Advertisement

Blog

Duct Tape, Spit & Baling Wire

As a UC Santa Barbara student, I was stationed at SLAC, the Stanford Linear Accelerator Center, working on the positron-electron project (PEP).

The PEP consisted of a circular electron-positron collider with a diameter of about half a mile. Electrons propagated in one direction and positrons, the positively charged antimatter electron equivalent, in the other direction through a beam pipe in a tunnel about twenty feet below ground. Each beam consisted of bunches of electrons or positrons that overlapped at six different points around the ring called interaction regions. The electron and positron beams crossed every 2.4 microseconds and their current was about 50mA.

When the beams crossed there was a good chance that an electron and a positron from each beam would “interact” by either annihilating into pure energy or by throwing hard photons at each other and sort of ricocheting away.

Anyway, in the experiment I worked on, a detector shaped like a cylinder lying on its side, about the size of a three-story building, surrounded the electron-positron beam so that it could track whatever happened when electrons met positrons.

When I got to SLAC, the experiment had already been running for a few years and they were just installing an upgraded version of the detector. The upgrade featured two improved components, a “vertex chamber,” at the detector's center close to the point where the electrons and positrons interacted, for improved spatial resolution of particles emerging from the reaction, and, my responsibility, sodium iodide crystal end caps with fancy new electronics.

My first poignant experience in particle physics was quite similar to my first poignant experience working for a large company: Dude, these meetings are BORING.

So there I am, half-asleep, sitting along the wall of the conference room, safe from the huge elliptical adult table where professors and postdocs were trying to figure out why the new inner detector was heating up.

One guy said, “We can run the electronics for hours without any heating. But once the accelerator comes on and we have beams, it starts heating. If we let it run more than an hour it's going to melt.”

Though I hadn't told anyone yet, the new end cap electronics suffered huge jolts of noise — 2V bursts of junk every 2.4 microseconds — the same frequency as the beam crossings. It had to be related, but the system was designed to accommodate big doses of synchrotron radiation — hard X-rays emitted by the charges as focusing magnets direct them around the circle. What else could it be?

As for the vertex chamber, the accelerator had no thermal or electrical connection to the detector. Plus, the beampipe wasn't heating up, just the walls of the new vertex chamber. I stared at the overhead screen. It showed a diagram like this:

The detector geometry. 
(Source: SLAC publication 4585; author Eliott Bloom)

The detector geometry.
(Source: SLAC publication 4585; author Eliott Bloom)

Having survived graduate-level electrodynamics the year before, I couldn't look at a diagram like that without regurgitating solutions. The first thing that came to mind was the method of images. If you have a charge on one side of a conductor, the charge behaves as if there is an opposite charge on the other side of the conductor. It works for landing airplanes, too.

As bunches of electrons and positrons zoomed by, their images would have to zoom past, too. But there's a problem for the images. At the point where the inner detector closes in, the radius of the beampipe decreases by a factor of two over a few centimeters. The image current has to accelerate to keep up with the real current; a huge increase in the current flowing though the outer edges of the inner detector sees.

Without thinking, I mumbled, “The image current blows up at the discontinuity. That'll heat you up.”

It generated a few frustrated looks and then they got into the schematics and started talking about a ground loop.

I'd never heard of a “ground loop” before. A more senior student sitting next to me explained what it meant and everything made sense. The “fancy new electronics” on my end caps were floating on common ground, but only connected to something resembling “earth” through the power supply. The whole end cap was nothing but a big antenna picking up RF radiation from the 50mA electron/positron current at 417kHz.

After the meeting I went down to the equipment pool and asked the grizzled old engineer who signed out equipment what I needed to simulate high frequency noise. This guy looked like a shop teacher, missing a couple of fingers, face riddled from arc welding without a mask, and he sounded like an AM disc jockey, “That ain't high frequency.”

I explained what I thought was going on and he gave me a high voltage power supply. Then he took a 50Ω cable from a rack, pulled a knife from his belt, and cut it about six inches from the connector. He twisted it, taped it up, connected it to the power supply, and said, “Don't touch this part.”

“Okay.”

As I walked out of his office, more like a compound, really, he called me back. “You'll probably want this, too.”

He handed me a spool of about 14 gauge wire with no insulation, just shiny steel wire.

