Imperial College London 2. Particle sources 1. Electron sources Thermionic sources Field emitters Laser sources 2.
Download ReportTranscript Imperial College London 2. Particle sources 1. Electron sources Thermionic sources Field emitters Laser sources 2.
Slide 1
Imperial College London
1
2. Particle sources
1. Electron sources
Thermionic sources
Field emitters
Laser sources
2. Ion sources
2.1 Production of high currents of single charge state ion beams
Penning sources
Hot cathode sources
RF sources
2.2 Production of high charge state ions
ECR sources
EBIS sources
Laser sources
Slide 2
Imperial College London
2
2. Particle sources
2.3 Production of negatively charged ion beams
Surface Production
Volume Production
3. Extraction of particle beams
3.1 The space charge limit and Child-Langmuirs law
3.2 External and internal fields in the extractor, laminar flow and
pierce angle
3.3 The beam emittance, the acceptance of the extraction
system and the conservation of phase space
Slide 3
Imperial College London
3
Electron sources
Only very little energy is necessary to free electrons from the bound state or the
Upper levels of the “electron gas” in solids. This can be done by :
1) Thermionic emission
The heated electron must
have an energy higher than
the workfunction
2) Photoemission
The photon energy must
exceed the work function
3) Field emission
high external electric fields
alter the potential barrier,
and allow electrons to be
extracted by the tunneleffect
Slide 4
Imperial College London
4
Current density as a function of Binding energy and
temperature
J A T e
2
Diode characteristic
current
Richardson-Dushman
equation
o
kT
Temperature limited
Space charge limited
voltage
Material
A
F(eV)
Temp (° K) J (A/cm2)
Tungsten
60
4.54
2500
0.3
Thoriated W
3
2.63
1900
1.16
Mixed oxides
0.01
1.
1200
1.
Caesium
162
1.81
Tantalum
60
3.38
2500
2.38
Cs/O/W
0.003
0.72
1000
0.35
Slide 5
Imperial College London
5
Thermionic guns
Slide 6
Imperial College London
6
Field emission of electrons from surfaces
Fowler Nordheim :
J B E2 e
1.5
7 0
6.810
E
J: emission current density (A/cm2)
B: field-independent constant [A/V2]
E: applied field (V/cm)
F0: work function (eV)
Slide 7
Imperial College London
Field emission of electrons from surfaces
Single carbon nano tube (CNT) and CNT arrays for
the production of high brightness electron beams
Field emitter arrays, designed for the production of large panel plasma screens
7
Slide 8
Imperial College London
8
Photo effect
E photon h f light
light
hc
light
1
E pot. Ekin. F 0 melectronv 2
2
photo cathode
free electrons
Light ()
ring anode
pA
I
+ -
bound electrons
U
U (I=0)
uv- lamp
zincplate
h
glass
0
ff
Slide 9
Imperial College London
9
Photo effect and laser sources
J
PLaser QE
390 r
2
Laser
Slide 10
Imperial College London
10
Photo effect and laser sources
DESY PITZ 2 source (LC / XFEL)
Slide 11
Imperial College London
11
Production of high currents of single charge state ion beams
The impact of electron with gaseous
atoms is mostly used for the
production of ion beams.
For efficient ion production the electron
energy should be app. 2-4 times the
ionization energy of the ion.
Slide 12
Imperial College London
12
Production of high currents of single charge state ion beams
A Townsend gas discharge using an avalanche
effect is an very effective way to produce a high
amount of ions. Therefore the Paschen criteria
has to be fulfilled. To improve the gas
discharge and to enhance plasma confinement
magnetic fields are used.
Slide 13
Imperial College London
13
Penning sources
The Penning Ion Source or PIG
source (Philips Ionization
vacuum Gauge) invented by
Penning in 1937 uses a a dipole
field for plasma confinement .
The strong magnetic dipole field
gives high efficiency as
electrons oscillate inside the
hollow anode between the the
two cathodes at each end.
The Lifetime of the source limited
by sputtering of the cathodes,
especially for highly charged,
heavy ion operation.
Slide 14
Imperial College London
14
Magnetron sources
The Magnetron ion source which was
first presented by Van Voorhis in
1934 uses a solenoidal magnetic
field for plasma confinement The
field of ~ 0.1 T is generated with
an external solenoid surrounding
the ion source. The chamber wall
serves as anode, while the
cathode provides electrons
through thermionic emission. The
filament mounted parallel to the
magnetic field forces the electrons
to spiral. As with Penning sources
the Lifetime of the source limited
by sputtering of the cathodes,
especially for highly charged,
heavy ion operation.
