Mass analyser

Abstract

A mass analyser for a time-of-flight mass spectrometer comprises an ion mirror 105 formed from a plurality of electrodes, wherein said electrodes are slidably engaged with a plurality of electrical co

Description

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority from and the benefit of United Kingdom patent application No. 2411414.2 filed on 2 August 2024. The entire contents of this application are incorporated herein by reference. FIELD OF THE INVENTION The present invention relates generally to a mass analyser and a mass spectrometer comprising the mass analyser. BACKGROUND Mass analysers comprising a plurality of electrodes are known. However, it can be relatively difficult to remove the electrodes of such devices, e.g. for cleaning, and then return them so that they are accurately positioned. SUMMARY According to an aspect of the present invention, there is provided a mass analyser comprising: an ion mirror comprising a plurality of electrodes; and a plurality of electrical contacts configured to couple the plurality of electrodes to one or more voltage supplies; wherein the plurality of electrodes are slidably engaged with the plurality of electrical contacts. The electrical contacts may be spring biased towards the electrodes. Each of the electrical contacts may directly contact a conductive trace on a surface of a printed circuit board (“PCB”) assembly. The electrical contacts may be provided directly on the surface of the PCB assembly. The electrical contacts may be connected to the conductive traces on the PCB assembly and / or electrodes without wires. The mass analyser may comprise at least one electrical insulator block, the electrical contacts mounted to the at least one electrical insulator block with the PCB assembly provided between the electrical contacts and the at least one electrical insulator block. The at least one electrical insulator block may comprise a plurality of recesses, each recess configured to receive one of the electrical contacts. The PCB assembly may be shaped to have a plurality of fingers that are separated by slotted gaps, wherein the electrical contacts are provided at the distal ends of the fingers. In embodiments in which the at least one electrical insulator block comprises a plurality of recesses, each recess may receive one of the fingers. The mass analyser may comprise a retainer configured to hold the electrodes in position. Multiple electrodes of the plurality of electrodes may be electrically connected to each other via a rigid electrically conductive member, wherein only one of the multiple electrodes is connected to one of the electrical contacts. The rigid electrically conductive member may be a plate or rod. The electrodes and electrical contacts are preferably not secured to each other, such that the electrodes can be slid out of contact with the electrical contacts. The mass analyser may comprise a plurality of guides configured to guide the electrodes into alignment with the electrical contacts. The mass analyser may be a Time of Flight mass analyser comprising an ion accelerator and an ion detector, wherein the mass analyser is configured to determine the mass to charge ratio of ions based on a time of flight of the ions from the ion accelerator to the ion detector. The mass analyser may be a multi-reflecting Time of Flight mass analyser comprising at least two ion mirrors for reflecting the ions between them as the ions pass from the ion accelerator to the ion detector. Each of the ion mirrors may have a plurality of electrodes that are connectable with a plurality of electrical contacts in the manner described above. According to another aspect of the present invention, there is provided a method of assembling or disassembling a mass analyser as described above, comprising: sliding ion mirror electrodes into a housing of the mass analyser until they engage with a plurality of electrical contacts that are connected to one or more voltage supplies; or sliding ion mirror electrodes out of a housing of the mass analyser so as to cause them to disengage from a plurality of electrical contacts that are connected to one or more voltage supplies. The mass analyser may have guides, such as guide rails, along which the electrodes slide. When the electrodes are slid into the housing so as to engage with the plurality of electrical contacts, the electrodes may be fixed in place. The contacts may be biased towards the electrodes, such as by using spring biased contacts. The electrodes are preferably not secured to each other, such that they can engage and disengage from each other simply by sliding the electrodes towards and away from the contacts, respectively. According to another aspect of the present invention, there is provided a mass analyser comprising: an ion mirror having a plurality of electrodes; a first, high voltage power supply; and a second, lower voltage power supply; wherein the mass analyser is configured such that a first of the electrodes is supplied with a first potential from the first power supply, and a second of the electrodes