56 (10) (1990) 973. [2] Q.D. Jiang et al., Supercond. Sci. Technol., 6 (1993) 567-572. [3] H.R. Griem, Plasma Spectroscopy, McGraw-Hill. [4] R.H. Tourin, Spectroscopic Gas Temperature Measurement, Elsevier.
Surfaceand Coatings Technology74- 75 (1995) 580-585
Investigation of plasma produced by laser and electron pulse ablation T h . W i t k e a, A. L e n k a, B. S c h u l t r i c h a, C. S c h u l t h e i s s b a Fraunhofer-Institutffir Werkstoffphysik und Schichttechnologie (IWS), Dresden, Germany b Kernforschungszentrum, Karlsruhe, Germany
Abstract Short laser pulses with high energy are a very promising tool for controlled ablation of materials, both for structurization and for deposition. They are especially suited for the ablation of non-conducting and of complex materials. The plasmas induced by the laser irradiation are distinguished by their high degree of ionization and excitation. A new source with comparable pulse properties is represented by the channel spark device. It delivers pulsed high current and self-focused electron beams (about 15 keV, 1 kA, 100 ns). The plasma produced by lasers and by electron pulses has been compared for an aluminium target. The state of the plasma has been characterized by the emitted optical radiation, spectral resolved. Considering the short pulse times, the high expansion velocity and the small dimensions, a special device combining high temporal, high spatial and high spectral resolution has been applied. The identity and the fraction of the dominating species have been determined in this way. Keywords: Plasma technology; Thin film deposition; Electron pulse ablation; Plasma spectroscopy; Short time investigation
1. Introduction Short laser pulses with high energy are a very promising tool for controlled ablation of materials, both for structurization and for deposition. They are especially suited for the ablation of non-conducting and of complex materials. The peculiarities of the laser induced ablation are mainly based on two effects, by introducing the pulse energy in the target over a small area and over a small penetration depth (10 to 100nm for metals) within a short time. The result is a very high energy density produced in the surface region leading to very high instantaneous ablation rates. Furthermore, the ablated material may gain additional energy by interaction with the laser beam. The resulting plasma beam with its high degree of ionization and with kinetic energies between some 10 eV and some 100 eV is very well suited for the deposition of high quality films. The same principle, the ablation by concentrated irradiation with high intensity pulses, is realized by a new source, the channel spark [-1,2]. It works with pulsed high current and self-focused electron beams. The energetic pulse parameters are comparable to the laser light pulses. However, there are two important differences: the larger penetration depth (above the order of a micrometer) and the modified interaction with the 0257-8972/95/$09.50© 1995ElsevierScienceS.A. All rights reserved SSDI 0257-8972(95)08308-1
ablated vapour. The advantages of the channel spark are its simple construction and the effective energy transfer into the target surface. For deposition technology the plasma beam is the decisive element of the PVD equipment. Hence, the characteristics of the plasma and its dependence on the primary irradiation are of principal importance. In the following, the plasmas produced by two lasers are compared with those by electron ablation.
2. Experimental arrangement The principal arrangement for the channel spark is given in Fig. 1. The electron pulse is produced by a triggered hollow cathode discharge and is extracted by an accelerating high voltage via a glass capillary. For the acceleration of the electron beam a gas filled tube is necessary. Positive gas ions, generated by impact ionisation, compensate the negative space charge of the electrons and prevent a splitting of the electron beam. Pulse times below 100 ns and power densities of about 10 8 W cm -2 are achieved. By use of a digitizing oscilloscope the current in the electron accelerator was mea-
Th. Witke et aL /Surface and Coatings Technology 74-75 (1995) 580-585
581
p h o t o by IRO-camera
(DiCAM-2) with objective(f = 180 mm)
aluminiu
target
I,mm electron beam accelerator d~c=
tube (gLas)
x ~ 100ns
Im~<~1 kA E ~ 10keY power density < 10s W/cm 2
L - digitizing
.2
p~2Pa
I
electron source gas discharge
t
oscilloscope
°
-HV R1
T
trigger discharge
Fig. 1. Experimental arrangement of the channel spark.
sured a n d the trigger signals for the high speed m e a s u r i n g devices were generated. C o n s i d e r i n g the short pulse times, the high e x p a n s i o n velocity a n d the small dimensions, a special device c o m b i n i n g high temporal, high spatial a n d high spectral resolution has been applied which allows new insights
into the energetical c o n d i t i o n s within the fast propagating p l a s m a plume. The so called high speed framing camera is s h o w n in Fig. 2. A long-distance microscope (Questar Q M 1 ) is c o m b i n e d with four electronic highspeed cameras. The object is pictured by the microscope with high magnification over a long w o r k i n g distance.