Slide 15
Imperial College London
15
Hot cathode sources
Filament Ion Source
Discharge in the plasma chamber is
driven by the electrons delivered
by the filament.
single charged ions up to 100 mA
•
filtermagnet
CoSmmagnets
solenoid
Bz
Plasma enclosure by magnets.
plasmaelectrode
Bx
groundelectrode
cathode
•
•
•
Pressure range 10-1 - 10-3 mbar.
Discharge voltage 20 - 200 V
(depending on ionization
voltage)
Discharge current 10 - 500 A
gasinlet
copper
isolator
water
steel
brass
magnets
100 mm
screeningelectrode
Slide 16
Imperial College London
16
RF sources
Non resonant excitation of plasma
by RF. Only lower charge states
available (low electron energy)
but high beam currents
possible.
Internal antenna to feed RF power
into plasma => limited lifetime of
antenna due to sputtering, strong
coupling of RF into plasma and
good plasma confinement.
Slide 17
Imperial College London
17
RF sources
Production of large ion currents (I>1 A)
of single charged ions for surface
treatment or plasma heating
(tokamaks). Multiaperture extraction
therefore difficult to feed beam into
conventional accelerator structure.
External antenna to feed RF
power into plasma => Long
lifetime of antenna, but chamber
has to be of non conducting
material.
Slide 18
Imperial College London
Production of high charge state ions
The PLASMA created is increased
in density by electron
bombardment. The maximum
charge state that will be
obtained depends on the
incident electron energy.
e + X = X+ + 2e
For multi-charge states
e + X i+ = X (i+1)+ + 2e
higher electron energies are
required since electrons have to
be removed from inner shells.
The maximum charge state is
limited by the incident electron
energy.
18
Slide 19
Imperial College London
19
Electron Cyclotron Resonance Source
whf = wcyc = (e/m) B
Radial and axial magnetic field distribution
for the confinement of the source plasma.
Only at the centre of the source the cyclotron
condition for the electrons is full filled.
(0.1-1 kW)
Extracted
ion currents
for different
charge
states of
Argon
Slide 20
Imperial College London
20
Electron Cyclotron Resonance source
Schematic layout of an ECR source
for the production of radioactive
ion beams
By variation of the longitudinal
enclosing magnetic mirror
configuration the charge
distribution can be influenced.
Slide 21
Imperial College London
21
Electron Beam Ion Source
CRYogenic Stockholm Ion Source
17 cm
Upper: First EBIS IEL-1 build by Donets in 1968,
lower : Evolution of charge state distribution
Parameters of CRYSIS
of nitrogen ions at Ee=5.45 keV
nominal values
max
values
units
electron beam current
350
1300
mA
electron beam energy
20
27.5
keV
trap length
1.2
-
m
magnetic field
1.5
5
T
charge per pulse
1-2
4
nC
ion pulse length
0.05-100
-
µs
containment time
20-2000
-
ms
Slide 22
Imperial College London
22
Electron Beam Ion Source
Total ion charge trapped
in an EBIS as a function
of confinement time, for
different electron beam
currents
Ion current extracted from
an EBIS as a function of the
charge state for Na ions
(Ne gas was added)
Comparison of the
extractable (electric)
current between
ECR and EBIS.
Slide 23
Imperial College London
23
Laser Ion Sources
Schematic drawing of the experimental set up of a laser ion source. By
the impact of the laser with the target, a plasma is created which expands
in the drift chamber and then is accelerated in the extraction gap.
Slide 24
Imperial College London
24
Laser Ion Sources
By interaction of the plasma electrons
with the electric field of the laser
pulse the electrons are heated. This
leads to a shift of the charge state
distribution of the plasma ions
towards higher charge states.
The different charge states of
the plasma are separated by
the drift longitudinally and
reduce space charge in the
acceleration gap
Slide 25
Imperial College London
25
Laser Ion Sources
The different charge
states of the plasma
are separated by the
drift longitudinally
(higher charge states
first) and reduce
space charge in the
acceleration gap.
The available laser power density on the
target (for fixed total laser power
influenced by beam size on target)
strongly influences the charge state
distribution of the plasma
Slide 26
Imperial College London
26
Production of negatively charged ion beams
g
Conserving energy when forming a negative ion through
direct electron attachment, the excess energy has to be
dissipated through a photon. A + e = A¯ + g. But radiative
Capture is rare (5•10-22cm2 for H2).