is connected to a first line extending between the first and second power supplies such that the second electrode is supplied with a second potential that is lower than the first potential. The line between the first power supply and the first electrode may not contain any resistors. One or more resistors may be provided in the line between a node at which the second electrode is connected to the line and the first power supply. Additionally, or alternatively, one or more resistors may be provided in the line between a node at which the second electrode is connected to the line and the second power supply. The second power supply may be tuneable so as to vary the voltage that it outputs and hence vary the second potential supplied to the second electrode. The mass analyser may be configured such that a third of the electrodes is supplied with a third potential from the second power supply. The line between the second power supply and the third electrode may not contain any resistors. The third potential may be lower than the first and / or second potentials. The mass analyser may comprise a third power supply and may be configured such that a fourth of the plurality of electrodes receives a fourth potential from the third power supply. The line between the third power supply and the fourth electrode may not contain any resistors. The mass analyser may be configured such that a fifth of the electrodes is connected to a line extending between the third power supply and the second power supply such that the fifth electrode is supplied with a different potential to the fourth potential. The fifth potential may be higher than the fourth potential and / or lower than the third potential. One or more resistors may be provided in the line between a node at which the fifth electrode is connected to the line and the third power supply. Additionally, or alternatively, one or more resistors may be provided in the line between a node at which the fifth electrode is connected to the line and the second power supply. As mentioned above, the second power supply may be tuneable. The second power supply may be tuneable so as to vary the potential supplied to the fifth electrode. The second electrode may be closer to the entrance to the ion mirror than the first electrode. The third electrode may be closer to the entrance to the ion mirror than the second electrode. The fourth electrode may be closer to the entrance to the ion mirror than the third electrode. The fifth electrode may be closer to the entrance to the ion mirror than the fourth electrode. The mass analyser may comprise a further power supply configured to supply a voltage to a drift electrode adjacent to the entrance to the ion mirror. The line between the further power supply and the drift electrode may not contain any resistors. The mass analyser may comprise an orthogonal accelerator arranged and configured to accelerate packets of ions into the ion mirror, wherein the orthogonal accelerator comprises one or more steering electrodes for steering the ion packet in one or more respective direction that is orthogonal to the direction that the ion packet is accelerated in; wherein each of the one or more steering electrodes is connected to a line extending between the second power supply and one of the first, third or further power supplies. One or more resistors may be provided in the line between a node at which each steering electrode is connected to the line and the second power supply. Additionally, or alternatively, one or more resistors may be provided in the line between a node at which each steering electrode is connected to the line and the first, third or further power supplies. Each line described above may be a conductive trace on a PCB assembly. BRIEF DESCRIPTION OF THE DRAWINGS Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 illustrates a schematic of a mass spectrometer according to an embodiment of the present invention; Fig. 2 illustrates a schematic of a section of an MRTOF mass analyser according to an embodiment of the present invention; Fig. 3 illustrates a schematic of a part of an MRTOF mass analyser according to an embodiment of the present invention; Fig. 4 illustrates an electrical contact for a mirror electrode according to an embodiment of the present invention; Fig. 5 illustrates a perspective view of part of an MRTOF mass analyser according to an embodiment of the present invention; Fig. 6 illustrates an electrical diagram of an MRTOF mass analyser according to an embodiment of the present invention; and Fig. 7 illustrates an electrical diagram of an MRTOF mass analyser according to an embodiment of the present invention. DETAILED DESCRIPTION Fig. 1 illustrates a schematic of a mass spectrometer 100 according to an embodiment of the present invention. The mass spectrometer 100 comprises an ion source 101 configured to generate analyte ions, an ion guide 102 located in a first vacuum chamber, an ion transfer lens 103 located in a second vacuum chamber, and a multi-reflecting Time of Flight (“MRTOF”) mass analyser 108 located in a third vacuum chamber. As