离子推进器结构及应用

Ion thruster

An

ion

thruster

is

a

form

of

electric

propulsion

used

for

spacecraft

propulsion

that

creates

thrust

by

accelerating

ions.

The

term

is

strictly

used

to

refer

to

gridded electrostatic ion thrusters, but may often more loosely be applied to all

electric

propulsion

systems

that

accelerate

plasma,

since

plasma

consists

of

ions.

Ion

thrusters

are

categorized

by

how

they

accelerate

the

ions,

using

either

electrostatic

or

electromagnetic

force.

Electrostatic

ion

thrusters

use

the

Coulomb

force

and

accelerate

the

ions

in

the

direction

of

the

electric

field.

Electromagnetic

ion

thrusters

use

the

Lorentz

force

to

accelerate

the

ions.

In

either

case,

when

an

ion

passes

through

an

electrostatic

grid

engine,

the

potential difference of the electric field converts to the ion's kinetic energy.

Ion thrusters have an input power spanning 1

–

7 kilowatts, exhaust velocity 20

–

50

kilometers per second, thrust 20

–

250 millinewtons and efficiency 60

–

80%.

The Deep Space 1 spacecraft, powered by an ion thruster, changed velocity by

4.3 km/s while consuming less than 74 kilograms of xenon. The Dawn spacecraft

broke the record, reaching 10 km/s.

Applications include control of the orientation and position of orbiting satellites

(some

satellites

have

dozens

of

low- power

ion

thrusters)

and

use

as

a

main

propulsion engine for low-mass robotic space vehicles (for example Deep Space

1 and Dawn).

Ion thrusters are not the most promising type of electrically powered spacecraft

propulsion

(although

in

practice

they

have

been

more

successful

than

others).The

ion

drive

is

comparable

to

a

car

that

takes

two

days

to

accelerate

from zero to 60 miles per hour; a real ion engine's technical characteristics, and

especially its thrust, are considerably inferior to its literary cal

capabilities

of

the

ion

engine

are

limited

by

the

space

charge

created

by

ions.

This limits the thrust density (force per cross-sectional area of the engine). Ion

thrusters

create

small

thrust

levels

(for

example

the

thrust

of

Deep

Space

1's

engine

approximately

equals

the

weight

of

one

sheet

of

paper)

compared

to

conventional

chemical

rockets,

but

achieve

very

high

specific

impulse,

or

propellant mass efficiency, by accelerating their exhaust to high speed. However,

ion

thrusters

carry

a

fundamental

price:

the

power

imparted

to

the

exhaust

increases with the square of its velocity while thrust increases linearly. Chemical

rockets,

on

the

other

hand,

can

provide

high

thrust,

but

are

limited

in

total

impulse

by

the

small

amount

of

energy

that

can

be

stored

chemically

in

the

the

practical

weight

of

suitable

power

sources,

the

accelerations given by ion thrusters are frequently less than one thousandth of

standard gravity. However, since they operate as electric (or electrostatic) motors,

a greater fraction of the input power is converted into kinetic exhaust power than

in a chemical rocket. Chemical rockets operate as heat engines, hence Carnot's

theorem bounds their possible exhaust velocity.

Due

to

their

relatively

high

power

needs,

given

the

specific

power

of

power

supplies and the requirement of an environment void of other ionized particles,

ion thrust propulsion is currently only practical on spacecraft that have already

reached

space,

and

is

unable

to

take

vehicles

from

Earth

to

space.

Spacecraft

rely on conventional chemical rockets to initially reach orbit.

Origins

The

first

person

to

publish

mention

of

the

idea

was

Konstantin

Tsiolkovsky

in

r,

the

first

documented

instance

where

the

possibility

of

electric

propulsion

was

considered

is

found

in

Robert

H.

Goddard's

handwritten

notebook

in

an

entry

dated

September

6,

first

experiments

with

ion

thrusters were carried out by Goddard at Clark University from 1916

–

1917. The

technique

was

recommended

for

near-vacuum

conditions

at

high

altitude,

but

thrust was demonstrated with ionized air streams at atmospheric pressure. The

idea appeared again in Hermann Oberth's

ays to

Spaceflight),

published

in

1923,

where

he

explained

his

thoughts

on

the

mass

savings

of

electric

propulsion,

predicted

its

use

in

spacecraft

propulsion

and

attitude control, and advocated electrostatic acceleration of charged gases.

A working ion thruster was built by Harold R. Kaufman in 1959 at the NASA Glenn

Research

Center

facilities.

It

was

similar

to

the

general

design

of

a

gridded

electrostatic ion thruster with mercury as its fuel. Suborbital tests of the engine

followed during the 1960s and in 1964 the engine was sent into a suborbital flight

aboard the Space Electric Rocket Test 1 (SERT 1).It successfully operated for the

planned

31

minutes

before

falling

back

to

Earth.

