Comments About Current Plasma Studies at UIUC Dr. George H. Miley, Hugo Leon, Atanu Khan, Ben Ulmen, Guilherme Amadio, William Matisiak, George Chen, Paul Keutelian

Research

Description of the IEC Jet Thruster Concept Spherical plasma diode Ground potential on an outer sphere Negative potential on transparent inner spherical grid Ions generated in discharge region between vacuum chamber wall grid Electron emitters at boundary and additional electrical grids near grounded outer sphere can increase and control efficiency better localize generation of ions Allows device to operate at much lower gas pressures

Background NASA and other laboratories have worked to develop advanced Hall Thrusters for future satellite applications. IEC-jet thruster are able to address both issues- scalability and plasma jet control for maneuverability. Conventional plasma thrusters have undergone much more experimental study than the IEC-jet thruster. Simplicity of IEC-Jet thruster design and thermal scalability- feasible to quickly develop and test a range of parameters and exhaust channel designs.

Application of Jet Mode for Thrust Novel plasma jet thruster, based on Inertial Electrostatic Confinement (IEC) technology, -for ultra maneuverable - space thruster for satellite and small probe thrust operations. IEC Jet design potential - cover a wide range of powers with good efficiency while providing plasma jet that can start with large diameter but be narrowed directionally to focus. Analogous to planar electrostatic ion thruster "folded" into spherical form. Electrical efficiency match conventional plasma thrusters; design simplicity reduced erosion giving long life time reduced propellant leakage losses high power-to-weight ratio Low gas leakage & good heat removal make it possible to scale the design to low power or high power.

IEC Thruster Configuration Valley or trough created in electrostatic potential, and hole cut into the ground sphere. Allows high-speed ions to escape in the form of a plasma jet. Ions generated near the ground potential with aid of electron emitters and additional grids. Central spherical electrical grid accelerates ions to core region. Cylindrical "channel" grid with same electrostatic potential as the central spherical grid creates passage for ions to escape to outside.

IEC Thruster Configuration

Performance Parameters Planar and IEC Ion Thrusters Planar Ion Thruster Propellant Xenon Molecular Weight ( amu ) 131.3 Specific Impulse (s ) 3000 Thrust (mN) 34 Jet Power (W) 500 Net accelerating Potential 600 593 Beam Current 832 842 Power Loss to Grid ≤50 50 Power Loss to Bremsstrahlung Radiation Greater than planar but still negligible negligible Power loss to Ionization of Propellant (W) 200-250 W 256 W Input Power 750-800 806 Thruster Efficiency 62-68 62

Performance & Design Losses of power to electrical grids should be lower Higher densities and temperatures in the central core plasma, but increased losses due to Bremsstrahlung radiation still negligible . Energy expenditure per ion for IEC device has not been established experimentally, but can be estimated. IEC ion thruster - totally new concept: little testing has been carried out on it exact performance is not firmly established several issues need to be investigated experimentally and theoretically to develop a reliable thruster for high power applications. directly applicable to many basic underlying thruster issues   Performance of the IEC thruster is dependent on three major processes: ionization, microchannel formation and, redirection of ions into the plasma jet. The exact thrust and velocity of the plasma jet as well as power losses due to ionization, thermal radiation, Bremsstrahlung radiation, and grid dissipation need to be measured in the experimental device.

Additional IEC Jet Applications Waste Treatment Separation Elements can be separated by mass, charge, electronic state, or by combinations Several separation processes applicable, but further study needed to select optimal approach considering plasma components and direct energy conversion

Dipole Assisted IEC

The Concept of Dipole-assisted IEC An opportunity to enhance the Inertial Electrostatic Confinement (IEC) fusion exists by introducing magnetic dipole to the IEC chamber. The dipole fields will increase the plasma density in the center region of the IEC and the combined IEC and dipole confinement properties will reduce plasma losses. The DaIEC uses a dipole magnet in the center of two hemispherical grids. Two ion sources inject 40 keV deuterium and helium-3 ion beams toward the center of the DaIEC. Magnetic field focus the ion beams by trapping ions along field lines. Hence, a high density beam-beam fusion region within the center of the DaIEC. The charged fusion products of D-He3 fusion reaction (14.7 MeV protons,4 MeV a particles). Plus the background plasma ions exit through a magnetic diverter (nozzle) to achieve thrust.