Table 1 Parameters of the lasers, the channel spark and the produced plasmas
Pulse energy Pulse time Spot diameter Wavelength Power density Electron temperature Degree of ionization AI in% A1+ in% A1++ in% Electron density
Nd-YAG-laser Rofin-Sinar RSY 25Q
Excimer-laser Siemens XP 2020
Channel spark KFK TPA-CSM 20/1.5/6
6 mJ 120 ns 120 gm 1.06 gm 4.5 x 10s W cm -2 1.2 eV 1.13 2 83 15 24 x 1016cm 3
2J 50 ns 2.5 mm 308 nm 8 x 10s Wcm -2 1.3 eV 1.08 0 92 8 -~1 x 1017cm-3
1J 100 ns 22 mm g5 x 108 Wcm z 1.3 eV 1.4 2 56 42 ~ 2 x 1016cm 3
582
Th. Witke et aL /Surface and Coatings Technology 74 75 (1995) 580-585
image splitter 22 % intensity per channel 12 % intensity loss
/ Queatar QM1 Long-Dlatance Microscope
"
e
/
/
PCO Camera DICAM-2
working range: 0,5 ... 1,5 m magnification: max. 12 format: 18 mm diffraction limited
with: MCP, Taper, Videomodule
I
t
[
channel t channel 2
J
channel 3 channel 4
For all channels independent adjustable: delay: 0 ns... 1 ms (1-ns-steps) width: 5 ns ... 1 ms (1-ns-steps) gain
PC with frame-grabber
Fig. 2. Schematic representation of the high speed framing camera.
By a beamsplitter this picture is identically projected on the inputs of the four cameras. Over micro-channelplates and tapers of optical fibres, the images are transmitted to CCD-videomodules storing the pictures (780 × 580 pixel, 8 bit). The delay after the trigger signal, the width and the gain are separately adjustable for every channel. The performance figures of the camera system are as follows: (a) widths adjustable in 1 ns steps from 5 ns to 1 ms; (b) delays adjustable in 1 ns steps from 0 ns to 1 ms; (c) usable lateral resolution maximum 10 ~tm (with 2 additional lenses); (d) observation distance between 50cm and 150cm; and (e) filter insertion possible in every channel.
3. Results
For restricting the broad range of complex processes, the discussion is concentrated on experiments with a Nd-YAG-laser and an excimer-laser in the intensity range around 108W cm 2, especially interesting for many technological applications. The standard conditions are summarized in Table 1. The processes are
intensity 4 I
Si
3
IAI ++
A, + +
[AI++
ltl
1
si+
A,+tl A
II
2
s,+ ll.;/'//t
1 0
-
. 400
I
,
I
500
u
,
UUUL I
600
,
I
700
wavelength / nm Fig. 3. Emission spectrum of the channel spark plasma.
discussed for aluminium because of its clear emission spectrum of the plasma. The irradiation was done in vacuum or in a controlled atmosphere. By estimating average values for the electron temperature, the degree of ionisation and the electron density, it is possible to get a first impression of the ablation process. The plasma temperature and the degree of ionisation of the channel spark are in the same range as the values for the lasers. These parameters were estimated by Boltzmann-plot from the emission spectrum of the plasma (Fig. 3) I-3,4].
Th. Witke et al./Surface and Coatings Technology 74-75 (1995) 580-585
timing:
de~,:
1 ch~nl~ ~) no
I
2
3
4
c;heunnel 2 ~ no
¢~tJurtn~ 3 ~0 ne
oh~ned 5 0 rim
583
4
Fig. 4. Dynamics of a channel spark plasma.