illustrated in Fig. 1, the MRTOF mass analyser 108 comprises an orthogonal accelerator 104 and an ion detector 107 that are separated by a time of flight region. The orthogonal accelerator 104 comprises one or more pusher electrode(s) and / or one or more puller electrode(s). A first ion mirror 105 and a second ion mirror 106 are provided in the ion flight path intermediate the orthogonal accelerator 104 and the ion detector 107, and the ion detector 107 is arranged to detect ions reflected by the first and second ion mirrors 105, 106. Each ion mirror comprises a plurality of apertured electrodes and an end-cap electrode. In use, gas is pumped out of the vacuum chambers by one or more vacuum pumps so as to reduce the pressure inside them. The third vacuum chamber may be maintained at a lower pressure than the second vacuum chamber and the second vacuum chamber may be maintained at a lower pressure than the first vacuum chamber. The first vacuum chamber may be maintained at lower pressure than ambient. Ions are generated by the ion source 101 and pass into the first vacuum chamber. The ions are guided through the first vacuum chamber by the ion guide 102 and into the second vacuum chamber. The ions pass through the ion transfer lens 103 in the second vacuum chamber and into the third vacuum chamber. The ions then arrive at the orthogonal accelerator 104 of the MRTOF mass analyser 108. At the orthogonal accelerator 104, the ions pass between two of the electrodes of the orthogonal accelerator 104. A pulsed voltage is applied to one or both of the electrodes in order to generate a potential difference between the electrodes that causes the ions to be accelerated orthogonally as an ion packet. The electrodes of the orthogonal accelerator 104 towards which the ion packet is accelerated each comprise an aperture so that the ion packet can pass through the electrodes and out of the orthogonal accelerator 104. These electrodes have voltages applied to them so as to maintain an electric field along the ion path that spatially focuses the ions. The packet of ions is pushed into the time of flight region by the orthogonal accelerator 104 towards the first ion mirror 105. Voltages are applied to the electrodes of the first and second ion mirrors 105, 106 in order to generate electric fields that reflect the packet of ions. The packet of ions enters into the first ion mirror 105 and is reflected back towards the second ion mirror 106. The packet of ions then enter the second ion mirror 106 and are reflected back to the first ion mirror 105. The first ion mirror 105 then reflects the packet of ions back to the second ion mirror 106. This continues and the packet of ions is continually reflected between the first and second ion mirrors 105, 106 as they drift along the device until the ions impact upon the ion detector 107. The packet of ions therefore follows a substantially sinusoidal mean trajectory between the first and second ion mirrors 105, 106. The ions are mass analysed by the MRTOF mass analyser 108 in the known manner, i.e. by determining the mass to charge ratios of the ions based on their flight time from orthogonal accelerator 104 to the ion detector 107. The mass spectrometer 100 shown in Fig. 1 is illustrative and may comprise different or additional ion-optical devices to those shown, and / or a different number of vacuum chambers. For example, according to various embodiments the mass spectrometer 100 may additionally comprise one or more ion separation devices, one or more ion traps, one or more collision, fragmentation or reaction cells, or one or more mass filters. Fig. 2 illustrates a schematic of a section of an MRTOF mass analyser according to an embodiment of the present invention. As described above, the mass analyser 100 comprises a first ion mirror 105 and a second ion mirror 106. Each of the ion mirrors 105, 106 comprises a plurality of electrodes in a stacked arrangement, and is electrically connected to a printed circuit board (“PCB”) assembly 110. The PCB assembly 110 is illustrated more fully in Fig. 3, which shows the mass analyser with the electrodes of the ion mirrors 105, 106 removed. As shown, the PCB assembly 110 is mounted to a housing 112 of the mass analyser 100. This may be in a manner that allows the housing 112 to be electrically insulated from at least the conductive portions of the PCB assembly 110. The PCB assembly 110 may be formed from a first PCB 110a and a second PCB 110b that are connected together. However, it is also contemplated that the PCB assembly 110 may be formed from a different number of boards, such as a single PCB or more than two PCBs that are connected together. The PCB assembly 110 is configured to supply voltages to the plurality of electrodes of the ion mirrors 105, 106. This may be achieved by providing one or more resistor divider circuits 114 on the PCB assembly 110 so that different voltages can be applied to different electrodes, as will be described further below. The PCB assembly 110 comprises a plurality of fingers 116 that are separated by slotted gaps. This arrangement