This invention relates to RV stand offs or breaks and in particular, but not exclusively not to stand offs which are suitable for use in plasma generating apparatus. A stand off 14 includes a resistive or insulating cylinder 15 , through which gas feeds are drilled and a close coupled helically wound combination, generally indicated at 16 . This combination 16 comprises an RF conductive tube 17 in the form of a braided cable; a thermocouple 18 having twisted pair supply cables 19 and a heater element 20 in the form of a cable loop or pair.
1. An RF stand off including a resistive or insulating gas feed for extending between a grounded part of an apparatus and a live part and a close-coupled combination of an electrical heater, an electrical temperature sensor having supply cables and an RF conductor for extending between the grounded part and the RF live part; the combination being generally helically wound around the gas feed and the heater and/or its cables and the supply cables being respectively substantially co-extensive and close-coupled such that voltage induced by RF in one cable is substantially balanced out by an equal and opposite voltage in another.
2. A stand off as claimed in claim 1 wherein the temperature sensor projects from the close-coupled combination at or adjacent the point at which it senses temperature.
3. A stand off as claimed in claim 2 wherein the point is at or adjacent the midpoint of the stand off.
4. A stand off as claimed in claim 1 wherein the combination is in the form of a twisted trio.
5. A stand off as claimed in claim 1 wherein the combination is formed by parallel tapes.
6. A stand off as claimed in claim 1 wherein the RF conductor is formed at least in part by a conductive tube and the electrical heater and the sensor and cables are at least in part contained therein.
7. A stand off as claimed in claim 6 wherein the end of the tube which is to be connected to the RF live part is closed.
8. A stand off as claimed in claim 7 wherein the tube is in the form of a braided cable.
9. A stand off as claimed in claim 1 furthering including a DC bias blocking capacitor for connection in series between the RF conductor and the RF live part.
10. An RF stand off including a resistive or insulating gas feed for extending between a grounded part of an apparatus and a live part and a close-coupled combination of an electrical heater, an electrical temperature sensor having supply cables and an RF conductor for extending between the grounded part and the RF live part; the combination being generally helically wound around the gas feed and the heater and/or its cables and the supply cables being respectively substantially co-extensive and close-coupled such that voltage induced by RF in one cable is substantially balanced out by an equal and opposite voltage in another wherein the temperature sensor projects from the close-coupled combination at or adjacent the point at which it senses temperature; the point is at or adjacent the midpoint of the stand off; and combination is in the form of a twisted trio.
11. A stand off as claimed in claim 10 wherein the RF conductor is formed at least in part by a conductive tube and the electrical heater and the sensor and cables are at least in part contained therein.
12. A stand off as claimed in claim 11 wherein the end of the tube which is to be connected to the RF live part is closed.
13. A stand off as claimed in claim 11 wherein the tube is in the form of a braided cable.
14. RF apparatus including a grounded part and an RF live part interconnected by a stand off as claimed in claim 1 .
15. Apparatus as claimed in claim 14 wherein the grounded part defines a recess or well for receiving the RF stand off and thereby shielding it.
16. Apparatus as claimed in claim 15 wherein the grounded part and RF stand off together form a transmission line.
17. RF apparatus including a grounded part and an RF live part interconnected by a stand off as claimed in claim 10 .
18. Apparatus as claimed in claim 17 wherein the grounded part defines a recess or well for receiving the RF stand off and thereby shielding it.
19. Apparatus as claimed in claim 16 wherein the grounded part and RF stand off together form a transmission line.