This

test

was

followed

by

an

orbital test, SERT-2, in 1970.

An

alternate

form

of

electric

propulsion,

the

Hall

effect

thruster

was

studied

independently in the U.S. and the Soviet Union in the 1950s and 1960s. Hall effect

thrusters had operated on Soviet satellites since 1972. Until the 1990s they were

mainly used for satellite stabilization in North-South and in East-West directions.

Some 100

–

200 engines completed their mission on Soviet and Russian satellites

until

the

late

thruster

design

was

introduced

to

the

West

in

1992

after a team of electric propulsion specialists, under the support of the Ballistic

Missile Defense Organization, visited Soviet laboratories.

General description

Ion

thrusters

use

beams

of

ions

(electrically

charged

atoms

or

molecules)

to

create

thrust

in

accordance

with

momentum

conservation.

The

method

of

accelerating the ions varies, but all designs take advantage of the charge/mass

ratio of the ions. This ratio means that relatively small potential differences can

create very high exhaust velocities. This reduces the amount of reaction mass or

fuel required, but increases the amount of specific power required compared to

chemical

rockets.

Ion

thrusters

are

therefore

able

to

achieve

extremely

high

specific impulses. The drawback of the low thrust is low spacecraft acceleration,

because the mass of current electric power units is directly correlated with the

amount

of

power

given.

This

low

thrust

makes

ion

thrusters

unsuited

for

launching

spacecraft

into

orbit,

but

they

are

ideal

for

in-space

propulsion

applications.

Various

ion

thrusters

have

been

designed

and

they

all

generally

fit

under

two

categories.

The

thrusters

are

categorized

as

either

electrostatic

or

electromagnetic. The main difference is how the ions are accelerated.

Electrostatic

ion

thrusters

use

the

Coulomb

force

and

are

categorized

as

accelerating the ions in the direction of the electric field.

Electromagnetic ion thrusters use the Lorentz force to accelerate the ions.

Electric

power

supplies

for

ion

thrusters

are

usually

solar

panels

but,

at

sufficiently large distances from the Sun, nuclear power is used. In each case the

power

supply

mass

is

essentially

proportional

to

the

peak

power

that

can

be

supplied,

and

they

both

essentially

give,

for

this

application,

no

limit

to

the

energy.

Electric thrusters tend to produce low thrust, which results in low acceleration.

Using 1 g is 9.81 m/s2; F = m a

?

a = F/m. An NSTAR thruster producing a thrust

(force) of 92 mN will accelerate a satellite with a mass of 1,000 kg by 0.092 N /

1,000 kg = 0.000092 m/s2 (or 9.38×10?6

g).

thrust = 2*η*power/(g * Isp)

Where

thrust is the force in N

η is the efficiency, a dimensionless value between 0 and 1 (70% efficiency is 0.7)

power is the electrical energy going into the thruster in W

g is a constant, the acceleration due to gravity 9.81 m/s2

Isp is the Specific impulse in s

Electrostatic ion thrusters

Gridded electrostatic ion thrusters

See also: electrostatic ion thruster

Gridded electrostatic ion thrusters commonly utilize xenon gas. This gas has no

charge and is ionized by bombarding it with energetic electrons. These electrons

can

be

provided

from

a

hot

cathode

filament

and

when

accelerated

in

the

electrical field of the cathode, fall to the anode. Alternatively, the electrons can be

accelerated

by

the

oscillating

electric

field

induced

by

an

alternating

magnetic

field of a coil, which results in a self-sustaining discharge and omits any cathode

(radio frequency ion thruster).

The positively charged ions are extracted by an extraction system consisting of 2

or 3 multi-aperture grids. After entering the grid system via the plasma sheath the

ions are accelerated due to the potential difference between the first and second

grid (named screen and accelerator grid) to the final ion energy of typically 1

–

2

keV, thereby generating the thrust.

Ion thrusters emit a beam of positive charged xenon ions only. To avoid charging

up

the

spacecraft,

another

cathode

is

placed

near

the

engine,

which

emits

electrons (basically the electron current is the same as the ion current) into the

ion beam. This also prevents the beam of ions from returning to the spacecraft

and cancelling the thrust.

Gridded electrostatic ion thruster research (past/present):

NASA

Solar

Technology

Application

Readiness

(NSTAR)

-

2.3

kW,

used

on

two

successful missions

NASA’s

Evolutionary

Xenon

Thruster

(NEXT)

-

6.9

kW,

flight

qualification

hardware built

Nuclear Electric Xenon Ion System (NEXIS)

High Power Electric Propulsion (HiPEP) - 25 kW, test example built and run briefly

on the ground

EADS Radio-Frequency Ion Thruster (RIT)

Dual-Stage 4-Grid (DS4G)

Hall effect thrusters

See also: Hall effect thruster

Hall

effect

thrusters

accelerate

ions

with

the

use

of

an

electric

potential

maintained

between

a

cylindrical

anode

and

a

negatively

charged

plasma

that

forms the cathode. The bulk of the propellant (typically xenon gas) is introduced

near the anode, where it becomes ionized, and the ions are attracted towards the

cathode;

they

accelerate

towards

and

through

it,

picking

up

electrons

as

they

leave to neutralize the beam and leave the thruster at high velocity.

