1. & Figures Descriptions
Magnetic Field Effect on a PIG Type Ion Source for a Neutron Generator
June-Woo Juhn*, Yeong-Shin Park, Yong-Seok Hwang
Energy System Engineering, Seoul National University, Korea *
&
Figures
Descriptions
I. Introduction
#I.3. Inhomogeneous Magnetic Field Good to Axial Extraction2
#I.2. Basic Structure
#I.1. Simple Diagram of an NDI System
Gas Inlet
Radially Extracted Ion Beam Current
Electron
Ion
Cathode
Anode
Insulator
Axial Magnetic Field
Magnet
PIG Ion Source : Penning- or Phillips- type Ionization Gauge
Requirements
PIG Ion Source
Baggage
with a Explosive
Tritium Target
Ion Source
Deuterium Ion Beam
Neutron
Helium
Gamma-ray
Gamma
Detector
Alpha Detector
High Current
Stability
Compactness
Long Life-time
· Relatively Simple and Compact Design with Low Cost for Manufacture
Axially Extracted Ion Beam Current
· Most Popular Ion Source for Sealed Neutron Tube
as a Portable Detector of Implosives at Airports and Seaports
Fig. #I.1
Main Structure
Fig. #I.2
Basic Operation Mechanism
% From the calculation result using Monte-Carlo method, this shape of magnetic field gave higher ionization event number2
· Reflection Discharge : Electrical well for electron trapping by two opposite cathodes
· Axial Magnetic Field by Magnets : Prevention of electron’s radial motion to the hollow anode
#II.1. Parts Details of the PIG Ion Source
Cathode Cathode Adaptor
+ Insulator
+ Anode
+ Cathode
+ Magnets Chamber Mate
+ Cap
#II.2. Assembly Procedure
#II. 3. CX of the PIG Ion Source
Insulator [Alumina]
Cathode Adaptor [Stainless Steel]
Anode [Stainless Steel]
Cathode [Stainless Steel]
Permanent Magnet [NdFeB]
Cathode [Stainless Steel]
Extractor [Stainless Steel]
Extractor Spacer [Teflon]
Motivation and Objectives
· Studies on Magnetic Field Effect to PIG Ion Sources Especially Used for Neutron Tube
- Insufficient information of magnetic field effect for design and construction of PIG Ion Source
General Guide Line : Homogeneous magnetic field parallel to cylindrical axis1
· Previous Studies about Magnetic Field Structure Effect to Performance of Multiply Charged Ion Source
- Narrower magnetic field line in the middle of an axial chamber : Advantageous for PIG ion source of axial extraction system2
- Importance of magnetic field distribution near by cathode surface3
· Experiment of Hydrogen Ion Beam Current Varying Magnetic Field Structure Mainly on the Region of Cathode
Fig. #I.3
40mm
II. Experimental Setup
% Anode I.D. : 45mm
Cathode Surface Radius : 40mm
Cathode Gap Distance : 40 mm
Extraction Hole Size : 3mm
PIG Ion Source as a Test Bed of Magnetic Field Effect
Fig. #II.1
Fig. #II.2
Fig. #II.3
% Simple and Low-Cost PIG Ion Source
- No electrical vacuum feed-through by anode as a vacuum boundary
- Common materials
Overview of Apparatus
Fig. #II.4
#II. 4. Apparatus Diagram
#II. 5. Magnetic Field Structure Simulation
Experiment of Magnetic Field Structure
Anode
Permanent Magnet
Cathode
Extractor
Insulator
Mated To the Vacuum Chamber
Gas Feeding
Up to 500V
(1kV, 500mA)
for Plasma Discharge
Ion Beam
190mm
105mm
for Ion Beam Acceleration
Vacuum Pumping
Ion Gauge
Power Supply
Limiting Resistance
500Ω
Up to 20kV
(50kV, 80mA)
(a) 45mm
Cathode
Anode
Magnet
(b) 50mm
(c) 55mm
· Ring-type Permanent Magnets
- Material : NdFeB Dimensions : O.D. 80mm x I.D. 50mm x T. 10mm or 9mm
· 45, 50, 55mm Thicknesses Magnets by the Different Combinations
- Cathode Distance : 40mm
- Magnetic field on the cathode surface of beam extracting side is maintained (Magnitude of axial magnetic field ≈ 2000gauss)