spaces the electrodes apart sufficiently so as to prevent short circuits, and also reduces the amount of PCB material used. The number of fingers 116 corresponds to the number of mirror electrodes to which different voltages are supplied. An electrical contact 118 is mounted to the distal end of each of the fingers 116. These contacts 118 provide an electrical connection between the PCB assembly 110 and the mirror electrodes. As such, the contacts 118 may be formed from any suitable electrically conducting material. The contacts 118 are mounted to the side of the fingers 116 that faces the mirror electrodes. An insulator block 120 may be provided on the opposite side of the fingers 116 and the contacts 118 secured to the fingers using securing means that pass through the contacts and fingers and into the insulator block 120, as best shown in Fig. 4. The insulator block 120 is also mounted to the housing 112 of the mass spectrometer 100. Although, a single insulator block 120 is shown for a plurality of fingers 116, each of the fingers 116 may comprise its own insulator block 120. The insulator block 120 is electrically insulating and may be formed from any suitable electrically insulating material. Fig. 4 shows a view of part of one of the electrical contacts 118. Each contact 118 comprises two free ends that are configured to contact an electrode of the ion mirrors 105, 106 when the electrodes of the ion mirrors are inserted into the housing, and a central portion through which a screw or other fastening mechanism 122 extends. The free ends may be spring biased towards the electrode, i.e. away from the finger 116. That is, the contacts 118 may be spring biased contacts. The screw 122 extends through the central portion of the contact 118, the finger 116, and into the insulator block 120, with the finger 116 positioned between the insulator block 120 and the contact 118. The head of the screw therefore holds the contact 118 against an electrical trace on the finger of the PCB assembly 110 so that a voltage can be supplied to the contact via the PCB assembly. The insulator block 120 may comprise a recess in which the central portion of the contact 118 and a portion of the finger 116 sit. Other means of mounting the contacts 118 to the PCB assembly 110 may be used. The PCB assembly 110 comprises conductive traces on its surface. These conductive traces extend from voltage supplies to each of the fingers 116 of the PCB assembly 110 and are in direct contact with the contacts 118, i.e. without wires therebetween. The conductive traces electrically connect the contacts 118 to the voltage supplies. Fig. 5 illustrates a perspective view of part of the MRTOF mass analyser, showing the first ion mirror 105. All of the electrodes of the ion mirror 105 except for the end-cap electrode comprise a central aperture for ions to pass through them from a drift region 124. As shown, the plurality of electrodes are in contact with the electrical contacts 118 on the PCB assembly 110. However, it will be appreciated that not every electrode needs to be in contact with an electrical contact 118. For example, when it is desired to provide the same voltage to two or more of the mirror electrodes, only one of the electrodes may be in contact with an electrical contact 118. The other of these electrodes may be electrically coupled to the one electrode via an electrically conductive plate 126. Other means of coupling the electrodes are also contemplated, such as rods or wires. The electrical contacts 118 provide an electrical connection between the voltage supplies and the electrodes of the ion mirrors (via the PCB assembly) that is easy to engage and disengage, e.g. without having to screw and unscrew the electrical contacts to and from the electrodes. The embodiments described herein also allow voltages to be applied to the mirror electrodes without the use of wires. This is advantageous as when wires are used to apply voltages to the electrodes, variations in the position and lengths of the wires can cause variations in capacitance and the electrostatic effect on the electrodes. The use of electrical contacts 118 may therefore allow the electrodes to be replaced without having to re-tune the voltage supplies to obtain the same mirror performance. Thus, the present invention provides an easier and more consistent electrical connection when replacing the mirror electrodes. The electrodes of the ion mirror 105 may be arranged to slide in and out of the housing 112. The electrodes and the contacts 118 are preferably not secured to each other in any manner. A plurality of guides, such as guide rails, may be provided in the housing to ensure that the electrodes align correctly with the contacts 118 when they are inserted into the housing. The electrodes may be slid into position so as to mate with the electrical contacts 118 and urged against the biased, free ends of the contacts 118. This may cause the free ends of the contacts to bend, but they remain biased towards the electrodes so as to maintain a good electrical connection thereto. A retainer 128 may then