LOW PRESSURE WIRE ION PLASMA DISCHARGE SOURCE, AND APPLICATION TO ELECTRON SOURCE WITH SECONDARY EMISSION

Abstract

Disclosed is a low pressure wire ion plasma discharge source including an elongated ionization chamber housing at least two parallel anode wires extending longitudinally within the ionization chamber. A first of the at least two anode wires is connected to a DC voltage supply and a second of the at least two anode wires is connected to a pulsed voltage supply.

Claims

1. Low pressure wire ion plasma discharge source comprising an elongated ionization chamber (1) housing at least two anode wires (2, 3) extending longitudinally within the ionization chamber, a DC voltage supply (4) and a pulsed voltage supply (5), wherein a first (2) of said at least two anode wires is connected to the DC voltage supply (4) and a second (3) of said at least two anode wires is connected to the pulsed voltage supply (5).

2. Low pressure wire ion plasma discharge source according to claim 1, further comprising several anode wires connected to the DC voltage supply (4) and/or several anode wires connected to the pulsed voltage supply (5).

3. Low pressure wire ion plasma discharge source according to claim 1, wherein the direct current generated by the DC voltage supply (4) is equal to or lower than 1 m A/cm.

4. Low pressure wire ion plasma discharge source according to claim 1, wherein the pulsed voltage supply (5) generates a pulsed large current of 1 to 5 A/cm or more.

5. Low pressure wire ion plasma discharge source according to claim 1, wherein the ionization chamber (10) comprises a main elongated chamber (11) and an elongated auxiliary chamber (12) in fluidic communication along their entire lengths through a slit (13), the at least one pulsed voltage supplied anode wire (14a, 14b) extending longitudinally in the main chamber (11) and the at least one DC voltage supplied anode wire (15) extending longitudinally in the auxiliary chamber (12).

6. Electron source with secondary emission under ion bombardment in a low pressure chamber, further comprising a low pressure wire ion plasma discharge source as set forth in claim 1.

Description

[0026] The present invention will now be described in detail with reference to the drawings which represent:

[0027] FIG. 1, a schematic representation of the functioning of a classical secondary electron emission X-ray generator using a wire ion plasma discharge;

[0028] FIG. 2, a schematic representation of the ion plasma confinement using a single pulse WIP discharge or a multiple pulses WIP discharge;

[0029] FIG. 3, the configuration of DC wire plasma discharges depending on DC current value and gas pressure;

[0030] FIG. 4, a schematic representation of an ionization chamber according to the invention;

[0031] FIG. 5, a schematic representation of the DC voltage supply and the pulsed voltage supply;

[0032] FIG. 6, a schematic representation of an embodiment of the ionization chamber according to the invention, comprising main and auxiliary ionization chambers;

[0033] FIG. 7, a representation of a sequence and waveforms for operation of the ionization chamber according to the invention; and

[0034] FIGS. 8A and 8B, voltage and current curves on the pulsed anode wire with application of a DC current according to the invention (7A) and without DC current (7B).

[0035] In FIG. 4, there is schematically represented an ionization chamber 1 according to the invention. The ionization chamber 1 is of elongated shape (typically of 1 m or more length) and houses two parallel anode wires 2, 3 extending longitudinally within the ionization chamber 1.

[0036] A first anode wire is connected to a DC voltage supply 4 intended to apply to the wire a high DC voltage (typically 0.5 to 1 kV) and a low DC current (typically 1 mA/cm).

[0037] The second anode wire is connected to a pulsed voltage supply 5 intended to apply a single high voltage (typically 1-5 kV) and high current (typically 1 A/cm; <10 s) pulse.

[0038] By continuously applying a high voltage to one anode wire, thus creating a continuous current through said wire, when subsequently applying a high DC voltage to the other wire, a stable WIP discharge with almost no jitter is safely obtained. Of course, number and positioning of the anode wires of each type (DC and pulsed) can be chosen to optimize ion density and uniformity. Also, when several anode wires supplied with a pulsed high voltage are used, pulsed high voltage can be supplied to a same single end of the wires, both ends of the wires or an opposite end of each wire.

[0039] In a specific embodiment, as shown in FIG. 6, the ionization chamber 10 comprises a main elongated chamber 11 and an elongated auxiliary chamber 12, auxiliary chamber 12 being in fluidic communication with main chamber 11 through an elongated slit 13 extending longitudinally along the length, preferably the entire length of the main and auxiliary chambers.

[0040] Main chamber 11 houses two parallel anode wires 14a, 14b extending longitudinally within the chamber (of course, only one anode or more than two anode wires may also be used).

[0041] Auxiliary chamber 12 houses an anode wire 15 extending longitudinally therein (of course, more than one anode wire may be disposed within the auxiliary chamber 12.

