Evolution of beam energy distribution as a diagnostics tool Alexey Burov, Bill Ng and Sergei...

Evolution of beam energy distribution as a Evolution of beam energy distribution as a diagnostics tooldiagnostics tool

Alexey Burov, Bill Ng and Sergei Nagaitsev

November 13, 2003

Beam energy distribution - Nagaitsev 2

What affects coasting-beam energy distribution?What affects coasting-beam energy distribution?

IBS: changes the energy spread, does not change the average energy.

Broad-band impedance: does not change the energy spread, changes the average energy.

Ionization losses and coulomb scattering with electrons: changes both the energy spread and the average energy.


Short historyShort history

In August, 2003 at the Recycler Shutdown Work Review, I have presented a beam-based “average” vacuum model. This model uses the measured transverse emittance growth rate and the energy loss data to derive the partial pressures. The model neglected the IBS and the impedance.

I have then asked Bill Ng to estimate the contribution of the impedance to the energy loss.

I have also asked Alexey Burov to include the IBS in the model and then extract the impedance from the measured energy distribution.

All Recycler measurements were performed by Martin Hu, the Accumulator data were obtained by Paul Derwent.


Observations (coasting beam) Aug 2, 2003Observations (coasting beam) Aug 2, 2003

1 hour, 9E10 p’s, long. Schottky at 1.75 GHz (no MI ramps) The average revolution frequency has decreased by 32 mHz

This corresponds to a 0.37-MeV energy shift

The rms momentum spread has increased (1.0 to 1.3 MeV/c) The low energy tail has developed

-12 MeV/c 12 MeV/c -12 MeV/c 12 MeV/c


Summary of beam-based measurements (as of Aug.’03)Summary of beam-based measurements (as of Aug.’03)

Emittance growth rate:

Beam average energy loss: 0.40±0.04 MeV/hrThis corresponds to a mean energy loss of 0.42±0.04

MeV/hr

m/hr 1 10)(

)(12 22

%95,

i i

ieiniiave

p

Z

ZZZn

crn

i ii

i

i

I

Tx

A

ZMeV 0.040.42ln][g/cm

307.0MeV 2max2

2


Partial pressures modelPartial pressures model

Assume that the residual gas consists only of Z=1 and Z=8 atoms (H2 and H2O)

Solving equations for 10±1 μm/hr and 0.42±0.04 MeV results in: pH = 3.3±1.7x10-9 Torr

pW = 1.0±0.2x10-9 Torr





pW = 1.0±0.2x10-9 Torr

If I assume that one of the gases is hydrogen with a know concentration nH, and then try looking for another gas with a new Z (A = 2Z), nZ ≤ nH, I am unable to find any solution unless pH > 1.5x10-9 Torr and Z ~ 5.





pW = 1.0±0.2x10-9 Torr

“Water” alone contributes 8.3 μm/hr to the emittance growth





pW = 1.0±0.2x10-9 Torr

“Water” alone contributes 8.3 μm/hr to the emittance growth

Before Jan 2003 shutdown the measured emittance growth rate was 5 μm/hr. There were no beam energy loss measurements.

Present measurements are consistent with the “water” content doubled after the shutdown.


The low-energy tail originThe low-energy tail origin

1.9E11 protons before scrape 0.9E11 protons after scrape

-12 MeV/c 12 MeV/c


The low-energy tailThe low-energy tail

The scrape was done with a horizontal scraper at a location with a horizontal beta-function of 52 m, zero dispersion (~20 cm) and with equal tunes.

The scraper was stopped 6.2 mm away from the beam center, which corresponds to a 7-μm acceptance, and then withdrawn.

How can one scrape the low energy tail at zero-dispersion location?


The low-energy tailThe low-energy tail

The scrape was done with a horizontal scraper at a location with a horizontal beta-function of 52 m, zero dispersion (~20 cm) and with equal tunes.

The scraper was stopped 6.2 mm away from the beam center, which corresponds to a 7-μm acceptance, and then withdrawn.

How can one scrape the low energy tail at zero-dispersion location?

The answer is: proton-electron collisions.


Proton collision with a stationary electronProton collision with a stationary electron

Max. energy transfer Tmax= 91 MeV

0 20 40 60 80 1000

1

2

3

4

5

Tra

nsve

rse

mom

entu

m, M

eV/c

pperp ( )

t ( )

Energy transfer per collision, MeV

Outside of ±0.3% momentum acceptance


Proton collision with a stationary electronProton collision with a stationary electron

avep

n cM

pJ

22

2

2

1

0 5 10 15 200

10

20

30

40

Bet

atro

n ac

tion,

mm

-mra

d

J ( )

t ( )

Energy transfer per collision, MeV

Scraped to 7 mm-mrad

Evolution of beam energy distribution as a diagnostics tool Alexey Burov, Bill Ng and Sergei...

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