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Full discharges in the Pelletron

Full discharges in the Pelletron. Recycler department meeting April 12 th , 2006 A. Shemyakin, L. Prost, G. Saewert. Interruptions and full discharges.

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Full discharges in the Pelletron

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  1. Full discharges in the Pelletron Recycler department meeting April 12th, 2006 A. Shemyakin, L. Prost, G. Saewert

  2. Interruptions and full discharges • Sometimes the protection system turns the gun off, interrupting the beam recirculation. At normal conditions, the terminal voltage drops by about 10 kV and there no vacuum bursts. Operation can resumed in a matter of 15 sec. • However, in some occasions the Pelletron voltage goes down all the way to zero in microseconds with a large pressure jump in one of the tubes. Usually one needs to wait 0.5 – 2 hours before a normal operation can be resumed. In addition, the tube ability to hold HV deteriorates; almost all damage to the Pelletron electronics occurs in the time of such full discharges.

  3. The tube • Each of the Pelletron tubes is a 6-m stack of 252 ceramic rings with a titanium electrodes in between. The tube consists of 12 modules, 2 modules between each separation box. • The voltage is distributed along each tube by a chain of 0.5 GOhm resistors. The current through resistors (“the tube current”) is 39 A at he nominal voltage of 4.32 MV. • A boundary electric field, where vacuum changes at an increase of the high voltage, is 8 -20 kV/cm depending on the length of tube portion under voltage and history

  4. A full discharge • A discharge begins in vacuum with a typical rise time of ~100 ns. • Capacitance from the terminal to ground ( 300 pF) holds the terminal voltage nearly constant. As a result, discharging of one section results in overvoltage at other sections. • If the overvoltage is too high, the discharge spreads out. • If the average electric field over a section reaches ~ 35 kV/cm, the gas spark gaps fire with the time rise of ~ ns, preventing further energy deposition in vacuum

  5. A full discharge - numbers • Total charge at the terminal= 4.32 MV * 300 pF ~1.3 mC • Total stored energy 3 kJ • If during a discharge each electron of stored charge releases 1 atom, the pressure jumps to 4.E-6 Torr. • Typically, a full discharge trips several ion pumps

  6. Relevant diagnostics • A Pearson coil measures a current flowing between the deck and the cathode flange through a braid. All other connections are slowed down by ferrite rings. • Fast changes of the Pelletron voltage are measured by a capacitive pickup (CPO) installed across the terminal.

  7. Diagnostics - Tube Monitors • We can record AC-coupled signals from the lowest tube electrodes, so-called Tube Monitors (TMs). TMs are sensitive to • Changes of the beam current • Decrease of terminal voltage • Discharges in the tubes • Direct charging by electrons • TMs, Pearson coil, and CPO are recorded by a Transient Recorder (Jordan Wilberding & Greg Saewert) and saved on a hard drive Oscilligram of pulsing to ground. Yellow trace- Pelletron voltage, 5 kV/div; White- cathode current, 0.12 A/div; Green- Acc.TM, Blue- Dec.TM, both 2 mV/div

  8. A simplified electric circuitry

  9. Beam – related full discharges • With no beam and after a good conditioning, the voltage can be raised up to 5.2 MV without any vacuum activity, 20% above the operational voltage. Therefore, all our full discharges are beam – related. • Mechanisms of a beam – tube interaction • Direct redistribution of the potential along the tubes • Beam –produced particles (secondary electrons and ions, vacuum ultraviolet, X-rays) are additional “igniters” of discharges • We have identified several mechanisms of full discharge provoking: • DC beam scraping in a tube • High DC tube losses • Beam scraping in a tube during interruptions

  10. DC beam scraping in a tube • As soon as the beam approaches tube electrodes, the potential distribution changes dramatically. It changes the beam envelope, an increasingly larger portion of the beam is lost, and the discharge is fast. • Observed primarily at the initial tuning

  11. High DC tube losses Losses as functions of the beam current before and after adjustments of focusing and steering in the deceleration tube. October 18, 2005 Tubes are prone to full discharges if the tube currents change by more than 1 – 2 A (out of 39) or electric strength is low. We put significant efforts in minimizing the losses. Also, a stable operation requires a careful conditioning.

  12. “Fast” full discharges In both cases, the full discharges are “fast”. Namely, the Pelletron voltage decreases by hundreds of kV in a microsecond. Almost all fast full discharges observed in MI-31 were on the deceleration side. Oscillogram of a fast full discharge. July 9, 2005. I = 0.3 A. Time scale is 5s/div. Yellow- Pelletron voltage, 5 kV/div; White - Cathode current, 0.12 A/div; Green- Acc. TM, 0.2 V/div; Blue- Dec. TM, 0.2 V/div

  13. Recirculation interruption • Present understanding of “natural” (i.e. occurring at fixed settings) interruptions • A partial discharge changes the potential distribution along tubes. • Modified beam envelope results in beam scraping in the collector anode • The anode voltage drops that changes the beam envelope even further • The beam begins to scrape either at the ground or near the collector entrance, producing a large amount of secondary electrons • The protection system sees a Pelletron voltage drop and turns off the gun Oscillogram of a recirculation interruption in a “good” time. Yellow trace- Pelletron voltage, 5 kV/div; White- cathode current, 0.12 A/div; Green- Acc.TM, Blue- Dec.TM, both 200 mV/div

