# US 6,719,909 B2 · Band gap plasma mass filter

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- Number: US 6,719,909 B2
- Document ID: US6719909B2
- Status: Granted
- Kind: B2
- Application number: 10/114,900
- Filed: 2002-04-02
- Published: 2003-10-02
- Granted: 2004-04-13
- Earliest claimed: 2002-04-02
- Assignees: Archimedes Technology Group, Inc.
- Inventors: Tihiro Ohkawa
- Web page: https://patentgrep.com/patent/US6719909B2

## CPC

- H01J 49/421: {Mass filters, i.e. deviating unwanted ions without trapping}
- H01J 49/328: {with a cycloidal trajectory by using crossed electric and magnetic fields, e.g. trochoidal type}
- H01J: ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS (spark-gaps H01T; electric arc lamps H05B31/00; particle accelerators H05H)

## Abstract

A device and method for selectively establishing predetermined orbits, relative to an axis, for ions of a first mass/charge ratio (m1), requires crossing an electric field with a substantially uniform magnetic field (E×B). The magnetic field is oriented along the axis and the electric field has both a d.c. voltage component (∇Φ0) and an a.c. voltage component (∇Φ1). In operation, voltage Φ0 is fixed to place the ions m1 on confined orbits around the axis when Φ1 is zero. On the other hand, when Φ1 is tuned to a predetermined value, the ions m1 are ejected away from the axis. With E×B established in a chamber, the ions m1 will pass through the chamber when on confined orbits (Φ1=0), and they will be ejected into the wall of the chamber when on unconfined orbits (Φ1=predetermined value).

## Claims

### Claim 1

A band gap plasma filter for selectively passing ions of a first mass/charge ratio (m1) therethrough, wherein m1 is less than a predetermined cut off mass, Mc, said filter comprising:
a means for introducing a plasma, including said ions m1, into a hollow, substantially cylindrical-shaped chamber, said chamber defining an axis and being surrounded by a wall;
a magnetic means for establishing a substantially uniform magnetic field (B), said magnetic field being oriented along said axis in said chamber;
a means for creating an electric field (E), wherein said electric field is oriented in a substantially radial direction relative to said axis to cross with said magnetic field (E×B), and wherein said electric field has a d.c. voltage component (∇Φ0) and an a.c. voltage component (∇Φ1) (E=∇ (Φ0+Φ1);
a means for fixing said d.c. voltage component (∇Φ0) to confine said ions m1 for passage through said chamber and subsequent exit therefrom when said a.c. voltage component (∇Φ1) is substantially zero; and
a means for tuning said a.c. voltage component (∇Φ1) to eject said ions m1 from said chamber and into collision with said wall thereof to prevent passage of said ions m1 through said chamber.

### Claim 2 (depends on 1)

A filter as recited in claim 1 wherein said plasma is a multi-species plasma and includes ions of a second mass/charge ratio (m2).

### Claim 3 (depends on 2)

A filter as recited in claim 2 wherein said first mass/charge ratio (m1) is greater than said second mass/charge ratio (m2).

### Claim 4 (depends on 2)

A filter as recited in claim 2 wherein said first mass/charge ratio (m1) is less than said second mass/charge ratio (m2).

### Claim 5 (depends on 1)

A filter as recited in claim 1 wherein said cut off mass, Mc, is determined by the expression:
Mc=zea2(B)2/8Vctr
where “e” is the elementary charge, “z” is the charge number, “a” is the distance between the axis and the wall of the chamber, and the voltage has a positive value (Vctr) along the axis, which decreases parabolically to zero at the wall of the chamber.

### Claim 6 (depends on 1)

A filter as recited in claim 1 wherein said tuning means selects a radio frequency, ω, for said a.c. voltage component (∇Φ1)) according to values of α and β wherein:
α=[Ω2/4−λ0]/ω2
β=λ1/[4ω2]
and
λ=2eV(t)/ma2
with λ=λ0+λ1 cos ωt, where “e” is the elementary charge, V(t) is the applied voltage, Φ0+Φ1, as a function of time, “a” is the distance between the axis and the wall of the chamber and Ω is the cyclotron frequency of the ions m1.

### Claim 7

A device for selectively establishing predetermined orbits for ions of a first mass/charge ratio (m1) relative to an axis, which comprises:
a means for crossing an electric field (E) with a substantially uniform magnetic field (B), wherein said magnetic field is oriented along said axis and said electric field is oriented in a substantially radial direction relative to said axis, and further wherein said electric field has a d.c. voltage component (∇Φ0) and an a.c. voltage component (∇Φ1) (E=∇(Φ0+Φ1));
a means for introducing the ions m1 into said crossed magnetic and electric fields;
a means for fixing said d.c. voltage component (∇Φ0) to place said ions m1 in confined orbits around said axis when said a.c. voltage component (∇Φ1) is substantially zero; and
a means for selectively tuning said a.c. voltage component (∇Φ1) to establish unconfined orbits for ejection of the ions m1 away from said axis when said a.c. voltage component (∇Φ1) has a predetermined value.

### Claim 8 (depends on 7)

A device as recited in claim 7 wherein said crossed electric and magnetic fields are established in a hollow, substantially cylindrical-shaped chamber, with said chamber defining said axis and being surrounded by a wall.