LTE within the plasma is the assumption for this procedure. According to Ref. [3], a transient and inhomogeneous plasma has to fulfil the validity criteria of a stationary and homogeneous plasma. For the channel spark plasma an electron density of approximately 2 x 1016 cm -3 can be estimated from the line width of the Al+-line at 559nm. This electron density is high enough for formation of LTE [3]. Moreover, the channel spark plasma, as an inhomogeneous and transient plasma, has to fulfil the LTE conditions of an inhomogeneous, stationary plasma and of a homogeneous, transient plasma. Using the channel spark plasma parameters, an interaction constant of about 0.3 mm and an equilibration time near 50 ns can be estimated. In its mean lifetime (50 ns-250 ns after eruption) the plasma parameters do not change rapidly in this area and time. Moreover, the lines of the Boltzmann plot of the different ionisation stages are approximately parallel. Therefore, the assumption of LTE is possible. Since the plasma production by channel spark shows some fluctuations, the plasma parameters are average values of more than 20 discharge processes. Especially, the acceleration and the production of the electron beam is not exactly the same process for every run. The target position and the target material also influence the plasma parameters because the target is a part of the anode of the accelerator. The emission spectrum shown in Fig. 3 contains lines of nitrogen, carbon and silicon besides the aluminium spectrum. Silicon stems from the glass tube of the
accelerator system, nitrogen is the working gas, and carbon (as hydrocarbons) is a main part of the rest gas. Fig. 4 shows the dynamics of a channel spark plasma. The camera channels were activated in a successive mode. The timing of the channels is given in the legend. It seems that the plasma plume moves rather slowly with velocities below 1000 m s-1 compared with typically 10000 m s -1 for laser ablation under these conditions. At the end of the ablation process arc discharges between the plasma and the target can be observed. They occur about 300ns after the electron pulse. In this time, a positive space charge has developed in front of the target surface leading to unipolar arcs. The HSFC was completed with Barlow lenses for higher magnification. A characteristic series is shown in Fig. 5 giving an impression of the formation and life of the arc discharges near the target surface. The lifetime of the discharges is in the range 200-600 ns and the size varies between 50 and 300 lam. In this way target material is emitted from the erosion areas of the arcs on the target surface for much longer than the electron pulse duration. Insight into the plasma composition can be gained by running the four cameras at the same time with suitable filters (Fig. 6). The filters were selected according to the emission spectra of the plasma. Filter choice and camera timing are given in the legend of the figure. A clear distinction is revealed between the propagation of the excited and luminating species ablated from the target (picture 2 and 3) and those from other sources (picture
584
Th. Witke et aL/Surjitce and Coat&gs Technology 74 75 (1995) 580-585
timing:
2eons
1
2
3
4
ch~ne~ I lOOns
chennet 2 lOOns
chanrte~3 lOOns
channe~4 lOOns
Fig. 5. Observation of plasma target discharges with higher lateral resolution of the high speed framing camera.
4). The latter are overwhelmingly ejected from the capillary as is proved by the considerable fraction of silicon ablated from the walls of the glass capillary. This plasma stream with a pressure much higher than the basic pressure of only 2 Pa is sufficient to deflect the flux of the evaporated material.
4. Conclusions
Short and intense laser or electron pulses are effective means in thin film technology. They are distinguished by the high excitation of the impinging coating particles, the possibility for transition of stoichiometry (even of complex compositions) and the missing restriction on conducting materials. Laser pulsed deposition based methods are now well established and widely investigated. New fields are opened by using equivalent electron pulses realised in the channel spark device. Its most important advantage is the much lower instrumental
expense. However, besides much similarity some differences in the formation and propagation of the depositing plasma stream must be considered. For laser or electron pulses of about 100 ns and 108 W cm 2 directed on a target, the material ablates mainly by thermal evaporation. The corresponding surface temperatures are of the order of the boiling temperature and amount to some thousand kelvin. By interaction with the incident laser or electron beam the vapour is highly excited and nearly completely ionized. Plasma temperatures beyond 1 eV (corresponding to temperatures above 10000 K) are achieved and an essential fraction of the particles is in higher ionization states. In vacuum or low pressure atmosphere the laser induced plasma plume propagates with a high velocity of about 10000 m s -1. Under comparable conditions for the channel spark device, the plasma propagation is suppressed by some space charge effects. These are manifested by the appearance of unipolar arcs long after the end of the electron pulse. Furthermore, the influences of the plasma stream
Th. Witke et al./Surface and Coatings Technology 74-75 (1995) 580-585
1: channel 1 without fllte~
timing:
delay: 150ns
50ns
.................. , ~ .................* 1
,, ......... ~ 2 "~ ............
585
2: channel 2 fllte~ 570 nm; A ~ 5 nm light from AI ++ -ions
3: chann~ 3
filter 620 nm; A ; 5 nm light from hi + -ions
-~ 3
• ......... ~ 4
4: channel 4 filter 500 nrn; A ~ 20 nm light from N 2 (working gas), s i ( a c c ~ ' a t o r tube),
C 2 (rest gas)
Fig. 6. Composition of a channel spark plasma.
expelled and the electron pulse from the accelerator capillary on the ablation plasma must be considered. These hydrodynamic effects together with some plasma irregularities inside the capillary tube are responsible for the observed fluctuations of the channel spark discharge and are the object of further investigation and optimization.
References [ 1] M. H6bel, J. Geerk, G. Linker and C. Schultheiss, Appl. Phys. Lett., 56 (10) (1990) 973. [2] Q.D. Jiang et al., Supercond. Sci. Technol., 6 (1993) 567-572. [3] H.R. Griem, Plasma Spectroscopy, McGraw-Hill. [4] R.H. Tourin, Spectroscopic Gas Temperature Measurement, Elsevier.