& 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 * hahaha13@snu.ac.kr & 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)