Higher cross sections (~10-20cm2 for H2 and Ee >10 eV) can
be realized when the excess energy can be transferred to a
third particle, M + e = A + B + e and sometimes = A + B¯
Even better are processes which excite a molecule to the
edge of breakup (vibrationally excited 4
Three Types of H- Ion Sources
Cs can be used as an
are in use
electron donator, but the
• Surface conversion sources
ionisation energy of 3.9 eV is
much higher than the 0.75 eV
• Volume production sources
electron affinity of H
• Hybrid production sources
=> Surface treatment
Slide 27
Imperial College London
27
Production of negatively charged ion beams
anode
driver
efast->H2
extractor
eslow->H-
cathode
gasinlet
Cross sections for
different production
and destruction
mechanisms
solenoid
filter
Magnetic dipole fields
can be used as filters
to create areas of
different electron
temperatures
Slide 28
Imperial College London
28
Sources using surface production
Picture of the LANCE
surface source
Schematic layout of the LANCE
surface source using a filament
driven gas discharge for plasma
production and surface
conversion on a Cs target for Hformation
Slide 29
Imperial College London
29
Sources using volume production
copper
magnet
isolator
water
screeningplasma- plasma- electrode
chamber electrode
electrondumping
grid
cathode
and
gasinlet
solenoid filter- bending- soleniod
magnet magnet
The small experimental H- source in Frankfurt used a hot
cathode driven gas discharge, dipole fields and a Pt surface
cover for the reduction of the dissociation of the H2 molecules
Slide 30
Imperial College London
30
Sources using hybrid production schemes
The RAL H- source uses a Penning
discharge (dipole for plasma
production and to influence electron
temperature) and Cs injection.
The SNS H- source developed in
Berkley uses RF to produce the
plasma, dipoles to influence the
electron density and a Cs collar.
Slide 31
Imperial College London
31
The extraction of particle beams
Slide 32
Imperial College London
The space charge limit and Child-Langmuirs law
32
Slide 33
Imperial College London
The space charge limit and Child-Langmuirs law
33
Slide 34
Imperial College London
External and internal fields in the extractor, laminar flow and
pierce angle
34
Slide 35
Imperial College London
External and internal fields in the extractor, laminar flow and
pierce angle
35
Slide 36
Imperial College London
The beam emittance, the acceptance of the extraction
system and the conservation of the phase space
36
Slide 37
Imperial College London
The beam emittance, the acceptance of the extraction
system and the conservation of the phase space
37
Slide 38
Imperial College London
The beam emittance, the acceptance of the extraction
system and the conservation of the phase space
38
Imperial College London
1
2. Particle sources
1. Electron sources
Thermionic sources
Field emitters
Laser sources
2. Ion sources
2.1 Production of high currents of single charge state ion beams
Penning sources
Hot cathode sources
RF sources
2.2 Production of high charge state ions
ECR sources
EBIS sources
Laser sources
Slide 2
Imperial College London
2
2. Particle sources
2.3 Production of negatively charged ion beams
Surface Production
Volume Production
3. Extraction of particle beams
3.1 The space charge limit and Child-Langmuirs law
3.2 External and internal fields in the extractor, laminar flow and
pierce angle
3.3 The beam emittance, the acceptance of the extraction
system and the conservation of phase space
Slide 3
Imperial College London
3
Electron sources
Only very little energy is necessary to free electrons from the bound state or the
Upper levels of the “electron gas” in solids. This can be done by :
1) Thermionic emission
The heated electron must
have an energy higher than
the workfunction
2) Photoemission
The photon energy must
exceed the work function
3) Field emission
high external electric fields
alter the potential barrier,
and allow electrons to be
extracted by the tunneleffect
Slide 4
Imperial College London
4
Current density as a function of Binding energy and
temperature
J A T e
2
Diode characteristic
current
Richardson-Dushman
equation
o
kT
Temperature limited
Space charge limited
voltage
Material
A
F(eV)
Temp (° K) J (A/cm2)
Tungsten
60
4.54
2500
0.3
Thoriated W
3
2.63
1900
1.16
Mixed oxides
0.01
1.
1200
1.