- Opposite Cathode surface is filled with different shape of magnetic field line at each set of magnets
45mm T. : Relatively intense radial magnetic field with direction to the center of the circular cathode
50mm T. : Moderate radial magnetic field
55mm T. : Almost no radial magnetic field ≈ Considered as a parallel field
- Variation of Hydrogen Gas Flow Rate at Each Magnetic Configuration
- Fixed Discharge Power at 100W
Fig. #II.5
Table. #II.1
% The extracting side feels little changes of magnetic field
% Cathode surface region where the magnetic field variations mainly occurs
III. Result
#IV.1. Magnetic Field Line Corresponding to Extraction Hole
#III.1. Discharge Conditions
Plasma Discharge Condition
Fig. #III.1
T. #II.1. Gas Flow Rate Conditions
· Dominant Variation by Gas Pressure
- As flow rate goes down, discharge current increases in fixed discharge power (100W) except the case
Because of reflect discharge, PIG ion source can operate in the range of relatively low pressure (<mTorr )
· Remarkable Changes at 5sccm
- Different from the cases of higher flow rate, Less discharge current was measured as the radial magnetic field was intensified
Cathode
Anode
Magnet 50 55
45mm T. Magnet
Flow Rate [sccm] 5 10 20 30 40
Chamber Pressure [Torr]
2 x 10-5
4.8 x 10-5
9.9 x 10-5
1.4 x 10-5
2.6 x 10-5
% Gas pressure of beam extracted region was checked by Ion Gauge
% Pressure of plasma chamber may be an order of higher value
Extraction of Hydrogen Ion Beam
Fig. #III.2
· Much Higher Ion Beam Current Especially at Low Gas Flow Rate
- More than 2 times higher beam current was extracted at the cases of 5sccm and 10sccm
- Difference of beam current decreases as the pressure rises
· Large Increment of Beam Current with Small Alteration of Magnetic Field Structure
- 2.5mA at 12kV extraction voltage with 100W of input power
#III.2. Extracted Beam Current
40sccm
30sccm 50 55
45mm T. Magnet
5sccm
10sccm
20sccm
#IV.3. OOPIC Pro © Simulation
IV. Discussion & Conclusion
Anode
Cathode
Increment of Beam Current
· Generation of Denser Plasma in the Region of Extraction Hole
- Plasma density around the extraction hole can be calculated from the extracted beam current
· Mechanism of the Intensified Ion Beam Current
- Electrons are basically confined by the magnetic field and trapped by electric potential well
- Electrons leaving the opposite cathode of extraction side are Constricted by the negative radial magnetic field component
- Constricted electrons travel for longer time producing more ions around the extraction hole
- Diffusion rate proportional to the gas pressure could make this effect lower
Parallel and Homogenous B
B = Bz = 2000gauss
Ion Current Density
Plasma Ion Density
Eq. #IV.1
Fig. #IV.2
(a)
Eq. #IV.2
Diffusion Coefficient across the Magnetic Field
Mean Free Path
Gas Pressure
#IV.2. Employed Magnetic Field
(a) Intense Br
(b) Moderate Br
(c) Weak Br
Fig. #IV.2
#IV.3
(b)
(c)
(a)
· OOPIC Pro© Simulation
- Comparison of results from the magnetic field variation including the case of parallel field
- It is easily observed that the main particle motions and ionizations are generated along the constricted field line
(b)
Enhancement of Hydrogen Beam Current of PIG Ion Source by the Negative Radial Magnetic Field in the Cathode Surface Region at Low Gas Pressure
V. Reference
A. S. Pasyuk et al., At. Energy., 39 (2), 139 (1975)
G. Hadinger et al., IEEE Trans. Nucl. Sci., NS-19(2), 137 (1972)
Z. Song et al., IEEE Trans. Nucl. Sci, NS-32(5), 1826 (1985)
(c)