provided over the electrodes in order to hold the electrodes in position and therefore maintain a consistent spacing between the mirror electrodes. The retainer may be screwed in position with screws 130 which extend through the retainer 126. The retainer 126 is formed from an electrically insulating material. Although only the first ion mirror 105 is illustrated in Fig. 5, it will be appreciated that a corresponding configuration is used to apply voltages to the electrodes of the second ion mirror 106. Additionally, or alternatively, the electrodes of other ion-optical devices, such as an orthogonal accelerator, may be electrically connected to the PCB assembly 110 in a corresponding manner to that described above in relation to the electrodes of the ion mirrors 105, 106, i.e. using spring biased contacts. Fig. 6 illustrates an electrical diagram of an MRTOF mass analyser according to an embodiment of the present invention. As described above, the mass analyser 100 comprises an orthogonal accelerator 104, a first ion mirror 105, a second ion mirror 106, and an ion detector 107. The mass analyser also comprises a first power supply unit (“PSU 1”) 132a, a second power supply unit (“PSU 2”) 132b, a third power supply unit (“PSU 3”) 132c, and a low voltage power supply 134 that operates as an offset unit. Table 1 below illustrates how the voltages from the power supply units 132a-132c and the low voltage offset unit 134 may be applied to five ion mirror electrodes (M1-M5), a drift electrode (M6) and three orthogonal accelerator steering lens electrodes (Y1, Y2 and Z). The steering electrodes (Y1, Y2, and Z) guide the ion beam in the y- and z- dimensions, i.e. in a direction orthogonal to the dimension extending between the ion mirrors 105, 106. Electrode Nominal voltage (V) Voltage input M1 +4970 PSU 1 M2 +1545 Potential divider M3 +325 Low voltage offset unit M4 -8200 PSU 2 M5 -3985 Potential divider M6 -5850 PSU 3 Y1 -5220 Potential divider Y2 -5220 Potential divider Z -4200 Potential divider As illustrated in Figs. 6 and 7, the first ion mirror electrode (M1) receives a voltage directly from the first power supply unit 132a, while the second ion mirror electrode (M2) receives a voltage by being connected to a node in a line that extends between the first power supply unit 132a and a low voltage power supply 134 (i.e. an offset unit). As is shown, a first resistor divider circuit 136a is provided in the line, where the resistors in the circuit are selected such that the second ion mirror electrode (M2) is provided with the desired voltage. The third ion mirror electrode (M3) receives a voltage directly from the low voltage power supply 134. The fourth ion mirror electrode (M4) receives a voltage directly from the second power supply unit 132b, while the fifth ion mirror electrode (M5) receives a voltage by being connected to a node in a line that extends between the second power supply unit 132b and the low voltage power supply 134. As is shown, a second resistor divider circuit 136b is provided in the line, where the resistors in the circuit are selected such that the fifth ion mirror electrode (M5) is provided with the desired voltage. The drift electrode (M6) adjacent to the entrance to the ion mirror receives a voltage directly from the third power supply unit 132c. The voltage of the low voltage power supply 134 may be tunable so that the voltage supplied to the second and fifth ion mirror electrodes (M2 and M5) can be varied, e.g. to compensate for component tolerances. It will be appreciated that the provision of the low voltage power supply 134 allows the MRTOF mass analyser to be manufactured at a reduced cost since there is no need for each of the electrodes to be provided with its own high voltage power supply unit. Each of the orthogonal accelerator steer lens electrodes (Y1, Y2, and Z) receives a voltage by being connected to a line that extends between the second power supply unit 132b and the low voltage power supply 134. As is shown, a resistor divider circuit 136c is provided in the voltage line, where the resistors in the circuit are selected such that the lens electrode is provided with the desired voltage. As described above, the voltage of the low voltage power supply 134 may be varied, which also adjusts the voltages applied to each of the steer lens electrodes (Y1, Y2, and Z), e.g. to compensate for component tolerance. As illustrated in Fig. 7, protection devices 138 may be provided between the low voltage power supply 134 and the electrodes to reduce noise and limit the voltage in the event of a fault. Although the present invention has been described with reference to various embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims. For example, although the mass analyser is illustrated as an MRTOF mass analyser, it will be appreciated that the mass analyser may not be limited this. The mass analyser may be any mass analyser that requires voltage supplies to be coupled to a plurality of electrodes. For example, the mass analyser may comprise a Time of Flight (”TOF”) mass analyser having only one ion mirror.

