J.B. Rosenzweig and Scott Anderson

1
The Role of Space-charge in Emittance Measurement
of High-brightness Photoinjector Beams

• J.B. Rosenzweig and Scott Anderson
• UCLA Dept. of Physics and Astronomy
• ICFA Sardegna, July 2002

2
Measuring the emittance of photoinjector beams

• The low emittance of, and huge forces (internal
  and external) applied to these beams makes them
  behave very differently (like plasmas) than
  emittance dominated beams
• In addition, investigation of the behavior of
  these beams, as well as optimization of the
  beams end use, requires accurate measurement of
  the beam emittance
• In order to produce accurate measurements, the
  emittance diagnostic must take into account the
  nature of photoinjector beams

3
Measuring Emittance

• Traditional emittance measurement techniques
  (e.g. quadrupole scan) use the envelope equation
  for a drifting beam ignoring space-charge,
• For typical photo-injector beams, this exclusion
  of the space-charge term is not appropriate
  except in a strongly focused waist. We see that
  these beams are space-charge dominated by the
  ratio of the space-charge to emittance terms in
  the full envelope equation.
• We used the LLNL/UCLA Thomson scattering
  photoinjector, with a short pulse, low energy
  beam to test different emittance measurement
  techniques.

4
Slit Based Emittance Measurement

• Beam phase space is reconstructed from the
  position and width of the beamlets on screen.
  Emittance calculated from phase space picture.
• Collimation makess emittance dominated beamlet
  expansion due to emittance, not space-charge.
  Plasma wavelength same b function becomes much
  smaller.

5
Quadrupole Scan Measurements

• Neglecting space charge we can write an equation
  for s2 based on the Twiss parameters of the beam.
• The procedure then, is to measure s2 (the mean
  square beam size) versus the focal length of the
  lens and fit the resulting curve to calculate the
  emittance.
• Thick lens treatment often necessary in compact
  beamlines.

6
Experimental Procedure

• Emittance was measured using both the quad scan
  and slits for different beam plasma frequencies.
• The plasma frequency was changed by changing the
  laser pulse length. This was done by altering
  the grating pair separation in the laser system.
• For each set of measurements, the laser spot size
  and energy, grating pair separation, beam charge,
  and injection phase were recorded in order to
  calculate the plasma frequencies.

7
Quad Scan Vs Slit Data
The strength of the space-charge forces are
parameterized in the scan by product drift length
between quadrupole and detector and the plasma
wave number.
8
Simulation of the Quad Scan

• Given the measurements of charge and laser pulse
  dimensions, we simulated the beam dynamics up to
  the quadrupole using PARMELA and HOMDYN.
• The quad scan procedure was then simulated with
  PARMELA using point to point space charge,
  HOMDYN, and by integrating the envelope equation.
• All three methods gave similar results. Linear
  transport models agreeing with PARMELA leads us
  to believe non-linear space charge effects are
  unimportant.

9
Quad Scan Simulation

• PARMELA simulations predict emittances in good
  agreement with slit measurements.
• Simulated quad scans with emittances set by the
  slit measurements give higher output emittance
  values that agree reasonably with quad scan
  measurements.

10
Space-charge in the Quad Scan

• There are two relevant normalized numbers two
  parameterize space charge strength, one measuring
  drift length, and the other measuring emittance
  v. space charge.
• The white plot points locate the positions of the
  experimental data. The normalized emittance used
  as input to the simulations was 5 mm mrad.
• kpb is a measure of the ratio of the space-charge
  to emittance forces at the quadrupole.

11
Interplay of Space-charge and Emittance
Simulation

• Data and simulation both show asymmetry about
  minimum spot size.
• Asymmetry is due to different emittance forces.
  If waist is emittance dominated, then envelope
  looks very different before and after waist.
• Asymmetry makes fitting to a parabola
  problematic.

12
Summary of Emittance Measurement Techniques

• Quad scans are ill-suited for highly space-charge
  dominated beams because the beam evolves under
  the influence of both space-charge and emittance
  effects.
• Rules of thumb
• Quad scan data may not be valid if
• Asymmetry in the data is an indicator of trouble.