[0042] The anode wire(s) 15 located within the auxiliary chamber 12 is connected to a high voltage/low current DC supply (as shown in FIG. 4). The anode wires 14a, 14b located within the main chamber 11 are connected to a pulsed high voltage/high current supply (for example, as shown in FIG. 5). In the embodiment of FIG. 6, anode wires 14a, 14b are connected to the pulsed high voltage/high current supply through opposite ends. Of course, they also could be connected through their same side ends or both ends.

[0043] The elongated main and auxiliary chambers may have any suitable shapes such as parallelepipedic or cylindrical shapes. The overall longitudinal length of the main and auxiliary chambers is typically 1 m or more.

[0044] In reference with FIG. 7, a typical operation sequence of the ionization chamber according to the invention will now be described when used for producing a secondary electron emission beam.

[0045] 1. Ionization Camber Characteristics [0046] Ionization chamber: The chamber has a parallelepipedic shape with the following typical dimensions: length 130 cm, width 4 cm and height 4 cm. [0047] Anode Wires [0048] DC voltage anode wire: one DC wire, typically 200 m in diameter [0049] Pulsed voltage anode wire: two Pulsed wires, typically 300 m in diameter [0050] DC supply (HVPS-1) with the following characteristics: [0051] Output high voltage typically 2 kV; [0052] Controllable Output current that can be limited to typically 0.3 mA for 1.30 m wire length (hence0.3 mA/m) thus keeping the DC plasma in constricted mode; [0053] Pulsed power supply with the following characteristic (see FIG. 5): [0054] a high voltage power supply (HVPS-2) with HV output of typically 5 kV; [0055] a capacitor C, typically 30 nF to store electrical energy and subsequently deliver it to the pulsed wire ; [0056] a switch S, capable of closing rapidly and handle voltage of up to 5 kV and current of up to typically 500 A in order to deliver a high voltage pulse to the pulsed wire (s). The switch can be formed of one or several IGBTs. Alternatively MosFET transistors can be used. Alternatively thyratron also can be used (it shall be noted that in the case of a thyratron, a transformer must be used)

[0057] 2. Operation [0058] Upon startup (T0), a high DC voltage (typically 2 kV) is applied to one wire. [0059] After some time (T1), the plasma is created around the wire and current flows. The power supply current limit is set at a value low enough to maintain the DC plasma in constricted mode and high enough to generate enough charges for the stable formation of the pulsed WIP. Typical current setpoint may depends on wire diameter and chamber geometry (distance wirewall, distance between wires). For a DC wire of 200 m diameter, 1.5 m length, positioned at 1 cm of the chamber walls, current setpoint is 0.1 mA. Once the said plasma is established, power supply voltage drops at a value depending on the plasma impedance, typically 1 kV. This DC plasma is sustained continuously during the operation of the device. [0060] At T2, HVPS-1 is charging capacitor C to a set high voltage, typically 5 kV. [0061] Once the capacitor C is charged, at T3, the switch S is closed and subsequently, the Pulsed wire is submitted to the same high voltage. The voltage rise time depends on the circuit physical characteristics, designed to be fast (typically <1 s) [0062] At T4, the high voltage appearing on the pulsed wire is forming the pulsed WIP plasma and a high current starts flowing in the ionization chamber, creating high ion density during a time that depends on the pulse power supply design (typically a few s). [0063] At T5, the electrical energy stored in capacitor C has been fully transferred to the plasma and the pulsed current stops [0064] At T6, after a time delay precisely controlled, a negative high voltage pulse (typically 100 kV) is applied to the cathode, accelerating the ion plasma, creating secondary electrons and subsequent X-ray emission. [0065] At T7, controlled depending on the desired repetition rate of the X-ray source, the cycle (starting at T2) is repeated

[0066] Typical Delays: [0067] DC plasma inception (T1-T0): non critical (only at startup), typically <1 s [0068] Charge time of the capacitor C (T3-T2): must be shorter than the desired time between successive X-ray pulses, typically <100 ms for 10 Hz operation. [0069] Rise time of the voltage across the pulse wire (T4-T3): must be fast enough to efficiently form the pulse plasma. Depends on the circuit parameters (switch closing time, inductance), typically <1 s. [0070] Duration of the WIP plasma (T5-T4): typically 2 s [0071] Delay Pulsed WIP plasmae-beam (T5-T6): typically 5 s [0072] Repetition rate (T7-T2): typically 1-100 Hz (0.01-1 s)

[0073] The sequence and waveforms for operation are shown in FIG. 7. FIGS. 8A and 8B show the ion source voltage and current on pulsed anode wire when a DC current is applied to the other wire (100 shots) (FIG. 8A) and when no DC current is applied (FIG. 8B). Without DC current, there is a large jitter and a poor stability of the WIP discharge.