Overview Negative Ion Sources: Magnetron and Penning Dan Faircloth Ion Source Section Leader Rutherford Appleton Laboratory History The caesium revolution Magnetron sources Penning sources Failure modes and sputtering ISIS Developments GE Research Lab, Schenectady, NY 96 Irving Langmuir Irving Langmuir Albert Hull Using magnetism to find alternatives to patented electrostatic control of valves E x B 920 Comet valves? Boomerang valves? Ballistic valves? MAGNETRON VALVES 920 s Starts adding gasses to his valves and going to high powers. Langmuir talks to his fellow New England scientists Magnetron Ion Source First reported in 934 as a proton source by Stanley Van Voorhis and his team in Princeton Louis Maxwell The Franklin Institute Philadelphia 930 Penning Ion Source 937 Penning Ionisation Gauge or Philips Ionisation Gauge (PIG) Frans Penning 927 Penning Ionisation: A m + B A + B + + e + E i.e. Add a sniff of argon Also developed by Overton Luhr and others at MIT and Union College Philips Physics Laboratory -Eindhoven
Spawn a series of variations Penning Source - Calutron source - Bernas source - Nielson source Magnetron Source - Freeman source The Penning style source (Calutron) starts the Cold War Fundamental Geometry B B Increasing Need for Negative Ion Beams Tandem accelerators Cyclotron extraction Magnetron Penning Multi-turn injection into rings Neutral Beams Stripping foil Protons H - from Linac Off Axis Duoplasmatron Extraction Off Axis Duoplasmatron Extraction Displacement 960 s George Lawrence Los Alamos 2
Off Axis Duoplasmatron Extraction Electron Current (ma) 300 250 200 50 00 50 Electron Current H Current 70 60 50 40 30 20 0 H Current (μa) 962 Victor Krohn + ions on a metal target increase yield of sputtered negative ions by an order of magnitude 0 0 0.2 0.4 0.6 0.8.2.4.6.8 2 2.2 Displacement (mm) 0 Space Technology Laboratories inc. Redondo Beach, California Early 970s Budker Institute of Nuclear Physics Novosibirsk Surface Plasma Sources (SPS) 5 g Caesium Ampoule Gennady Dimov, Yuri Belchenko, Vadim Dudnikov Production of H ions by surface ionisation with the addition of ceasium Electrodes......have work functions 3
Pure molybdenum 4.6 Caesium Coverage Fermilevels Work Function (ev) 2..5 Pure Caesium P 0.6 Thickness (monolayers) H H Pressure (mbar) Vary Caesium Vapour Pressure to Control Caesium Coverage 0.05 0.04 0.03 0.02 0.0 0.00 00 20 40 60 80 200 Oven Temperature (Degrees C) Pressure (mbar).0e-0.0e-02.0e-03.0e-04 Log Scale 00 20 40 60 80 200 Oven Temperature (Degrees C) Electrons B Electron Dump Negative Ion Extraction Negative Ion Beam Electrons will also be extracted Up to 000 times the H current! Use a magnetic field Dump must be properly designed SPS sources: only 0.5 to 0 times H current 970s Caesium Revolution! Anode Magnetron Source Hydrogen Hydrogen Cathode Anode B Cathode Soviets spread the word and develop sources BNL Krsto Prelec et al. develop the magnetron for NBI LANL Paul Allison et al. develop the Penning Berkley Ehlers+Leung develop Surface Converter sources Fermilab Chuck Schmidt et al. develop the BNL magnetron for accelerators Caesium Vapour Extraction Electrode H Beam Electrons 0 mm B Extraction Electrode Beam Magnetic Pole Pieces 4
980 BNL Developments BNL 2 A Beam H - Magnetron for NBI A Budker Semiplanotron Fermilab Magnetron 6cm Fermilab Magnetron 5
Fermilab Magnetron Fermilab Magnetron Fermilab Magnetron Caesium: Friend of H - but mortal enemy of high voltage 6
