Electron SourcesS. Bernal and I. Haber

USPAS 08

U. of Maryland, College Park

IREAPIREAP

IREAPIREAP

Most under R&DPlasma Ion source, cavity, optical trap,

etc.

Laser cooling, nano-physics,

etc.

Other: Plasma,

multipactor,

nano-tube, laser cooled, etc

Discharge tubes,

Field-Effect Microscopes

Cold-cathode,

Needle

Fowler-

Nordheim

Field-Emission>>> 1 A/cm2

Many accelerators, FELs, etc.RF Photocathode

Photoelectric effect

Photoemiss.>> 1 A/cm2

Microwave tubes, Incoherent X-Ray sources, etc.

Cathode + Electron Gun

Richardson-Dushman,

Child-Langmuir

ThermionicUp to 1 A/cm2

APPLICATIONSTYPICAL

CONFIG.PHYSICS

TYPE &Max. Current

Density

Electron Sources in General

IREAPIREAP

Cathode-driven,

Variable A/K gap

Pierce Geom., triode

10 keV, 100 mA, 100 ns

UMER Gun

rC= 4 mm

Cathode-driven,

Variable A/K gap

Pierce Geom., triode

5 keV, 120 mA

100 ns

LSE Gun

rC= 4 mm

Scaled-down replica of SLAC PEP gun

Pierce Geom., Convergent,

diode

8 keV, 200 mA, 1µs

Hughes Gun

rC=12.7 mm

DESCRIPTIONTYPEMax. Energy, Current, pulse

SOURCE

Electron Sources in UMER Lab

IREAPIREAPElectron Sources in UMER Lab

UMER E-Gun LSE E-Gun

Hughes E-Gun

Physics of Thermionic Emiss. and Diode E-Guns

IREAPIREAP

1See, for example, Leighton, Principles of Modern Physics, McGraw-Hill, 1959, p.355-56

2

6

2 2

exp ,

Amp1373K, 1.2 10 , 2.1V.

m K

CM C

B C

C

qWj AT

k T

T A W

= −

= = × =

Richardson-DushmanEquation1:

Child-Langmuir2:1 2 3 2

00 2

4 2,

9CL

Vqj

M dε

=

1-D diode withspace charge:

4 3

0( ) .x

x Vd

φ

=

V0

d

1-D diode w/ospace charge:

0( ) .x

x Vd

φ

=

V0

dx

y

2See Martin Reiser, Theory and Design of Charged Particle Beams, Sec. 2.5.2.

“Infinite” Diode “finite” Diode Pierce Diode

Pierce Electrode Geometry* IREAPIREAP

*Miroslav Sedlacek, Electron Physics of Vacuum and

Gaseous Devices, Sec. 6.2 (1996).

Assume V(x)∝x4/3, outside beam forfinite (in y-direction) diode.

Use complex formalism x→reiφ = ς+iξ,

V→W=VR+iVR.

Then VR∝r2/3cos[(4/3)arctan(ς/ ξ)].

Since cathode is grounded, VR=0, so

we need

(4/3)arctan(ς/ ξ)]=π/2, or slope at x=0 is

(3/4) π/2=67.50

UMER Electron Gun IREAPIREAP

Pierce

Electrode

TRIODE ELECTRON GUN: CATHODE DRIVEN, PIERCE TYPE.TRIODE ELECTRON GUN: CATHODE DRIVEN, PIERCE TYPE

UMER Electron Gun

-120

-100

-80

-60

-40

-20

0

20

-200 -150 -100 -50 0 50 100

Pearson_10kV - May 16, 2001

Nanoseconds Z=0

CATHODEAND

ANODE GRIDS8 mm

IREAPIREAP

UMER e-Gun Apertures IREAPIREAP

0.972.7852.85Full

1.5 0.875

0.25 mm

Aperture

Radius

0.983.0107

0.901.5240.810.77.20.280.3±20%0.55 mA

χ���� (����m)rms, norm.

Beam

Current

0.00

0.20

0.40

0.60

0.80

1.00

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Tu

ne D

ep

ressio

n,

σσ σσ/ σσ σσ

0

Inte

nsity

Para

mete

r, χχ χχ

Aperture Radius (mm)

σσσσ/σσσσ0

χχχχ

Ap=50BV=30V

Sol=0No Atten.

10 mm

Ap=50BV=42.2VSol=0No Atten.

Ap=50BV=45VSol=0No Atten.

Adjusting the

E-Gun Control

Grid Bias for

Optimal Output

6.2 mA,

10 keV