  14. Beam scraping in a tube during interruptions • In the WB run, there were many full discharges in the acceleration tube that were later identified as caused by beam scraping during interruptions, when the anode voltage drops. Simulations of the beam envelope at various anode voltages allowed to avoid the problem in Mi-31 run. • Similar discharges in the deceleration tube were unavoidable at the beginning, because optics was unknown to the necessary precision. Adjusting focusing and steering finally made the frequency of discharges in the deceleration tube tolerable. • Rate of full discharges significantly improved after a new permit box capable of closing the gun in 1 s was installed.

  15. History of full discharges There were no full discharges in the acceleration tube before Nov. 19, 2005. After that, almost all discharges were in the acceleration tube. Most of discharges were at I > 0.1 A

  16. A typical oscillogram of a full discharge Traces from the Transient Recorder showing a full dischargein two time scales. The full scale is 10 ms for the left plot and 26 s for the right plot. 12:08:05, March 29, 2006. I = 0.2 A. The traces and full scales for the left/right plots, from the top down, are 1. Pelletron voltage, 25/3.5 kV 2. Pearson coil (cathode) current, 6/6 A 3. Acceleration TM, 0.12/0.05 V 4. Deceleration TM, 2.5/1.4 V

  17. Zoology of the full discharges • All discharges are in the acceleration tube • All discharges are associated with a jump of the Pearson coil (“cathode”) current. The jumps go near or above the saturation level of the electronics of 4 A. • The discharges are slow. Often the traces captured by the Transient Recorder are indistinguishable from ones of an interruption. • Fast closing the gun doesn’t prevent full discharges

  18. Possible suspects • Damage of lenses or correctors in the acceleration tube • Mary Sutherland compared resistances of all elements and found no deviations from good times • We did not notice changes in cooling • Loss curves at I ≤ 0.5 A are similar • Aperture limitations in the acceleration tube • Dedicated scans both in pulse and DC modes did not show any significant limitations • Protection circuitry • Is the gun being closed during interruptions ? • At least, in most cases it is. • Implementing triggering the protection by a positive rise of the Pearson coil signal (Jim Crisp, Greg Saewert) did not help

  19. “Good” interruption recorded recently March 29, 2006. I = 0.2 A. Time: 155 s full scale. Full scale for traces: 1. Pelletron voltage- 6 kV 2. Pearson coil (cathode) current, 0.3 A 3. Acceleration Tube Monitor, 0.14 V 4. Deceleration Tube Monitor, 2.5 V In this case, correct closing the gun is clear.

  20. Possible indications of problems - interruptions • The Pearson coil current jumps are well above both space charge and emission limits of the gun. Similar bursts in WB were up to 15 A. • Typically TMs show signals that may indicate a recirculation of bursts of a high current (~ 1 A). • The voltage during interruption often drops by hundreds of kV. • In a test run with a shorted top section, there were no interruptions with positive P.c. jumps. Traces of a typical interruption. March 28, 2006, 4:07 PM. Jumps on TMs are shifted by one point (50 ns). The flight time is ~40 ns.

  21. Traces of an interruption with a shorted top section • All interruptions were with no positive Pearson coil jumps. • Many of interruptions started with a ~2 s jump of TMs and Pelletron voltage, with the same dU/dTM ratio of 100 kV/V. • The Pelletron voltage drop in all interruptions was les than 100 kV, in most < 20 kV. • The only full discharge was fast. Traces of a typical interruption with a shorted upper section. March 31, 2006, 3:36 PM. I = 0.25 A.

  22. A possible explanation • The discharge begins at the very top of the acceleration tube. • If the inductance between the anode flange and the terminal is large enough, the anode potential can jump, producing high-current bursts. • In addition, part of the Pearson coil signal can be a capacitive current between the gun flange and the anode. • It looks like the strength of the top section of the acceleration tube significantly deteriorated, and it became much more sensitive to the beam.

  23. Possible indications of problems – cont. • Pressure in the gun is worse than it used to be and depends much stronger on the beam current (by ~ 8 times). • No indications of possible SF6 leak have been found • Tube losses depend on the gun pressure • Sometimes, the first increase of the beam current in the morning gives a lower value maximum current, and the gun pressure jumps near this maximum. • The strength of the top section degrades most, and conditioning goes primarily on the acceleration side. • However, we still can condition the top section to 1.2 MV. The gas side is an unlikely culprit.

  24. What we can do • We may hope that eliminating dust and other work in the tank will help, or hope to find an explanation on the gas side. • We can decrease the electric field on the top portion of the acceleration tube • Changes optics • Increases gradient on other sections • May require moving the gun solenoid • A step back. Degradation may continue. • We can ventilate the tube with nitrogen and bake • It may help if the reason of the problem is a cathode sputtering • The gun IP may need a deep bake or replacement • Side note: if the tube is to be baked, we can discuss measuring the magnetic field in the cathode location

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