6cm BNL Magnetron 989 BNL Magnetron Lifetime, typically 9 months Very good power efficiency ~ 67mA/kW High beam currents ~ 00mA H- current 90-00mA Extraction 35kV Voltage Arc Voltage 40-60V Arc Current 8-8A Rep Rate 7.5Hz Pulse width 700µs Duty Factor 0.5% 0.5mg/hr consumption Gas Flow 3sccm RMS emittance 0.4 πmm.mrad (normalized) BNL Magnetron Extraction cone: 45deg angle 3.2 mm aperture BNL Magnetron.3 m 7
202 New Fermilab Magnetron (Based on BNL design) New FermilabMagnetron Fermilab HINS Magnetron ~in anode cover plate ceramic extractor standoffs 94 ma Vertical en rms = 0.8 mm.mrad extraction gap.090in gas valve extraction cone C. Schmidt Horizontal en rms = 0.2 mm.mrad tube cathode connections Magnetrons are noisy! C. Schmidt Plasma Sheath Magnetron Source H - produced on the cathode surface are accelerated by the cathode plasma sheath towards the extraction aperture 8
Resonant Charge Exchange DESY HERA Magnetron Source Leaving slow H - H - (fast) + H 0 (slow) H 0 (fast) + H - (slow) Slow thermal H 0 produced in the plasma ( 0. ev) Can undergo resonant charge exchange with fast H - ( 80 ev) produced at the cathode surfaces DESY HERA Magnetron Source Penning Ion Sources Invented by Dudnikov in the 970 s Very high current density > Acm -2 Low noise Will not work without caesium Almost Three Years! Key Design Points for H - Production:. Electrodes are made of Molybdenum 4.5 ev work function and a high melting point Key Design Points for H - Production: Electrodes are made of Molybdenum (4.5 ev work function) and a high melting point Electrons are emitted from the Cathode surface 9
Key Design Points for H - Production: Caesium vapour further lowers the cathode work function (.5 ev) Key Design Points for H - Production: Penning Field confines the electrons increasing the number of ionisations B Key Design Points for H - Production: Cathode geometry causes the electrons to reflex back and forwards Plasma Production Region H 2, H 0, H +, H 2+, H 3+,, +, e -, H - Plasma Potential 0 V -60 V Plasma Sheath Distance Resonant charge exchange near the extraction region H - (fast) + H 0 (slow) H 0 (fast) + H - (slow) Leaving slow H - Essential to producing low noise beams 0
The Overall Behaviour Not Well Understood! INR Moscow Penning Extraction Region Slow H - Production Region Plasma Production Region H 2, H 0, H +, H 2+, H 3+,, +, e -, H - Pulse beam current 40 ma Pulse repetition rate (PRR) 2 50 Hz Macro-pulse beam current duration 60 200 µs Normalized emittance 0.35 π mm mrad Novosibirsk design INR Moscow Penning Los Alamos Scaled Penning source
Los Alamos Scaled Penning source ISIS Penning source Penning Pole Pieces Aperture Plate Negative Ion Beam Extraction Electrode Discharge Region 20 ma 500 µs 60 Hz Hollow Anode Source Body Cathode Mica Air Cooling Channels Ceramic Spacer Copper Spacer Mounting Flange Water Cooling Channels 0mm +7 kv Extraction Voltage 50 A Discharge Negative Ion Beam Caesium Vapour Heated Transport Line Caesium Oven Hollow Anode Piezo Hydrogen Valve 0mm H 2 +7 kv Extraction Voltage 50 A Discharge Negative Ion Beam Caesium Vapour Heated Transport Line Caesium Oven + 7 kv Extraction Voltage 50 A Discharge Negative Ion Beam Caesium Vapour Heated Transport Line Caesium Oven Source Runs at 50 Hz Rep Rate Hollow Anode Source Runs at 50 Hz Rep Rate Hollow Anode 0mm 0mm Piezo Hydrogen Valve Piezo Hydrogen Valve H 2 H 2 2
Timing H 2 Gas Pulse Cathode Hydrogen Feed Hollow Anode Air Cooling Heated Caesium Transport Line ~ 200 μs ~ 600 μs 50 A Discharge Pulse ~ 250 μs 7 kv Extract Pulse H - Beam Source Body Discharge Power Feed Source Runs at 50 Hz Rep Rate Time Thermocouples Aperture Plate Extraction Mount Extraction Electrode Support Insulators Caesium Shields Caesium Oven Caesium Oven View Port Heater and Thermocouple Connectors Piezo Hydrogen Valve Connector Thermocouple Connectors Discharge Power Connector Air and Water Connectors Hydrogen Connector 3