The anode is at one end of a cylindrical tube, and in the center is a spike that is

wound to produce a radial magnetic field between it and the surrounding tube.

The ions are largely unaffected by the magnetic field, since they are too massive.

However, the electrons produced near the end of the spike to create the cathode

are far more affected and are trapped by the magnetic field, and held in place by

their

attraction

to

the

anode.

Some

of

the

electrons

spiral

down

towards

the

anode, circulating around the spike in a Hall current. When they reach the anode

they

impact

the

uncharged

propellant

and

cause it

to

be ionized,

before

finally

reaching the anode and closing the circuit.

Field-emission electric propulsion

Field-emission electric propulsion (FEEP) thrusters use a very simple system of

accelerating ions to create thrust. Most designs use either caesium or indium as

the propellant. The design comprises a small propellant reservoir that stores the

liquid

metal,

a

narrow

tube

or

a

system

of

parallel

plates

that

the

liquid

flows

through, and an accelerator (a ring or an elongated aperture in a metallic plate)

about a millimeter past the tube end. Caesium and indium are used due to their

high atomic weights, low ionization potentials, and low melting points. Once the

liquid

metal

reaches

the

end

of

the

tube,

an

electric

field

applied

between

the

emitter and the accelerator causes the liquid surface to deform into a series of

protruding cusps (

ions

are

extracted

from

the

tips

of

the

electric

field

created

by

the

emitter

and

the

accelerator

then

accelerates

the

ions.

An

external

source

of

electrons neutralizes the positively charged ion stream to prevent charging of the

spacecraft.

Electromagnetic thrusters

Pulsed inductive thrusters

Pulsed inductive thrusters (PIT) use pulses of thrust instead of one continuous

thrust, and have the ability to run on power levels in the order of Megawatts (MW).

PITs

consist

of

a

large

coil

encircling

a

cone

shaped

tube

that

emits

the

propellant

gas.

Ammonia

is

the

gas

commonly

used

in

PIT

engines.

For

each

pulse

of

thrust

the

PIT

gives,

a

large

charge

first

builds

up

in

a

group

of

capacitors behind the coil and is then released. This creates a current that moves

circularly in the direction of jθ. The current then creates a magnetic field in the

outward radial direction (Br), which then creates a current in the ammonia gas

that has just been released in the opposite direction of the original current. This

opposite

current

ionizes

the

ammonia

and

these

positively

charged

ions

are

accelerated away from the PIT engine due to the electric field jθ crossing with the

magnetic field Br, which is due to the Lorentz Force.

Magnetoplasmadynamic (MPD) / lithium Lorentz force accelerator (LiLFA)

Magnetoplasmadynamic

(MPD)

thrusters

and

lithium

Lorentz

force

accelerator

(LiLFA) thrusters use roughly the same idea with the LiLFA thruster building off

of the MPD thruster. Hydrogen, argon, ammonia, and nitrogen gas can be used as

propellant. In a certain configuration, the ambient gas in Low Earth Orbit (LEO)

can be used as a propellant. The gas first enters the main chamber where it is

ionized into plasma by the electric field between the anode and the cathode. This

plasma then conducts electricity between the anode and the cathode. This new

current

creates

a

magnetic

field

around

the

cathode,

which

crosses

with

the

electric field, thereby accelerating the plasma due to the Lorentz force. The LiLFA

thruster

uses

the

same

general

idea

as

the

MPD

thruster,

except

for

two

main

differences. The first difference is that the LiLFA uses lithium vapor, which has

the advantage of being able to be stored as a solid. The other difference is that

the cathode is replaced by multiple smaller cathode rods packed into a hollow

cathode tube. The cathode in the MPD thruster is easily corroded due to constant

contact with the plasma. In the LiLFA thruster the lithium vapor is injected into

the

hollow

cathode

and

is

not

ionized

to

its

plasma

form/corrode

the

cathode

rods

until

it

exits

the

tube.

The

plasma

is

then

accelerated

using

the

same

Lorentz Force.

Electrodeless plasma thrusters

Electrodeless

plasma

thrusters

have

two

unique

features:

the

removal

of

the

anode and cathode electrodes and the ability to throttle the engine. The removal

of the electrodes takes away the factor of erosion, which limits lifetime on other

ion

engines.

Neutral

gas

is

first

ionized

by

electromagnetic

waves

and

then

transferred to another chamber where it is accelerated by an oscillating electric