### Claim 9 (depends on 8)

A device as recited in claim 8 wherein the ions m1 pass through said chamber when on confined orbits, and are ejected into said wall of said chamber when on unconfined orbits.

### Claim 10 (depends on 8)

A device as recited in claim 8 wherein a cut off mass, Mc, is greater than m1 and is determined by the expression:
Mc=zea2(B)2/8Vctr
where “e” is the elementary charge, “z” is the charge number, “a” is the distance between the axis and the wall of the chamber, and voltage has a positive value (Vctr) along the axis, which decreases to zero at the wall of the chamber.

### Claim 11 (depends on 7)

A device as recited in claim 7 wherein the ions m1 are included in a multi-species plasma with ions of a second mass/charge ratio (m2).

### Claim 12 (depends on 7)

A device as recited in claim 7 wherein the first mass/charge ratio (m1) is greater than the second mass/charge ratio (m2), and wherein said d.c. voltage component (∇Φ0) places the ions m1 and the ions m2 in confined orbits around said axis when said a.c. voltage component (∇Φ1) is substantially zero and maintains said ions m2 on confined orbits when said a.c. voltage component (∇Φ1) is tuned to said predetermined value.

### Claim 13 (depends on 7)

A device as recited in claim 7 wherein the first mass/charge ratio (m1) is less than the second mass/charge ratio (m2), and wherein said d.c. voltage component (∇Φ0) places the ions m1 and the ions m2 in confined orbits around said axis when said a.c. voltage component (∇Φ1) is substantially zero and maintains said ions m2 on confined orbits when said a.c. voltage component (∇Φ1) is tuned to said predetermined value.

### Claim 14 (depends on 7)

A device as recited in claim 7 wherein said tuning means selects a radio frequency, ω, for said a.c. voltage component (∇Φ1) according to values of α and β wherein:
α=[Ω2/4−λ0]/ω2
β=λ1/[4ω]
and
λ=2eV(t)/ma2
with λ=λ0+λ1 cos ωt, where “e” is the elementary charge, V(t) is the applied voltage, Φ0+Φ1 as a function of time, “a” is the distance between the axis and the wall of the chamber and Ω is the cyclotron frequency of the ions m1.

### Claim 15

A method for selectively establishing predetermined orbits for ions of a first mass/charge ratio (m1) relative to an axis, which comprises the steps of:
crossing an electric field (E) with a substantially uniform magnetic field (B), wherein said magnetic field is oriented along said axis and said electric field is oriented in a substantially radial direction relative to said axis, and further wherein said electric field has a d.c. voltage component (∇Φ0) and an a.c. voltage component (∇Φ1) (E=∇(Φ0+Φ1));
introducing the ions m1 into said crossed magnetic and electric fields;
fixing said d.c. voltage component (∇Φ0) to place said ions m1 in confined orbits around said axis when said a.c. voltage component (∇Φ1) is substantially zero; and
selectively tuning said a.c. voltage component (∇Φ1) to establish unconfined orbits for ejection of the ions m1 away from said axis when said a.c. voltage component (∇Φ1) has a predetermined value.

### Claim 16 (depends on 15)

A method as recited in claim 15 wherein the ions m1 are included in a multi-species plasma with ions of a second mass/charge ratio (m2), wherein the first mass/charge ratio (m1) is greater than the second mass/charge ratio (m2), and wherein said d.c. voltage component (∇Φ0) places the ions m1 and the ions m2 in confined orbits around said axis when said a.c. voltage component (∇Φ1) is substantially zero and maintains said ions m2 on confined orbits when said a.c. voltage component (∇Φ1) is tuned to said predetermined value.

### Claim 17 (depends on 15)

A method as recited in claim 15 wherein the ions m1 are included in a multi-species plasma with ions of a second mass/charge ratio (m2), wherein the first mass/charge ratio (m1) is less than the second mass/charge ratio (m2), and wherein said d.c. voltage component (∇Φ0) places the ions m1 and the ions m2 in confined orbits around said axis when said a.c. voltage component (∇Φ1) is substantially zero and maintains said ions m2 on confined orbits when said a.c. voltage component (∇Φ1) is tuned to said predetermined value.

### Claim 18 (depends on 15)

A method as recited in claim 15 wherein said tuning step includes the steps of:
determining a cyclotron frequency for the ions m1; and
selecting a radio frequency, ω, for said a.c. voltage component (∇Φ1) according to values of α and β wherein:
α=[Ω2/4−λ0]/ω2
β=λ1/[4ω2]
and
λ=2eV(t)/ma2
with λ=λ0+λ1 cos ωt, where “e” is the elementary charge, V(t) is the applied voltage, Φ0+Φ1 as a function of time, “a” is the distance between the axis and the wall of the chamber and Ω is the cyclotron frequency of the ions m1.

### Claim 19 (depends on 15)

A method as recited in claim 15 wherein said crossed electric and magnetic fields are established in a hollow, substantially cylindrical-shaped chamber, with said chamber defining said axis and being surrounded by a wall.

### Claim 20 (depends on 19)

A method as recited in claim 19 wherein the ions m1 pass through said chamber when on confined orbits, and are ejected into said wall of said chamber when on unconfined orbits.