Pulsed 7kV Extraction - Power Supply + 90 Analysing Magnet Coldbox Caesium Trap Platform Ground Platform DC Power Supply 35 kv - 8 kv + Laboratory Ground 35 kev H - Beam Extraction Electrode, Coldbox and Analysing Magnet all Pulsed Post Extraction Acceleration Gap Extraction Voltage Feed Ion Source 35 kv Insulator Ion Source Coldbox Sector Magnet Coil Refrigerator Feeds Ion Source Extraction Power Supply Fridge Discharge Power Supplies + Temperature Controllers RFQ LEBT 4
SPS Failure Modes Blocked caesium transport Failed heaters Failed piezo hydrogen valve Ancillary equipment failure Sputtering Blocked Aperture Plate Shorted Electrodes Compare SPS Lifetimes DESY FNAL BNL ISIS Discharge Current (A) 47 50 8 55 Pulse length (us) 75 80 700 800 Rep rate (Hz) 6.25 5 7.5 50 Duty Factor (%) 0.047 0.2 0.525 4 Lifetime (Days) 900 200 270 30 Lifetime (Plasma Days) 0.42 0.24.42.2 Fermilab Magnetron Ageing beam current Hydrogen pressure Arc current H2 On timing ISIS Penning Ageing Fermilab Magnetron 5
Fermilab Magnetron Fermilab Magnetron Fermilab Magnetron Typical Source Failures Cathode material flakes blocking source extraction aperture beam current arc current Cathode material flakes off and causes cathode/anode shorts ISIS Penning 26 Day Electrode Wear 6
H 2 7
3 Sources at ISIS Ion Source Operational Source 24 x 7 operation 20 day average lifetime 200-300 μs pulse length 50 Hz 35 kev 35 ma @ RFQ Ion Source Development Rig Pre-test operational sources Problem solving FETS Source Experimental sources High current Long pulse 65 kev 9 kv extract power supply Cold Box Refrigerator Chiller Circulator Control Racks and Power Supplies ISDR 35 kv Insulator 3 Sources at ISIS Mark Y-Emittance Scanner Pepperpot and Profile Measurement Trev Diagnostics Vessel X-Emittance Scanner Operational Source 24 x 7 operation 20 day average lifetime 200-300 μs pulse length 50 Hz 35 kev 35 ma @ RFQ Ion Source Development Rig Pre-test operational sources Problem solving FETS Source Experimental sources High current Long pulse 65 kev Diagnostics and a LEBT are critical to Ion Source Development Optical Spectroscopy Toroid 0-4 mbar 6 0-5 mbar Isolating Column 4 800 Ls - & x 400 Ls - Turbo Pumps Laser Diagnostics Solenoid 2 2000 Ls - Turbo Pump 7 0-6 mbar Solenoid 3 Toroid 2 Toroid3 Solenoid Retractable Faraday Cup Differential pumping and laser profile vessel Beam shutter Slit-slit scanners 400 Ls - turbo pump 5 0-6 mbar Toroid 4 Diagnostics vessel Camera ENERGY ANALSER Pepper pot or profile scintillator head. Extend discharge duty cycle Finite Element Modelling Steady state calculation FETS source modifications 600 520 440 360 280 200 700 650 600 550 500 450 Computational fluid dynamic cooling calculation Temperature (C) Transient Calculation Cathode Surface Anode Surface 0 0.005 0.0 0.05 0.02 0.025 0.03 Time (seconds) ΔT= 73 ºC ΔT= 39 ºC 2.2 ms discharge at 50 Hz achieved with simple design changes + new PS 8
0.2 kvmm -.2 kvmm - - 5.5 kvmm 20 00 80 60 40 20 0-00 0 00 200 300 400 500 600 700 800 900 000-20 - 40-60 - 80-00.8 kvmm - - 8.5 kvmm 2. kvmm - 9.5 kvmm - E xt V (k V) Dis Cur (A) kv PA Voltage 7 kv PA Voltage 9 kv PA Voltage 25 kv PA Voltage Increasing Post Acceleration Voltage Constant 0 kv Extraction Voltage 23 ma H - Beam Current Measured 355 mm from Ground Plane of PA Gap H- (ma) 2.7 kvmm - 2.5 kvmm - 5/3/202. Extend discharge duty cycle 2. Discharge current Discharge Current Experiments For each extraction condition there is a range of discharge currents that give minimum beam divergence. Extend discharge duty cycle 