“What's this for?”

“It's baling wire.”

“Why?”

“You'll figure it out.”

Back at the interaction region, the accelerator was down so I was allowed into the detector chamber. With a scope connected to one channel of the end cap electronics, I set the HV supply about 20 feet away, tuned it on, on and sure enough, got a big blast of noise. I switched it off and on a few more times, until I got a spark off the cannibalized cable.

The end cap electronics consisted of several hundred photomultiplier tubes housed in stainless steel. I took the baling wire, wrapped it around one of the tube housings, and then twisted the end around the signal cable's BNC connector. When I flipped the HV switch, no more noise.

I spent that night wrapping every tube in baling wire and the next day, when the accelerator came up, I got pristine signals suitable for an upgrade.

A couple of weeks later, when the accelerator was down for maintenance and people were allowed into the detector chamber, I found six professors staring at the end cap. One said, “Who wrapped wire all over the sodium-iodide array?”

I said, “There was a ground loop.”

He said, “Yeah, but there's no duct tape.”

Feel free to share your experiences with ground problems, lack of grounds, and ground loops.

Related posts:

15 comments on “Duct Tape, Spit & Baling Wire

  1. eafpres
    June 6, 2013

    As a teenager I raced go-karts.  My father, retired 30-year Navy, had been a self-taught electronics instructor in the Navy before moving on to bigger things, and kept working on electrical and electronic systems his entire life.  He had a mental picture of analog voltages and waveforms that allowed him to literally look at problems and design something.

    Unfortunately for me, this also meant we had to have various home-grown electrical bits and enhancements on anything.  In my case, this meant that, although it was not required by rule and nobody else had one, we had to wire a “kill switch” on the go-kart, using a toggle switch mounted near the steering wheel and wiring running to the engine.  As I recall, this setup shorted out the points such that the coil was never discharged, thus no spark and the engine stops. 

    At one event we were having intermittent problems.  As obvious as it seems now, it took a while to consider the kill switch.  As a teenager who was amped up to race, I was going nuts.  At the last minute, we noticed that a single strand of the stranded, insulated wire we used to hook it up was kind of just hanging there.  The connections were not soldered (bad idea) and a strand had gotten loose and was touching ground during certain accelerations.  Hence, an intermittent problem.

    Fuming, I waited for my father to remove the kill switch and then I was off the races.  I never forgot my introduction to unwanted grounding.

  2. bjcoppa
    June 6, 2013

    UCSB is an excellent engineering institution and they have collaborated much in the past with my alma mater North Carolina State Univ. where I did my PhD in materials science engineering. When I was considering which path to follow as a teenager, I attended a lobbyist event in DC to support the Superconducting Supercollider which was deemed as the largest and most powerful particle accelerator in the world. It never got funded and the project was canceled during the Clinton Administration even though we had a budget surplus at that time. I wonder what knowledge was lost as a result in this field including spillover applications.

  3. bjcoppa
    June 6, 2013

    Particle accelerators led me to majoring in Physics in college at the University of Arizona. I did an internship the summer before at Oak Ridge National Lab in TN using a ~5MeV Van de Graaff for surface hardening of plastics for shatterproof window applications. UofA has always been strong in theoretical high energy physics which ties in well to their Astronomy program.

  4. Ransom Stephens
    June 6, 2013

    The kill switch.

    When I was in high school, I was in this gang/high performance car club. (Gang in the sense that we had matching jackets and interests, which also generated suspicion that we could be violent and dangerous–absolutely delicious for the 17 year-old). One of the guys installed a kill switch that disconnected the ignition, so we all did.

    I had a 69 Charger and put the switch under the dash. Then, when I wanted the car to run out of gas, or breakdown in some manner — after all, these hot rods are temperamental, right? I'd hit the switch. Then, if my passenger didn't mind an hour or so star gazing, I could get out later and pretend to “fix” the problem.

    One of the reasons I made my daughter watch Beevis and Butthead = an accurate portrayal of boys prior to discovering gentleman-hood.

  5. Ransom Stephens
    June 6, 2013

    Ugh. You had to bring up the SSC.

    Set out in Waxahachie, there was no infrastructure, no local expertise. It led to a mass migration of physicists from Fermilab (east of Chicago in Batavia) and SLAC (Menlo Park, CA).

