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The history of the RF ball

The very first prototype with a ping-pong ball (4/09/07)

It was just a standard 38 mm ping-pong ball with two electrodes made of a conductive paint and connected to an RF coaxial connector (LEMO 00 type) with two needles driven into the ball. The ball was connected to an RF signal generator and attached to a wooden stick. Then the ball was inserted into a lab model of an LHC arc BPM connected to its standard electronics. The generator amplitude and frequency were changed to find the trigger threshold of the BPM electronics and the frequency of its maximal sensitivity. With this setup it was found that 1 V on the ball electrodes gives some 5 mV on the BPM electrodes, which is the threshold for a reliable trigger for the LHC BPM electronics.
The BPM system had good sensitivity in the range 40 - 60 MHz. The lowest frequency was chosen to operate the RF ball in order to minimise the circuit power consumption.

The first prototype of the 40 MHz transmitter (6/09/07)

The generator was built on a piece of an FR4 substrate with a few Cu islands manually cut-out with a sharp tip tool. The islands were used to accommodate the generator electronic components as well as to connect the Cu electrodes and the battery holder.
The generator circuitry contained:
- a 40 MHz miniature quartz oscillator;
- 74AC04 CMOS inverters;
- a step-up transformer;
- a LED as a "power on" indicator;
- a miniature switch for turning the circuit off and on, as the battery could not be removed and put back for this purpose due to a delicate electrode construction.
The generator was powered from CR2032 lithium 3 V battery. Its black holder is seen on the photo placed behind the circuit.

The polycarbonate ball with the second 40 MHz transmitter prototype (12/09/07)

The project and manufacturing of the polycarbonate ball was done within only a few days.
Polycarbonate was chosen due to its high resistivity to mechanical shocks, from which the ball suffers when it hits discontinuities of the LHC beam pipe. Polycarbonate is also compatible with the LHC ultra-high vacuum (pressure in the order of 10^-10 mbar, that is 10¹³ times smaller than the atmosphere). This compatibility is very important as some small pieces of the RF ball could accidently get detached from the ball during its travel inside the LHC beam pipe and stay there for the beam operation.
The ball is produced from polycarbonate by fused deposition modelling.
The 40 MHz transmitter was re-built to fit into the new housing and got improved by making the copper electrodes part of a resonant circuit. This way most of the 40 MHz energy is sent back and forth between the capacitance of the resonant circuit (a discrete 10 pF, an adjustable small capacitor and the Cu electrodes) and a low loss RF inductor (also seen on the photo). The resonant circuit decreased drastically the power consumption of the transmitter. This way the operation with one battery was extended beyond 2 hours, which was by far enough for a 3 km travel inside the LHC beam pipe, typically lasting some 20 minutes.

The first test inside the LHC beam pipe (13/09/07)

On the photo the first "operational" RF ball is inserted into an input to the LHC beam pipe by Sylvain Weisz, the leader of the RF ball project.
The ball contained exactly the "bricolage" RF transmitter shown on the previous photo.
The test was done only 9 days after the idea on an active RF ball popped up (4/09/07). It was possible only due to hard work and determination of many people involved in the project, which in turn were the response to the gravity of the situation caused by the PIM crisis.
The test was a full success. In addition to demonstrate that the RF ball idea works, during this the very first test a sixth faulty PIM was found.
Some details of this test were described in a CERN bulletin article to lift the spirits of the CERN community, as many were worried by the challenge of finding the few damaged PIMs in the multitude of the magnet interconnects. The photo to the left and the one below were published with the article.

Members of the RF ball team present for the first RF ball rolling (13/09/07)

In the RF ball project was involved many people from quite a few CERN groups. On the photo there are shown some members of this community at the beginning of the project. The RF ball team was changing over years, according to the needs and maturity of the project.
It should be added that the RF ball was preceded by other techniques and ideas, which gave the ground for its development.
The CERN RF group was successfully using a microwave reflectometry for detecting obstacles in the beam pipe and in fact the first damaged PIM was found using this technique.
The microwave reflectometry was hoped to be used for localisation of faulty PIMs by introducing a metallic ball, blown in the LHC beam pipe. Then once this ball gets stopped by a broken PIM (or any other obstacle), it could be localised by the reflectometry.
The "active" RF ball with a built-in transmitter was a next step to make the tests faster and more practical by employing the LHC BPM system.
Some details of the microwave reflectometry are described in LHC Project Report 1113 mentioned on the RF ball home page and an earlier conference paper, including a few photos of objects found in the LHC beam pipe.

RF ball transmitters on a serious PCB (December 2007)

The RF ball idea got quickly confirmed with the "bricolage" transmitter shown earlier, which was also used for first "operational" tests of the LHC beam pipes. In the meantime a proper design was being done, based on a 4-layer printed circuit board (PCB) adequate for a small series production.
At this occasion a few improvements to the circuitry were added, as described in some detail in the circuit section.

Routine operation of RF ball tests (since spring of 2013)

For the period of large works of the long shut-down 2013 - 2015 the RF ball was foreseen to be used many times to verify the LHC beam pipes. The whole machine was checked many times after the warm-up of 2013 to detect the faulty PIMs requiring reparation. For that occasion the RF ball test procedures were optimised for efficiency.
On the photo Berthold Jenninger launches the ball for the test, opening a valve on the mobile vacuum pump station, built for the RF ball project. A twin pump station was connected to the other side of the arc, operated by another member of the vacuum group.

RF ball becomes a standard LHC diagnostic tool (since spring of 2013)

The photo shows the "RF ball suitcase" accommodating the balls. In the foam holes are seen small permanent magnets, switching the RF balls into the standby mode once they are put in their "nests". In this mode the balls can wait for months before they are used for tests.
The missing ball (# 1) was being used for the test of the arc 6-7 on 14/05/13. During this test the ball #1 passed both beam pipes of the arc without being stoped by anything. This way within some two hours the arc 6-7 was checked and declared free from any obstacles.
Due to its simplicity and speed, the RF ball technique is likelly to be used durig the whole lifetime of the LHC after each warm-up from crogenic temperatures and before colling down the machine after maintanance or upgrade works.