This answer is supplementary information I've collected in the past few days. I've accepted the other answer because it's excellent and identifies the actual source of the "1600" mystery value.
note: Cubesat-Friendly Developments are discussed at the end.
The Principle
The idea is to heat a propellant using electrical current instead of chemical reaction producing even higher temperatures and therefore higher specific impulse.
For a fixed temperature, higher thrust velocity comes from lower mass species, so hydrogen-rich propellants are desireable. Hydrazine is populare both because of its hydrogen content and good space experience and often pre-existing availability within spacecraft. Ammonia is another option.
The random kinetic energy of the hot species is converted to cold, directed kinetic energy upon expansion. For a given temperature, lower mass results in higher veolocity.
$$ E = \frac{1}{2}m v^2$$
$$v = \sqrt{2E/m}$$
Propellant at a rate of the order of 100 mg/sec passes through a restriction of the order of 1 millimeter in diameter where an electrical current of order 10-100 Amperes passes through the dense, almost completely ionized gas. The ohmic heating of the plasma through electron-ion (and electron-atom) collisions transfers roughly a third of the power into direct heating of the ions to temperatures of 10,000 to 20,000 K. The hot plasma then expands, cools, and passes out the nozzle as directed thrust.
Because of the complex interaction of the plasma, current, and heat flow, a "sweet spot" exits in the 1kw to 30 kW range, and this is where most of the work has been done. The engine requires significant (and heavy) power conditioning and thermal isolation from the spacecraft, since more than half of the electrical power results in heat of the engine components.
image above and text below: from Status and Prospects on Nonequilibrium Modeling of High Velocity Plasma Flow in an Arcjet Thruster, Hai-Xing Wang, Su-Rong Sun, Wei-Ping Sun, Plasma Chem Plasma Process (2015) 35:543–564 DOI 10.1007/s11090-015-9610-4
"The key physics of the constricted arc discharge in arcjet are depicted in Fig. 1. An arc is stuck between the central, conical-tipped cathode and the coaxial, nozzle shaped anode. Working gas, injected with high swirl velocity near the cathode tip, passes through the constrictor region and is ohmically heated by the arc. The energy transferred to the gas is thought to result predominantly from electron–ion or electron–neutral collisions as elec- trons are the dominant current carriers. Extremely small constrictor size, extremely high gas velocity at the nozzle exit and operation at relatively low arc current are a few of the primary features of these kinds of thrusters.
Typical low power arcjets have conical converging–diverging nozzle with constrictor diameter on the order of 0.5 mm, expansion half angle of 20°, and exit diameter of 3.5 mm [8–10]. The physical characteristics of the arcjet flow field vary from a nearly fully ionized plasma with temperature in excess of 20,000 K near the cathode tip to a relatively cold plasma (1,000–2,000 K) at the anode wall. Moreover, velocities vary from approximately 10 km/s on centerline to zero at the wall.*" from here (paywalled)
There are many examples of arcjet thrusters in the 500 to 2000 Watt range so I'll talk about those first.
above: From Performance Computation of a Low-Power Hydrogen Arcjet Kazuhisa Fujita & Yoshihiro Arakawa, J. Propulsion. and Power v15, n 1, Jan-Feb 1999 (paywalled)
NASA started the Arcjet Thruster Research and Technology (ATRT) program in 1984. For example:
above: illustration of the arcjet principle from here.
above: Illustration of an arcjet start-up process, including spiral-fed propellant. Figure 3-6 here.
The Wikipedia article List of spacecraft with electric propulsion currently lists a few spacecraft that carried arcjet thrusters. The Telstar 401 is the first commercial use, launched in 1993. Four arcjet thrusters were used for North-South and East-West station keeping in Geosynchronous orbit.
Telstar 401, MR-508, Hydrazine, Comms (AS-7000)
Telstar 402R (Telstar 4), MR-508, Hydrazine, Comms (AS-7000)
A2100, MR-510, Hydrazine, Comms
ARGOS (P91-1), ESEX, Ammonia, Experimental Military
AMSAT-Phase 3-D (OSCAR-40), ATOS, Ammonia, OSCAR Sat cold gas mode
According to Performance and preliminary life test of a low power hydrazine engineering design model arcjet (Tang et al. 2015, Aerospace Science and Technology, v 15, n 7, Oct–Nov 2011, 577–588)
Telstar 401 (a geostationary communication satellite of Lockheed Martin Corporation) equipped with PRIMEX Aerospace Company’s (originally Rocket Research Company, RRC) MR-508 hy- drazine arcjet propulsion system was successfully launched in 1993, which is the first application of thermal arcjet propulsion systems (11). From then on, hydrazine arcjet propulsion systems have operated successfully on more than 29 spacecrafts, showing substantial performance and reliability (12). (my emphasis)
MR-508, -509, -510 series Hydrazine Arcjets
The MR-508, -509, -510 series of Hydrazine arcjet thrusters can be seen at Aerojet Rocketdyne - here is a set of four MR-510 and the Power Conditioning Unit (PCU). They provided an Isp above 500 seconds using hydrazine, and therefore provided more efficient use of the hydrazine mass.
However, in this power range, the power supply and control electronics can be much heavier than the engine itself!
Flight Qualification of the 2.2 kW MR-510 Hydrazine Arcjet System
ARC-1, ARC-2, ARC-3 (use Lockheed Martin A2100 bus)
ESEX Ammonia Arcjet
ARGOS
ESEX Arcjet (ARGOS)
ESEX Arcjet report (ARGOS)
AIAA 99-2706, An Overview of the On-Orbit Results from the ESEX Flight Experiment
above: ESEX Arcjet from here
above: Propellant supply system for ESEX (Electric Propulsion Space Experiment) from here on ARGOS.
Cubesat-Friendly Developments
Discharge Characteristics of a Very Low-Power Arcjet, Hideyuki Horisawa, Hotaka Ashiya and Itsuro Kimura, presents results of an experiment to understand the issues of scaling an arjet thruster down to the 1 to 10 Watt range and about 4 centimeters in length. A quartz window was included so that the plasma in the restriction region could be observed as experimental parameters were varied. In these early studies, "typical thrust was 1.5 ~ 2.0 mN, Isp: ~ 100 sec for input power of 1 ~ 5 watts and propellant (N2) mass flow rate of 0.6 ~ 2 mg/sec", and based on these results the possibility of higher Isp in this power range can be explored.
Direct Drive
In Testing of an Arcjet Thruster with Capability of Direct-Drive Operation (A. K. Martin et al., NASA-Marshall Space Flight Center, American Institute of Aeronautics and Astronautics) an arject engine capable of being driven directly from a solar array (without substnatial power conditioning) has been demonstrated.
The testing indicated that an operating point exists within the I-V characteristics that is compatible with direct-drive solar-electric operation; for a flow rate of 20 mg/s (argon) the arc could be sustained at a voltage of about 20 V and a current of 25 A (500W).
Cubesats have several restrictions on chemical propellants and storage of chemical energy. One step towards addressing this problem has been discussed in Performance Characteristics of Low-Power Arcjet Thrusters Using Low Toxicity Propellant HAN Decomposed Gas Matsumoto et al. IEPC-2013-095, (33st International Electric Propulsion Conference, The George Washington University, USA October 6 – 10, 2013).
HAN or Hydroxylammonium nitrate is a potential new propellant. However early results showed significant corrosion with conventional materials.
There has been some work on production of hydrogen-containing vapor from solid Teflon [citation needed].