    It's also how I ended up in Texas, a prof at UT Arlington which was the closest university to the lab. Prior to the SSC, UTA didn't have a high energy physics group. When the SSC was canceled, after a good chunk of the collider tunnel had been dug and a couple of billion already spent, we swtiched from commuting to Waxahachie to Geneva Swtizerland and CERN. Which, I have to admit, was nicer.

    The SSC died for a lot of reasons, mostly because it was put in Texas as sort of a gift from George Bush (41). The budget that killed the SSC was the first of the Clinton administration. There was still a huge deficit left over from Reagan and Bush (et al) and a stagant post cold war recession. Members of congress who had been science friendly flipped, most egregiously in Illinois, where Fermilab had prospered, betraying what they really cared about. Clinton was behind it but had used up all his clout getting that first budget through which was the one that paved the way for those surplus years. Gore was lukewarm, always more of a fan of biology projects and technology (the information superhighway), and it was originally a Reagan proposal.

    Had it been placed in Illinois near Fermilab, in New York near Brookhaven, or California, near SLAC, it might have made it. Then, we'd have discovered the Higgs 5 or 10 years ago and we won't know what else until LHC runs at higher energies.

    The death-knell came in a Dateline program that “exposed” extravagant spending, parties and houseplants paid for by taxpayers. Parties? you call those parties? There were gatherings with food that included contractors, scientists, and engineers as well as wine-and-cheese seminars and academic-what-not, but those events can't be called parties.

    As the lab was being built, offices were in a big industrial park and some money was spent on house plants — I still have a ficus tree I nicked when everything was being shut down. You can begrudge scientists high salaries (I made less as a UTA professor than a baggage handler at DFW airport with the same years of experience in the field), year-end bonuses (never heard of), stock options, etc, but you can't begrudge them a fern next to the water cooler!

    But I'm not bitter…

     

  6. Ransom Stephens
    June 6, 2013

    At this very moment, I am drinking from a mug that says “D-zero Quantum Chromodynamics Workshop, University of Arizona, 1996.”

    Great school, neat area, Tucson is how deserts should look.

  7. Brad Albing
    June 6, 2013

    @Ransom >>One of the reasons I made my daughter watch Beevis and Butthead = an accurate portrayal of boys prior to discovering gentleman-hood . I used a similar method with my 2 daughters – but rather than educational MTV, I just told them to always remember that men are idiots. Seemed to work out OK.

  8. Brad Albing
    June 6, 2013

    @Ransom >>…neat area, Tucson is how deserts should look . We should all meet there – get Bonnie Baker involved too – discuss all this over drinks + Native American-Mexican fusion food.

  9. Scott Elder
    June 6, 2013

    Wow!  You used the words “gang” and “kill” in one post about 10 words apart.  Google will have a field day with this one.  SMILE.

  10. Brad Albing
    June 6, 2013

    Hmm… I'll need to word my posts carefully – either so that I create a post like Ransom did – or so that I don't . Also need to be careful of certain word clustering, else the NSA will take notice.

  11. eafpres
    June 6, 2013

    Now that you konw that the government is storing everything you blog, post, pin, like, share, email or otherise touch electronically, ya'll better mind yer manners online.

    PRISM

    Dang, I'll bet I get flagged as a radical redneck 'cuz of this post.

  12. Scott Elder
    June 6, 2013

    I take solice in the statement made many years ago by Scott McNealy, then of Sun Microsystems.

    “There is no privacy on the Internet.  Get over it.”

  13. DEREK.KOONCE
    June 7, 2013

    I am sure the intended 'dead car' would not work these days with cell phones. Unless one is in a very remote location where Verizon has no service. 🙂

  14. Ransom Stephens
    June 7, 2013

    Yes, my use of email downloaded to this box and contempt for use of, or storage, of anything the cloud provides a filmy illusion of privacy.

    (This'll throw 'em) EXPENSIVE

    Just put EXPENSIVE at the end of every sentence. The entire NSA facility in nowhere Utah will crash.

  15. Davidled
    June 8, 2013

    Well, carrier provider will build more towers.  Wireless is going to 5G with LTE. In the future, weakest signal area will be reduced except tunnel.  Back-end sever will be improved.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.