Summary of PoP Study, and a graph of Electron Density vs. B-field. Magnetic field increases the electron density by a factor of 16. Electron temperature decreases in the presence of a magnetic field Discharge voltage decreases in the presence of a magnetic field The magnetic coil can be used to impose a potential in the central plasma to control space charge build up Overall, the use of the dipole provides improved ion beam focusing, ion confinement, and also appears to favorably affect the discharge voltage characteristics.

Photographs verify the dipole focusing effect when the magnetic field is on

The DaIEC can be used in a hybrid mode to produce both thrust and electricity. A key advantage of D-3He DaIEC is that the charged fusion products can be collimated and directly converted to thrust. The charged particles can also be used in a direct energy conversion to produce the electricity. To combine these systems we use the fact that the magnetic field channels charged particles in two directions – forward and aft from the reactor. The forward direction is used for electrical production and aft for thrust. To prevent particles in the electrical converter from canceling the thrust, a radial direct collection scheme is proposed.

DaIEC The DaIEC is a highly attractive space power/ propulsion fusion concept. Due to its physical simplicity and projected high power density, DaIEC is competitive and exceeds the capability of most other proposed space fusion systems. The specific high power and impulse will make it attractive for cargo and manned missions throughout the solar system, most notably a manned Mars mission. However, there are significant research and physics issues that must be resolved before the complete system can become a reality. The next step should be the achievement of beam-beam fusion.

High-Current, Low-Energy Deuterium Glow Discharge for Studies of Non-Linear Effects in Plasma Facing Materials   Schematic diagram of glow discharge set up: 1-vacuum chamber; 2-cathode support, 3 – cathode, 4- Mo anode with holes; 5-15 m thick Be-foil; 6-CR-39 detectors; 7-discharge zone supply; 8-scintillator/X-ray detector.

High-Current, Low-Energy Deuterium Glow Discharge for Studies of Non-Linear Effects in Plasma Facing Materials   Evaluation of DD and DT-reactions at the first wall surface of fusion reactors like ITER has not considered non-linear processes during high current, low energy bombardment of the metal first wall.   Concentrations of D and T atoms embedded in the wall surface increases = a “target” for bombarding ions.  Conventional (free space) DD-reaction cross-sections predict the DD-reactions are negligible at the low energies (≤2 keV) involved.   But, the free space approximation is not accurate for the conditions involved. The DD-reaction yield can be orders of magnitude higher than predicted by extrapolation of the standard (free space) DD-reaction cross-section to lower deuteron energies.  These enhancement (non-linear) effects came from a drastic increase in the deuteron screening potential in the crystalline structure of the metal targets at Ed ~ 1.0 keV, especially at a high deuteron current density where the ion density in the target can become quite large. 

Astrophysical Connection Nuclear reactions in astrophysical objects also encounter screening conditions similar to this.  Consequently studies of metal targets bombarded by low energy accelerators has been strongly studied by groups such as the European Astrophysical Lab (LUNA) while time integrated yields become large (hence limiting wall lifetimes), the instantaneous yields are low.  Thus the key to accurate measurements involves using high current bombardment plus special detectors such as CR-39 tracking foils to measure charged particle emission during bombardment. Modified IEC Glow Discharge offers High Currents needed

Summary of Experimental Data In accelerator measurements with the Ti-target at 2.5 < Ed < 10.0 keV, the deduced screening potential is Ue = 65  10 eV . However, for the PGD experiment, the screening potential is as large as Us=620  140 eV Put another way, this experimental enhancement for GD in terms of DD-proton yield even at Ed=1.0 keV is about nine orders of magnitude larger than that predicted with bare (B&H) cross-section. This striking result illustrates the importance of the higher deuteron/electron densities in the target (due to the higher currents in the GD) In addition to fusion plasma wall effects, these densities are also representative of Astrophysical conditions. Thus the effect is not only of strong scientific interest, but of direct importance to ITER type fusion reactors and also to nuclear reaction rates under astrophysical conditions.

