# Presentation: 337920

## High Gradient RF Issues and 88 MHz test results

- M. Vretenar for the 88 MHz team:
- R. Garoby, F. Gerigk, J. Marques, C. Rossi, M. Vretenar

- The 88 MHz test cavity
- The test stand
- Conditioning results
- High gradient experience at CERN
- Some conclusions

## Cavity design for the CERN muon channel

- 88 MHz cavity designs
- (“old” and “new” schemes)

- CERN Muon Cooling Channel scheme (2000): 44 MHz + 88 MHz cavities

- 88 MHz cavity requirements:
- 	Real estate gradient 4 MV/m
- 	Peak RF power > 2 MW

## Challenges of muon cavities

- Challenges of muon cavities in CERN (and other) schemes:
- Low frequencies
- High gradients
- Poor RF power efficiency (ZT2)
- Presence of high B-field

- Need to start as soon as possible an experimental programme to assess the technological challenges.

- High gradients in a frequency range with little experience
- Construction and operation of large RF amplifiers with high peak power

## The 88 MHz Test Cavity

- Idea to put together a test stand using an old PS 114 MHz cavity (used for leptons), modified for 88 MHz, fed by an upgraded 200 MHz Linac amplifier, and driven by a modified 80 MHz PS amplifier.

**0.9m**

- New “gap” inserted in the old PS cavity
- Main parameters are equivalent to muon cavity

- R. Garoby, F. Gerigk, CERN-NUFACT-Note-87

## The 88 MHz Test Stand

- Generation and transport of > 2 MW power to the test cavity is a delicate task, requiring a large size installation.
- Main challenges:
- high drive power (>400 kW),
- high anode voltage (40 kV),
- large power and voltages (during reflection)
- 		in the final amplifier,
- large peak voltages
- 		in the transmission line.

- Due to lack of resources after 2001, the test stand preparation has progressed very slowly.
- Infrastructure preparation very time consuming.
- Results only in 2005.

## 88 MHz Test Stand

## 88 MHz Test Stand

## 88 MHz Final Amplifier

- Old Linac1 (1958) 200 MHz amplifier, rated for 2 MW, equipped with tube TH170R (max. power 2.5 MW)
- Improved for higher power:
- Kapton capacitor (anode blocker)
- Revised neutralization
- New output resonator (88 MHz)
- New (low-cost) filter
- Double coaxial output (~1 MW/arm).

## 88 MHz Cavity Conditioning

**300 ********s**

**250 ********s**

- About 160 hours of effective conditioning time at 1 Hz
- Maximum power out of final amplifier 2.65 MW reached after about 100 hours of conditioning
- Increasing pulse length during test. Filling time ~250 s
- Test stopped due to failure of anode supply with 300 s pulse length (now repaired).
- Tests will be restarted soon but we are limited by sparking in the final amplifier and there is no need for longer pulses.

- Preparation of test stand finished end July 2005.
- Conditioning started on August 16th and continued until September 6th.

## Conditioning results

- Sparking in the output amplifier cavity (designed for 2 MW!) limited at 2.65 MW the power that could be sent to the test cavity.
- At this power level, all test stand components were close to their limit (HV supply, driver and cavity).
- Measured gradient in the test cavity at this power was **4.1 MV**, slightly higher than the design value (4 MV/m).
- At this field level, there were occasional remaining breakdowns in the cavity, causing loss of few % of the pulses. Further conditioning could reduce this breakdown rate
- Although the gradient was limited by the final amplifier, the conditioning experience shows that we were about at the highest gradient achievable in the cavity for reliable operation.

## Experience with multipactoring

- 3D computations of multipactoring up to 3rd order in the 88 MHz cavity were performed with the code MultP by C. Schulz (TU Berlin, Diploma Thesis, 2004).
- No significant electron activity was observed by the code at         V > 1.6 MV.
- 5 multipactoring levels were foreseen at V < 1 MV

- Experimental result:
- Some multipactoring activity was observed at gradients < 1 MV
- A disturbing multipactoring level was observed at 3.1 MV, _not predicted by simulations_ !

- Simulations were not extended to the power coupler...

## Limit values during test

- Highest field level reached during conditioning:
- 4.1 MV/m		E-field gradient
- 14.7 MV/m 	gap field
- 25.9 MV/m 	peak field (=2.4 Kilpatrick)
- 2.65 MW		output power
- 1 Hz 		repetition frequency
- 170 s 		pulse length

- Gap field

- Peak field

- Computed Q=50’000 (Superfish)
- Measured Q=33’300 (67%)

## Field emission at high gradient

- The amount of available power is limited by dark current. Field emission at high gradient generates electron current that absorbs power from the generator.

- Electrons generated by multipactoring

- Electrons generated by field emission at high gradient

## Field emission measurementsFowler-Nordheim plot

- Field emission current is computed from the “additional power” going to the cavity at high gradient.
- F.E. current follows the Fowler-Nordheim formula
- Plotting ln(I/E_s**2.5) = f(1/E_s), the slope of the straight line gives the field enhancement factor , depending on surface conditions.

- Fowler-Nordheim plot of field emission current for 88 MHz cavity.
- Enhancement factor =170, quite normal for copper plated cavities out of the workshop, without particular cleaning procedures.

## Experience with the CERN RFQ2

- Enhancement factor  can greatly affect voltage holding and maximum gradient.
- It is increased by dielectric impurities on the electrodes.
- It decreases slowly with steady operation.

- CERN RFQ2 operates at high gradient (35 MV/m peak field) injecting into Linac2.
- Original (from workshop) =220
- After an oil pollution from the vacuum system =920
- After slow reconditioning and 4 years of operation =67.

## Maximum gradients achieved at 88 and 200 MHz w.r.t. Kilpatrick limit

- In terms of _conditioning time_ and _residual breakdown rate_, the levels reached by RFQ2 and 88 MHz cavity are comparable (as for repetition rate and pulse length).
- Conclusion is that peak and operating field seem to follow Kilpatrick law (square root of frequency).

## Another high gradient experiment at 200 MHz – the IH2

- After 240 hours of conditioning, fields >50 MV/m (~3.5 times the Kilpatrick level) were reached on the drift tube face ( 6 cm2 per tube) in more than 20 gaps. Local field maxima were as high as 75 MV/m (on  0.5 cm2 per tube).
- Measured  was about 100.

- Steady operation cleans the electrodes and improves field enhancement factor:
- A conditioning test was done in 1997 on the 200 MHz Interdigital-H Tank2 at Linac3, which had been in operation (1’500 hrs/yr) since 1994.

- Reasons for the very high field: small high field regionelectrodes, cleaned by 6’000 hrs 	of operation.

## Some conclusions

- Field gradients of 15 MV/m (and real estate gradients of 4 MV/m) are achievable at 88 MHz, with peak fields exceeding 2.4 Kilpatrick.
- At low repetition rate and pulse length, effective conditioning times of about 200 hours are sufficient to reach the field limit.
- Cleaning procedures for electrodes (similar to what is done for SC cavities) or long-term conditioning by the RF could possibly improve the field enhancement factor and allow reaching higher gradients – but then the limitation would be in size and reliability of the RF system !
- Maximum field as function of frequency seems to scale accordingly to a Kilpatrick-like law (number of emitting impurities proportional to surface and inversely proportional to frequency?).
- Multipactoring calculations tend to be unreliable (phenomena probably occur in the regions more difficult to simulate).
- No conclusion can be drawn on the effect of magnetic fields on high field operation.