3. Permanent magnet Penning field 2. Discharge current B Nd 2 Fe 4 B Permanent Magnets To allow different extraction voltages the Penning field must be decoupled from the sector magnet field 4 kv extraction voltage 2.2 mm extraction gap FETS source modifications FETS source modifications Permanent magnets are used to produce the produce the 0.5 0.25 T required for a stable discharge. Extend discharge duty cycle 3. Permanent magnet Penning field 2. Discharge current 4. Extraction Voltage, Geometry and Meniscus Widen plasma electrode aperture Increase voltage from 7 to 25 kv Child-Langmuir 3 2 V I B d FETS source modifications Work in progress 2 Meniscus Studies 78 ma. Extend discharge duty cycle 3. Permanent magnet Magnet Redesign Beam expands under space Penning chargefield Dipole has a focusing component Exact degree of compensation unknown 2. Discharge current R 4. Extraction e db Field gradient index Optimum field gradient index n =.2 n = Be dr determined by experiment Magnetic Field Gradient Index, n FETS source modifications 5. Analysing magnet Size of good field region increased Field must be adequately terminated 0 0.5T Significant improvement in emittance. Extend discharge duty cycle 3. Permanent magnet Optimize Gap Penning field 0.70 0.3 kvmm 0.68 2. Discharge current - 0.4 kvmm - 0.5 kvmm - 0.66 4. Extraction 0.64 0.62 6. Post 0.60 0.58 xx' emittance acceleration norm. rms-emittance, 00% 0.56 0.54 0.52 0.50 yy' emittance 0 0 20 30 40 Electric Field [kv/mm] Minimum emittance growth occurs for a post acceleration field of 9 kvmm - 55.0 mm PA Gap Increasing Post Acceleration Gap Length 2.0 mm PA Gap 2.5 mm PA Gap 9.2 mm PA Gap 4.4 kvmm - 6.8 kvmm - 7.6 kvmm - 0 kvmm - Experimental Source Configurations Top Loading Ion Source Ion Source Assembly Pole tip extensions on the 90 Analysing Magnet Separate Penning Field Penning Field B FETS source modifications 5. Analysing magnet Magnet Assembly 9
Many Experiments Energy Spread Extraction Voltage Extraction Geometry Post Acceleration Gap Multi BeamlettExtraction ma/cm 2 2.5 2.0.5.0 0.5 0.0 Post Acceleration Voltage Anode Geometry Optical Spectroscopy Operating Conditions 62 ma ms 50 Hz Operation Emittances Vertical 0.29 πmm.mrad (rms norm.) Horizontal 0.49 πmm.mrad (rms norm.).2 ms 60 A discharge, 9.6 kv extraction voltage, 65 kev beam, 80 C caesium oven, 6 mlmin - H 2 60 ma 2ms 25 Hz Operation Droop is unavoidable at 50 Hz 2 ms H Beam Current Hydrogen timing has been fully investigated including double pulses Neither caesium or hydrogen settings can mitigate this droop 2.2 ms, 64 A discharge, 9.6 kv extraction voltage, 65 kev beam, 90 C caesium oven, 6 mlmin - H 2 It is a fundamental problem with electrode surface temperature rise during the pulse 20
ISIS Source Around the World IHEP China are using the ISIS source on CSNS ISIS Source Around the World University of the Basque Country are developing an Ion Source Test Stand in collaboration with ISIS Chinese Spallation Neutron Source ESS Bilbao Future Plasma and Extraction Test Stand: Detailed understanding of plasma Detailed understanding of extraction Scaled source How the Penning Source Ended the Cold War MAD Strategy: Mutually Assured Destruction Star Wars 23 March 983: Regan announces the Strategic Defence Initiative (SDI) 2
Beam Experiment Aboard Rocket (BEAR) Less than 4 months Later... 9 November 989 3 July 989: H Ions from a Penning Ion Source 0 ma, 50 µs pulses at 5 Hz 425 MHz MeV RFQ Gas-cell neutralizer Los Alamos National Laboratory -minute flight to a maximum altitude of 95 km The End Thanks to: Dan Bollinger for magnetron slides Viktor Klenov for INR Penning photographs 22