Future Modifications Plasma probe diagnostics would be added to measure the energy distribution of deuterons in glow discharge. This is important to monitor bombardment conditions for accurate rate calculations. Temperature measurements of the target surface versus discharge power (current, voltage and pulse on-to-off ratio) would be performed using a IR sensor.  This data is needed to accurately calculate the deuteron diffusivity, which appears to be an important parameter affecting the reaction rate. The deuterium concentration in the target would be estimated using a four probe electrical resistivity measurement at the cathode. A dE-ES: surface barrier type detector would be added to supplement CR-39 measurements of emitted proton energies.  Due to noise problems, this detection will be best used at higher discharge voltages.

In Development Electrostatic analyzer Interactive 3-D model of IEC

Thank You

DD-reaction enhancement factor calculated with formula (4) for the Ti target during deuteron bombardment with accelerator [16] (curve 1) and glow discharge (curve 2). The solid parts of the curves are corresponded to the deuteron energy ranges where DD-reaction yield was measured experimentally. Experimental yield of 3.0 MeV protons at 0.8 < Ed < 2.45 keV, normalized to that at Ed = 2.45 keV. The bare cross-section corresponded to Bosch and Halle approximation to Ed  2.45 keV is marked by a solid line. The dashed line is a DD-reaction yield =in accordance with a screening potential value Ue = 610 eV.

Comparison of High Current, Low Energy D Accelerator and Pulsed GD Table 1 Comparison of High Current, Low Energy D Accelerator and Pulsed GD Parameter I, range Ed(lab), keV, range Wmax, [W] P, mm Hg T,K, target D+ energy spread *High Current Accelerator 10-40 μA 100.0- 2.0 2.0 5*10-7, vacuum 100-350 ± 1.0% **Pulsed Glow Discharge (PGD) 100-600 mA 2.5- 0.40 200.0 2.0-10.0, D2 200-2000 ± 10.0% *The accelerator uses a Duoplasmatron (Ed = 50 keV) ion source, decelerating system and magnetic focusing installation ]. **Power supply given a periodic rectangular current pulse. The pulse duration can vary within 100 - 600 μs. The distance between cathode and anode is varied between 4.0 and 6.0 mm.

Dipole-Assisted IEC An opportunity to enhance the Inertial Electrostatic Confinement (IEC) fusion exists by introducing magnetic dipole to the IEC chamber. The dipole fields will increase the plasma density in the center region of the IEC and the combined IEC and dipole confinement properties will reduce plasma losses. To demonstrate that a hybrid Dipole-IEC (DaIEC) a first model DaIEC experiment was benchmarked against a reference IEC. A triple Langmuir probe was used to study the electron temperature and density.

Adaptation to Propulsion Unit Achievement of a beam-beam device requires two key principles: beam compression via focusing reduction of the background gas pressure The first objective can be achieved based on the results obtained in the PoP project and applies a dipole magnetic field to create sharply focused beam ions in the IEC chamber. External control of the potential of the virtual anode formed in the compressed beam region is achieved by biasing the dipole coil potential to keep it near the accelerating grid potential. The second goal of achievement of low background pressures will be achieved using external ion injectors.

Ion Injection can be used to achieve low background pressure, enabling Beam-Beam reactions. An RF Ion Gun developed earlier at UIUC forms the basis for this concept. A schematic of the Ion BeamTM is shown on the right. The photo on the left shows a 3-D mm hole cut into solid stainless ball placed 20 cm from the nozzle of the Ion BeamTM.

Fusion Ship II Concept

Clearly, the higher specific power and specific impulse offered by a DaIEC system significantly reduces mission times. Specific Impulse Variations for a (a) manned Mars mission; (b) Triton sample return mission

Mission Considerations The thrust-to-weight ratio of a DaIEC propulsion system is too low for surface-to-orbit missions. Launched from a space platform is envisioned. The DaIEC is ideally suited for cargo and manned interplanetary missions due to the very high specific power (thrust power/ propulsion system mass) and specific impulse (average exhaust velocity normalized by earth’s surface gravity).

Star Mode Under certain grid voltage, gas pressure, gas type, and grid configuration conditions, high density ion and electron beams will form in the IEC device, initiating the "star mode" of operation. In this mode, high-density space-charge-neutralized ion beams, "Microchannels," pass through open spaces between grid wires. Discovered by U of I Researchers