James R. Luke

Diode Laser-Driven Microthrusters: A New Departure for Micropropulsion

We developed an entirely new type of orientation thruster for micro- and nanosatellites. The laser plasma thruster is based on the recent commercial availability of diode lasers with sufficient brightness and 100% duty cycle to produce a repetitively pulsed or continuous vapor or plasma jet on a surface in vacuum. A low-voltage semiconductor switch can drive the laser. A lens focuses the laser diode output on the ablation target, producing a miniature jet that provides the thrust. Single-impulse dynamic range is nearly five orders of magnitude, and the minimum impulse bit is 1 nN/s in a 100-„s pulse. Even with diffraction-limited focusing optics, at least 0.5-W optical power is needed to produce thrust from selected ablator materials. Thrust-to-power ratio Cm is 50 to 100„N/W and specific impulse Isp is 200‐500 s with a 1-W laser, depending partially on the illumination mode. Transmission and reflection (R) illumination modes are discussed. R mode gives about 50% better Isp and two times better Cm. Improved results are anticipated from higher laser power in the reflection mode. The prototype engine we are developing is intended to provide lifetime on-orbit steering for a 5-kg satellite, as well as reentering it from low Earth orbit.

MICROPROPULSION USING A LASER ABLATION JET

We investigated exothermic laser ablation “fuels” for the micro Laser Plasma Thruster (µLPT), a novel type of microthruster. Using ms-duration laser pulses, which are required for multi-mode laser diodes to exceed ablation threshold fluence in the smallest focal spots available with conventional optics, successful target materials were restricted to those of low thermal conductivity, i.e., polymers, not metals. Polymers studied included carbon-doped polyvinylchloride for a passive target baseline, and several carbon-doped exothermic photopolymers specifically designed for their task, including polyvinylalcohol, a triazene polymer and a proprietary exothermic polymer (EP). In our single-shot impulse test setup, millimetric fuel samples were evaluated using a tiny torsion pendulum. Promising polymers were then made into fuel tapes and tested for continuous thrust under repetitive-pulse excitation. Two-layer fuel tapes consisted of a transparent supporting layer through which the light passed to ignite an absorbing fuel layer which formed a jet on the opposite side of the tape from that illuminated by the laser, an example of confined ablation. Best results were obtained with EP-1 up to 680 µN thrust with 2.1 W average optical power incident and jet velocity of 2‐3 km/s. Repeatability of our thrust-measuring torsion pendulum was improved to 1 µN.

Laser Ablation Powered Mini-Thruster

We have developed a new type of miniature jet for pointing microsatellites. It is based on laser ablation produced by a multi-mode diode laser. The target is a specially prepared tape with a transparent layer through which the laser light passes and an absorbing layer which produces the thrust. We have achieved specific impulse up to 1000 seconds (greater than possible with chemistry), together with laser momentum coupling coefficients of order 6 dyne/W. The preprototype should achieve 100 dynes of thrust. We will discuss the target interaction physics, the materials science involved in creating the targets, and some of our measurements with the preprototype thruster.

Micro laser plasma thrusters for small satellites

The micro laser plasma thruster ((mu) LPT) is an efficient, long- life, low-thrust pulsed rocket engine which uses a high-brightness semiconductor or glass fiber laser as a source of energy. It uses a simple, low-voltage semiconductor switch to drive the laser, resulting in zero off-state electrical power. Results are presented of the first experimental demonstration of uLPTs. We measured single impulses covering 5 orders of magnitude from 40 micro dyn-s [< 1 nano newton-s] to 2 dyn-s, specific impulse up to 1,800 seconds and coupling coefficients up to 25 dyne-s/J. Several target materials were studied. Initial applications are orientation and re-entry at end of life for micro- and nanosatellites. Anticipated lifetime output of the prototype engine now under development is about 5E7 dyn-s [500 newton-s], sufficient to re-enter a 5 kg LEO satellite.