Claims

1. A mass analyser comprising: an ion mirror comprising a plurality of electrodes; and a plurality of electrical contacts configured to couple the plurality of electrodes to one or more voltage supplies;wherein the plurality of electrodes are slidably engaged with the plurality of electrical contacts.

2. The mass analyser of claim 1, wherein the electrical contacts are spring biased towards the electrodes.

3. The mass analyser of claim 1 or 2, wherein each of the electrical contacts directly contact a conductive trace on a surface of a printed circuit board (“PCB”) assembly.

4. The mass analyser of claim 3, wherein the electrical contacts are connected to the conductive traces on the PCB assembly and / or electrodes without wires.

5. The mass analyser of claim 3 or 4, comprising at least one electrical insulator block, the electrical contacts mounted to the at least one electrical insulator block with the PCB assembly provided between the electrical contacts and the at least one electrical insulator block.

6. The mass analyser of claim 5, wherein the at least one electrical insulator block comprises a plurality of recesses, each recess configured to receive one of the electrical contacts.

7. The mass analyser of any one of claims 3-6, wherein the PCB assembly is shaped to have a plurality of fingers that are separated by slotted gaps, wherein the electrical contacts are provided at the distal ends of the fingers.

8. The mass analyser of any preceding claim, comprising a retainer configured to hold the electrodes in position.

9. The mass analyser of any preceding claim, wherein multiple electrodes of said plurality of electrodes are electrically connected to each other via a rigidelectrically conductive member, and wherein only one of the multiple electrodes is connected to one of said electrical contacts.

10. The mass analyser of any preceding claim, wherein the electrodes and electrical contacts are not secured to each other, such that the electrodes can be slid out of contact with the electrical contacts.

11. The mass analyser of any preceding claim, comprising a plurality of guides configured to guide the electrodes into alignment with the electrical contacts.

12. The mass analyser of claim 11, wherein the plurality of electrodes are slidable along the plurality of guides in a first direction towards the plurality of electrical contacts so as to engage the electrodes with the plurality of electrical contacts, and wherein the plurality of electrical contacts are biased in a second direction opposite to the first direction.

13. The mass analyser of claim 12, comprising a retainer configured to hold the electrodes in position against the electrical contacts, optionally wherein the electrical contacts are spring biased in said second direction and the retainer is configured to hold the electrodes in position against the electrical contacts with the electrical contacts being compressed by the electrodes.

14. The mass analyser of any preceding claim, wherein the mass analyser is a Time of Flight mass analyser comprising an ion accelerator and an ion detector, wherein the mass analyser is configured to determine the mass to charge ratio of ions based on a time of flight of the ions from the ion accelerator to the ion detector.

15. The mass analyser of claim 14, wherein the mass analyser is a multireflecting Time of Flight mass analyser comprising at least two ion mirrors for reflecting the ions between them as the ions pass from the ion accelerator to the ion detector.

16. A method of assembling or disassembling a mass analyser as claimed in any preceding claim comprising:sliding ion mirror electrodes into a housing of the mass analyser until they engage with a plurality of electrical contacts that are connected to one or more voltage supplies; orsliding ion mirror electrodes out of a housing of the mass analyser so as to cause them to disengage from a plurality of electrical contacts that are connected to one or more voltage supplies.

17. A mass analyser comprising:an ion mirror having a plurality of electrodes;a first, high voltage power supply; anda second, lower voltage power supply;wherein the mass analyser is configured such that a first of the electrodes is supplied with a first potential from the first power supply, and a second of the electrodes is connected to a first line extending between the first and second power supplies such that the second electrode is supplied with a second potential that is lower than the first potential.

18. The mass analyser of claim 17, wherein the second power supply is tuneable so as to vary the voltage that it outputs and hence vary the second potential supplied to the second electrode.

19. The mass analyser of claim 17 or 18, wherein the mass analyser is configured such that a third of the electrodes is supplied with a third potential from the second power supply.

20. The mass analyser of claim 19, where the third potential is lower than the first and / or second potentials.

21. The mass analyser of any one of claims 17-20, wherein the mass analyser comprises a third power supply and is configured such that a fourth of the plurality of electrodes receives a fourth potential from the third power supply.

22. The mass analyser of claim 21, wherein the mass analyser is configured such that a fifth of the electrodes is connected to a line extending between the third power supply and the second power supply such that the fifth electrode is supplied with a different potential to the fourth potential.

23. The mass analyser of any one of claims 17-22, comprising an orthogonal accelerator arranged and configured to accelerate packets of ions into the ion mirror, wherein the orthogonal accelerator comprises one or more steering electrodes for steering the ion packet in one or more respective direction that is orthogonal to the direction that the ion packet is accelerated in;wherein each of the one or more steering electrodes is connected to a line extending between the second power supply and one of the first, third or further power supplies.

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