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

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 beam’s 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 Rosenzweig/Anderson ICFA Sardegna 2002

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. Rosenzweig/Anderson ICFA Sardegna 2002

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. Rosenzweig/Anderson ICFA Sardegna 2002

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. Rosenzweig/Anderson ICFA Sardegna 2002

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. Rosenzweig/Anderson ICFA Sardegna 2002

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. Rosenzweig/Anderson ICFA Sardegna 2002

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. Rosenzweig/Anderson ICFA Sardegna 2002

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. Rosenzweig/Anderson ICFA Sardegna 2002

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. Rosenzweig/Anderson ICFA Sardegna 2002

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. Rosenzweig/Anderson ICFA Sardegna 2002

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. Rosenzweig/Anderson ICFA Sardegna 2002