Caesium
162
1.81
Tantalum
60
3.38
2500
2.38
Cs/O/W
0.003
0.72
1000
0.35
Slide 5
Imperial College London
5
Thermionic guns
Slide 6
Imperial College London
6
Field emission of electrons from surfaces
Fowler Nordheim :
J B E2 e
1.5
7 0
6.810
E
J: emission current density (A/cm2)
B: field-independent constant [A/V2]
E: applied field (V/cm)
F0: work function (eV)
Slide 7
Imperial College London
Field emission of electrons from surfaces
Single carbon nano tube (CNT) and CNT arrays for
the production of high brightness electron beams
Field emitter arrays, designed for the production of large panel plasma screens
7
Slide 8
Imperial College London
8
Photo effect
E photon h f light
light
hc
light
1
E pot. Ekin. F 0 melectronv 2
2
photo cathode
free electrons
Light ()
ring anode
pA
I
+ -
bound electrons
U
U (I=0)
uv- lamp
zincplate
h
glass
0
ff
Slide 9
Imperial College London
9
Photo effect and laser sources
J
PLaser QE
390 r
2
Laser
Slide 10
Imperial College London
10
Photo effect and laser sources
DESY PITZ 2 source (LC / XFEL)
Slide 11
Imperial College London
11
Production of high currents of single charge state ion beams
The impact of electron with gaseous
atoms is mostly used for the
production of ion beams.
For efficient ion production the electron
energy should be app. 2-4 times the
ionization energy of the ion.
Slide 12
Imperial College London
12
Production of high currents of single charge state ion beams
A Townsend gas discharge using an avalanche
effect is an very effective way to produce a high
amount of ions. Therefore the Paschen criteria
has to be fulfilled. To improve the gas
discharge and to enhance plasma confinement
magnetic fields are used.
Slide 13
Imperial College London
13
Penning sources
The Penning Ion Source or PIG
source (Philips Ionization
vacuum Gauge) invented by
Penning in 1937 uses a a dipole
field for plasma confinement .
The strong magnetic dipole field
gives high efficiency as
electrons oscillate inside the
hollow anode between the the
two cathodes at each end.
The Lifetime of the source limited
by sputtering of the cathodes,
especially for highly charged,
heavy ion operation.
Slide 14
Imperial College London
14
Magnetron sources
The Magnetron ion source which was
first presented by Van Voorhis in
1934 uses a solenoidal magnetic
field for plasma confinement The
field of ~ 0.1 T is generated with
an external solenoid surrounding
the ion source. The chamber wall
serves as anode, while the
cathode provides electrons
through thermionic emission. The
filament mounted parallel to the
magnetic field forces the electrons
to spiral. As with Penning sources
the Lifetime of the source limited
by sputtering of the cathodes,
especially for highly charged,
heavy ion operation.
Slide 15
Imperial College London
15
Hot cathode sources
Filament Ion Source
Discharge in the plasma chamber is
driven by the electrons delivered
by the filament.
single charged ions up to 100 mA
•
filtermagnet
CoSmmagnets
solenoid
Bz
Plasma enclosure by magnets.
plasmaelectrode
Bx
groundelectrode
cathode
•
•
•
Pressure range 10-1 - 10-3 mbar.
Discharge voltage 20 - 200 V
(depending on ionization
voltage)
Discharge current 10 - 500 A
gasinlet
copper
isolator
water
steel
brass
magnets
100 mm
screeningelectrode
Slide 16
Imperial College London
16
RF sources
Non resonant excitation of plasma
by RF. Only lower charge states
available (low electron energy)
but high beam currents
possible.
Internal antenna to feed RF power
into plasma => limited lifetime of
antenna due to sputtering, strong
coupling of RF into plasma and
good plasma confinement.
Slide 17
Imperial College London
17
RF sources
Production of large ion currents (I>1 A)
of single charged ions for surface
treatment or plasma heating
(tokamaks). Multiaperture extraction
therefore difficult to feed beam into
conventional accelerator structure.
External antenna to feed RF
power into plasma => Long
lifetime of antenna, but chamber
has to be of non conducting
material.
Slide 18
Imperial College London
Production of high charge state ions
The PLASMA created is increased
in density by electron
bombardment. The maximum
charge state that will be
obtained depends on the
incident electron energy.
e + X = X+ + 2e
For multi-charge states
e + X i+ = X (i+1)+ + 2e
higher electron energies are
required since electrons have to
be removed from inner shells.
The maximum charge state is
limited by the incident electron
energy.
18
Slide 19
Imperial College London
19
Electron Cyclotron Resonance Source
whf = wcyc = (e/m) B
Radial and axial magnetic field distribution
for the confinement of the source plasma.
Only at the centre of the source the cyclotron
condition for the electrons is full filled.
(0.1-1 kW)
Extracted
ion currents
for different
charge
states of
Argon
Slide 20
Imperial College London
20
Electron Cyclotron Resonance source
Schematic layout of an ECR source
for the production of radioactive
ion beams
By variation of the longitudinal
enclosing magnetic mirror
configuration the charge
distribution can be influenced.
