AEM – Special ReportLinac Current Stability and Modulation

June 25, 2007

Presented by: Doug Moehs

Philippians 4:6-7 (Doug) Do not be anxious about anything, but in everything, by prayer and petition, with thanksgiving, present your requests to God. And the peace of God, which transcends all understanding, will guard your heart and your mind in Christ Jesus.

05/18/07 Douglas Moehs 2

Stability: Arc Feedback Loop

• Booster experts would like the Linac beam current to be stable at the 2-3% level Saves time on re-tuning Promotes higher transmission efficiencies over longer periods of time

• Historically the Linac current has followed the ion source arc current over long periods (days, weeks)

I- Pre-Accelerator3 weeks of data from April 2007

4 mA variation in To1in (8%)

2.5 A change in Arc current

and Bob Goodwin

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Stability: Arc Feedback Loop

• The only way to hold the arc current constant is to change the arc supply voltage and assume that the resistance is constant.

Second order effects: For any given change to the supply voltage the total power in the plasma changes and thus the cathode temperature changes.

Cs coverage couples to the cathode temperature which means the H- production might change. However, the temperature does not appear to change significantly enough that this becomes an issue.

• Set up a feedback loop to make adjustments to the arc supply voltage Set up safeguards to limit the loops ability to over drive the plasma

Limit how much the loop can vary the arc supply voltage (± 10 V) Disable the loop if the arc current is outside of some window (= tolerance setting)

and Bob Goodwin

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Stability: Arc Feedback Loop

• This feedback loop has been operating successfully for the last 2 months on both the H- and I- systems

First Results:

I- Pre-Accelerator~3 weeks of data from May 2007

2 mA variation in To1inA lot of extractor sparking caused extra instabilities

<1 A change in Arc current

and Bob Goodwin

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Stability: Arc Feedback Loop

• The present aim on the H- Pre-Accelerator is to hold the arc current to within 0.3 A

On going run:

H- Pre-Accelerator~3 weeks of data from May - June 2007

< 2 mA variation in To1in

<0.5 A change in Arc current

and Bob Goodwin

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Stability: Arc Feedback Loop

• Data from April 5,2007 to June 14, 2007

• Linac current stability has definite improved: in this data by a factor of ~4. With the loop on the current fluctuation is 2% or less.

and Bob Goodwin

All data:Ignore sparking

End of linac:< 1 mA variation in D7TOR

Front of linac:< 1 mA variation in To1in

LOOP

OFF(H-)

LOOP

ON (I-)

LOOP

ON (H-)

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Modulation: Beam Notching

• Booster charge throughput is limited by beam losses.

• One intentional 400 MeV beam loss occurs when the extraction gap is created.

~ 3 RF buckets or 3.6% of the captured charge in the Booster

• This “hole in the beam” could be created at lower energy, preferably in the Pre-Accelerator

With multi-turn injection this means creating 1 hole or “notch” ~ 78 ns wide in the Linac beam for every injected Booster turn which are ~2.2 μs apart.

Typically around 12 turns are injected into the booster. Beam rise and fall times should be ~10 ns Repetition rate 15 Hz

, Bill Pelico, Chris Jensen, Dave Wildman, Robyn Madrak

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Modulation: Beam Notching

• Effort are on going to achieve this at the ion source but…. The environment is tough on the pulsed power supply Space charge effects appear to limit the beam recovery time

, Bill Pelico, Chris Jensen, Dave Wildman, Robyn Madrak

61 mA

0 mA

Beam off

Beam deflection plates are part of a 50 Ohm transmission line.

75% beam extinction in 750 keV line

~2.2 microseconds between notches

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Modulation: Beam Notching

• Transmission of the notch to the Booster is good!

• A notch in the Booster 2.2 ms into the acceleration cycle.

, Bill Pelico, Chris Jensen, Dave Wildman, Robyn Madrak

• 5 Turns injected

• Beam extinction was 83%

• The notch fall time was 56 ns, which is consistent with the Pulsed HV power Supply used for this test.

• The rise time is consistent with that observed in the 750 keV line, 1-2 ms.

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Modulation: Beam Notching

• Possible Solution to over come space charge: Create the notches in the H- 750 keV transport line

Advantages: Beam is more rigged at this energy so space charge should play a smaller role in beam

recovery. Utilize a defocusing quad to assist in beam deflection

Disadvantages: Only available when running on the H- Pre-Accelerator

There is not enough space on the I- Pre-Accelerator transport line to install the necessary deflection plates

• DC studies have been carried out with test deflection plates 17” and 2 cm apart

These studies are to determine the aperture and voltage requirements Presently we loose 32% of the beam when the test plates are put in the beam.

A new Linac tune is necessary to get the full beam through this aperture and we are waiting for study time to determine if this is possible.

Nonetheless a 3kV gradient reduced the remaining current into tank 1 by 90%

, Bill Pelico, Chris Jensen, Dave Wildman, Robyn Madrak

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Modulation: Beam Notching

, Bill Pelico, Chris Jensen, Dave Wildman, Robyn Madrak

• Additional uses for a low energy (≤750 keV) beam notching system exist! Beam sweeps the Linac 400 MeV Lambertson upon extraction to Booster.

A small notch during the transition period would eliminate this localized beam loss. In slip stacking only 7 of the 84 capture buckets are sent on to the Main injector.

Reducing the intensity in the remaining 77 buckets at 250 KeV might reduce coupled bunch instabilities in the Booster.