Performance Test Results for the Laser‐Powered Microthruster

Abstract : Microthrusters are useful for orienting and repositioning small craft above the atmosphere. We report technical results obtained during a successful 5-year program to develop a commercially-viable laser-powered microthruster. Its main advantage is the ability to generate a broad thrust range under programmable electronic control with minimal electrical power. The device applies millisecond-duration diode-laser pulses to a fuel tape to produce an ablation jet. By employing laser-initiated energetic polymers in our ablation fuel tapes, we obtained momentum coupling coefficients as large as 2.0mN/W of incident laser power, giving a continuous thrust range from 50 N to 10mN. With our standard 30m x 8mm fuel tape, fueled thruster mass is 0.4kg and 40N-s lifetime impulse is achieved. With an order-of-magnitude greater fuel mass, the thruster could accomplish re-entry or substantial orbit-raising of a 10-kg microsatellite. In its usual configuration, specific impulse is 200 seconds, and ablation efficiency, the ratio of exhaust kinetic energy to incident laser optical energy is 180%. We compare performance of several laser-initiated micropropellants which we studied, including polyvinyl nitrate (PVN), glycidyl azide polymer (GAP), and nitrocellulose (NC). All were doped with a laser-absorbing component, either carbon nanopearls with 10nm mean diameter or dyes tuned to the 920-nm laser wavelength but transparent at visible wavelengths. Our demonstrated momentum coupling coefficient is sufficient to levitate a 0.1-kg object with a 400-W laser beam having appropriate characteristics.

Advantages of a ns‐pulse micro‐Laser Plasma Thruster

We describe a new project to develop an improved micro‐laser‐plasma thruster (LPT) using nanosecond pulses. As with the previous version driven by a ms‐duration diode laser, the purpose of this microthruster is to position and attitude control of micro‐ and nano‐satellites.

A ns‐Pulse Laser Microthruster

We have developed a prototype device which demonstrates the feasibility of using ns‐duration laser pulses in a laser microthruster. Relative to the ms‐duration thrusters which we have demonstrated in the past, this change offers the use of any target material, the use of reflection‐mode target illumination, and adjustable specific impulse. Specific impulse is adjusted by varying laser intensity on target. In this way, we were able to vary specific impulse from 200s to 3,200s on gold. We used a Concepts Research, Inc. microchip laser with 170mW average optical power, 8kHz repetition rate and 20μJ pulse energy for many of the measurements. Thrust was in the 100nN – 1μN range for all the work, requiring development of an extremely sensitive, low‐noise thrust stand. We will discuss the design of metallic fuel delivery systems. Ablation efficiency near 100% was observed. Results obtained on metallic fuel systems agreed with simulations. We also report time‐of‐flight measurements on ejected metal ions, which ga...

Giant Momentum Coupling Coefficients from Nanoscale Laser-initiated Exothermic Compounds

*† ‡ § ** †† ‡‡ We report here on results obtained with laser-initiated micro-propellants, such as PVN, PVC, GAP, NC, and mixtures of these. All samples were doped with a laser absorbing component. In some cases, this was carbon nanopearls with 10nm mean diameter, while, in others, it was a carbon-based ink with ∝m-size particles. We also report results of performance tests for absorbers tuned to the 935-nm laser wavelength.

Plasma generation and plume expansion for a transmission-mode micro-laser ablation plasma thruster

An end-to-end model is presented of the transient plume created by a micro laser-ablation plasma thruster. In this paper we describe a model of the plasma generation and expansion for a microlaser plasma thruster operated in transmission-mode (T-mode). The laser ablation and plasma formation processes are modeled using a kinetic ablation model. This procedure provides boundary conditions at the target surface for the plume model that is based on a particle computational approach. The present study considers a 2.5-8 W diode-based laser irradiating a poly-vinyl chloride target over a rectangular focal spot of 30 x 120 μm for a pulse length of 3-10 ms. The plume simulations reveal many details of the multi-component plasma expansion. The results are compared with experimentally obtained plume signatures. Generally good agreement between experimental and calculated flux profiles is found.

A Transient Model of a Cs‐Ba Diode

A transient model of a Cs‐Ba diode is developed assuming homogeneous plasma conditions. The diode model is coupled to a sub‐model of the control circuit that is being used to test the Cs‐Ba tacitron. Model calculations compare favorably with experimental results. The model predicts correct trends, such as increased voltage drop as either the Cs pressure in the interelectrode gap, or the discharge current increases or the decrease of minimum voltage drop with increasing Cs reservoir temperature. The model also determines the I–V curves that not only have the right shape but are numerically similar to the measured I–V curves.