Slide 21
Imperial College London
21
Electron Beam Ion Source
CRYogenic Stockholm Ion Source
17 cm
Upper: First EBIS IEL-1 build by Donets in 1968,
lower : Evolution of charge state distribution
Parameters of CRYSIS
of nitrogen ions at Ee=5.45 keV
nominal values
max
values
units
electron beam current
350
1300
mA
electron beam energy
20
27.5
keV
trap length
1.2
-
m
magnetic field
1.5
5
T
charge per pulse
1-2
4
nC
ion pulse length
0.05-100
-
µs
containment time
20-2000
-
ms
Slide 22
Imperial College London
22
Electron Beam Ion Source
Total ion charge trapped
in an EBIS as a function
of confinement time, for
different electron beam
currents
Ion current extracted from
an EBIS as a function of the
charge state for Na ions
(Ne gas was added)
Comparison of the
extractable (electric)
current between
ECR and EBIS.
Slide 23
Imperial College London
23
Laser Ion Sources
Schematic drawing of the experimental set up of a laser ion source. By
the impact of the laser with the target, a plasma is created which expands
in the drift chamber and then is accelerated in the extraction gap.
Slide 24
Imperial College London
24
Laser Ion Sources
By interaction of the plasma electrons
with the electric field of the laser
pulse the electrons are heated. This
leads to a shift of the charge state
distribution of the plasma ions
towards higher charge states.
The different charge states of
the plasma are separated by
the drift longitudinally and
reduce space charge in the
acceleration gap
Slide 25
Imperial College London
25
Laser Ion Sources
The different charge
states of the plasma
are separated by the
drift longitudinally
(higher charge states
first) and reduce
space charge in the
acceleration gap.
The available laser power density on the
target (for fixed total laser power
influenced by beam size on target)
strongly influences the charge state
distribution of the plasma
Slide 26
Imperial College London
26
Production of negatively charged ion beams
g
Conserving energy when forming a negative ion through
direct electron attachment, the excess energy has to be
dissipated through a photon. A + e = A¯ + g. But radiative
Capture is rare (5•10-22cm2 for H2).
Higher cross sections (~10-20cm2 for H2 and Ee >10 eV) can
be realized when the excess energy can be transferred to a
third particle, M + e = A + B + e and sometimes = A + B¯
Even better are processes which excite a molecule to the
edge of breakup (vibrationally excited 4
Three Types of H- Ion Sources
Cs can be used as an
are in use
electron donator, but the
• Surface conversion sources
ionisation energy of 3.9 eV is
much higher than the 0.75 eV
• Volume production sources
electron affinity of H
• Hybrid production sources
=> Surface treatment
Slide 27
Imperial College London
27
Production of negatively charged ion beams
anode
driver
efast->H2
extractor
eslow->H-
cathode
gasinlet
Cross sections for
different production
and destruction
mechanisms
solenoid
filter
Magnetic dipole fields
can be used as filters
to create areas of
different electron
temperatures
Slide 28
Imperial College London
28
Sources using surface production
Picture of the LANCE
surface source
Schematic layout of the LANCE
surface source using a filament
driven gas discharge for plasma
production and surface
conversion on a Cs target for Hformation
Slide 29
Imperial College London
29
Sources using volume production
copper
magnet
isolator
water
screeningplasma- plasma- electrode
chamber electrode
electrondumping
grid
cathode
and
gasinlet
solenoid filter- bending- soleniod
magnet magnet
The small experimental H- source in Frankfurt used a hot
cathode driven gas discharge, dipole fields and a Pt surface
cover for the reduction of the dissociation of the H2 molecules
Slide 30
Imperial College London
30
Sources using hybrid production schemes
The RAL H- source uses a Penning
discharge (dipole for plasma
production and to influence electron
temperature) and Cs injection.
The SNS H- source developed in
Berkley uses RF to produce the
plasma, dipoles to influence the
electron density and a Cs collar.
Slide 31
Imperial College London
31
The extraction of particle beams
Slide 32
Imperial College London
The space charge limit and Child-Langmuirs law
32
Slide 33
Imperial College London
The space charge limit and Child-Langmuirs law
33
Slide 34
Imperial College London
External and internal fields in the extractor, laminar flow and
pierce angle
34
Slide 35
Imperial College London
External and internal fields in the extractor, laminar flow and
pierce angle
35
Slide 36
Imperial College London
The beam emittance, the acceptance of the extraction
system and the conservation of the phase space
36
Slide 37
Imperial College London
The beam emittance, the acceptance of the extraction
system and the conservation of the phase space
37
Slide 38
Imperial College London
The beam emittance, the acceptance of the extraction
system and the conservation of the phase space
38